VetteC5RFrEak

Well the issue is that my car is that whenever I come to a stop my idle will hover at 1000rpms. If I am lucky, after a few seconds, the idle will drop to where it should be, just right above the 500 mark. However, most of the time I have to tap the throttle a couple times and then the idle will finally drop to 500.
Any ideas what is causing my idle to stick? I would really like to get this issue fixed. It is quite annoying sitting at a light tryign to get my idle to drop.

beatercoprice

First place I would look is the throttle body. See if it is sticking slightly open, maybe due to crud build up. Try replacing the TPS sensor. If the TBI has never been refreshed maybe you should consider getting a rebuild kit (and some torx bit screw drivers, you might also need the offset torx wrenchs cant remember if I needed them or not). I rebuilt my TBI no problem, easier than a carb. Made a HUGE differance in pedal feel. I bought the GP sorenson kit from advance auto I think it was 45 bucks, a new TPs was something like 20 and an AC delco IAC was 50 bucks, the torx screwdrivers and offset wrenches were about 10 bucks.

thedude

Although I agree that it's a good idea to rebuild the TBI, you should just try to lubricate the throttle and the throttle cable first, and see if that helps out.

VetteC5RFrEak

I know it ain't the throttle linkage or butterfly valve sticking. I have been under the hood and watched. You can't close the throttle anymore then it already is. I pulled the IAC out yesterday and it had minor build up on it, I cleaned it off, but my idle still seems to hang like usual. Unfortunately in the process I managed to break the head off the bolt that was holding my TPS on (turns out soemtime in the past the other bolt had it's head twisted off to; so last night I had to drill out one of the bolt studs and make-shift somethign for the time being. So I will most likely have to pull the TBI unit off soon and tap the hole. I will def look into a rebuilt kit.

As far as the IAC goes, I only looked once place last night, and that was Kragen's online store, they had one for $100, and thats the only one they had.

Thanks for the information guys, I really appreciate it. I am def gone check it out.

VetteC5RFrEak

Is it possible that the EGR could be causing all these issues? I checked out my EGR today and it doesn't move at all at any rpm. The vacuum line to it works when you give it some gas, but the diaphram inisde the EGR doesn't move at all.

thedude

When EGR valves fail, they usually fail in the closed position, which wouldn't affect idling at all. If it gets stuck open, it would cause a low, rough, hesitant idle. Although the valve could be bad, I don't think it is related to your problem.

91chevz71

could be TPS or IAC. try cleaning the IAC.

RFmaster

C5

I would check my P/N switch input to ECM just to make sure.
EGR will not be active below MPH limit, so reving engine in park should not activate EGR valve. On high mileage engines with mostly highway miles I have seen carbon build-up that may keep EGR in semi-open position. Taking EGR off and cleaning the passage may 'clear' this up.
Good Luck

//RF

VetteC5RFrEak

thedude, yea, what you are saying makes sense. When the car was idleing, I manually opened the EGR and the car almost died out on me.

91chevyz, the IAC is only 2 weeks old. New TPS soon to come though.

RF, what do you mean check the PN switch? Also, I was getting vacuum out of the line goign to the EGR when reving it in neutral. Is that supposed to happen?...

RFmaster

C5

Ignore my comment regarding P/N switch - your car is T5 and does not have Park/Neutral switch. I think, ECM uses VSS to determine when it comes to stop or when speed is below 3 MPH. Idle speed is affected if car is moving or not. But most importantly, the ECM cuts off fuel completely when decelerating. Without VSS signal BLM readings would drop to a very rich 114-116 when coming to an abrupt stop or slow down. The idle would also start to hunt up and down when coasting to a stop! Do you know your ECM type number (8746)? Can you read ALDL data stream?

EGR vacuum is controlled electronically through vacuum solenoid - it should have black-red and gray wire. If solenoid is stuck open you will have manifold vacuum all the time. During normal operation ECM will cycle EGR solenoid, while monitoring vacuum level reduction via MAP sensor. Typically EGR will be enabled only if vehicle speed is above 45 MPH, steady TPS, steady low map values, etc (highway cruise mode). EGR should not be active at idle, during acceleration, or WOT. see sticky
https://www.thirdgen.org/egrdiagnose

//RF

VetteC5RFrEak

I am still having this issue, almost a year later. Except now, I can't tap the throttle to get the idle to come down. After stopping or coming to a speed less then 5-10mph, the idle will slowly drop from 1500 down to 500-600.

In class I hooked it up to a scanner and it was reading 0 for IAC counts. My teacher said it had a vacuum leak then. We looked and looked, couldn't find one. He then asked if I have ever messed w/the idle screw. I have, but the plug was missing before I got the car, so it was F'ed w/before I got it. So I used the scanner as a tach and set the idle where the book said to set it. It said to set it at 475 plus or minus 25 with the EST disconnected and IAC disconnected with the pintle fully extended. After doing that, I am now getting IAC counts, yet the problem still exists. I have tried a different IAC, still the same problem.

Last night, just for reference, I was cruising about 70mph and put the clutch pedal in so I was in neutral. The car would idle no less then 1500. I cruised in neutral for a good minute and there was no fluctuation in idle.

The throttle linkage does not stick, nor does the butterfly valves in the TB. I can even simulate the problem when sitting still. If you open the throttle then let off, the rpms will slowly drop down to the 500-600 range.

At this point I am lost as what to do.

RFmaster

Sorry to hear that you still have this problem - I would also check for general vacuum leaks.

1) Check TB to intake manifold gasket. As gasket ages it tends to develop leaks.

2) TB shaft - check for worn shaft right where goes into throttle body. Any wobble would indicate that shaft has worn out its hole.

3) Power brake booster and vacuum hose leak. Disconnect PB hose from intake manifold and plug intake manifold tap for testing.

4) PCV valve and hose - check for leaks.

VetteC5RFrEak

I changed the TB to intake gasket. When my teacher mentioned the vacuum leak, he mentioned that the TBI's tend to suck the gasket in w/age. However, the old gasket was fine. But I did replace it.

The TB shaft, I know it has some play to it. I have another TB that I have been slowly rebuilding. I did the Ultimate TB mods to it by grinding off the bore lips. I finally found a place to ream the shaft and put new bushings in, but from what I hear, they charge $100. So I am still in limbo over that. I hate putting money into this engine since I plan on ditching it eventually for an LSx+T56 swap, yet at the same time, the idling up issues blow.

I will check the brake booster and it's related vacuum hose to see what happens.

As far as the PCV and it's hose, my teacher pulled that when searching for the vacuum leak, couldn't find anythign wrong.

He did mention that there could be an internal leak and that there is a test you can by blowing propane into the crankcase through the PCV vent and you would hear an RPM change if there was a leak.

I am just confused though, why does the engine hold the rpm's while moving, but lets them drop when I stop. That parts just confuses the crap out of me.

RFmaster

See if there is connection to brake usage. Coast to stop vs. braking to a stop - see if idle follows the same pattern. ECM increases idle speed when VSS reports vehicle speed above 3 MPH. Connect a vacuum gauge to the same port that PB booster is being tapped from. BTW vacuum taps are know to leak!

//RF

VetteC5RFrEak

Hopefully I can get on it manana and check it out. Thanks for the input.

VetteC5RFrEak

Well, I checked the brake booster and it wasn't leaking. I disconnected the rubber hose and plugged the port w/my thumb, rapped the throttle and it still hung before dropping (this is just idling still). However, for a few seconds it seemed if I pulled the throttle real hard and released real quick it would bring the idle down right away, but almost would kill the engine. Then if I slowly pulled the throttle and let it down slowly, it would hang before dropping. Unfortunately, it got to the point where it would hang while coming down if I let off the throttle quick or slow. So I decided I would investigate further.

If I unhook the purge canister vacuum line at the canister and plug it w/my finger, you hear a whistling sound. You get the same sound if you have it hooked up to the canister. However, if you just let it suck air freely, there is no whistling sound. So I grabbed the spare TB I have and wanted to know where that wound up inside the TB since the sound was coming from inside the TB. Turns out, that port is also connected to the two little holes right above the butterfly valves which is what I supposed allows the fuel into the engine when at idle.

At this point I decided to pull the TB off of my engine and see if maybe it needed cleaning in the vacuum ports. I sprayed some brake clean, no clogs. I then pulled the brake booster vacuum line out of the intake manifold, wrapped it with teflon tape and re-installed it. I did the same thing w/the fitting that screws into that. I then pulled the other vacuum fitting off of the back of the manifold and wrapped it's thread w/teflon tape, then re-installed it.

After that, I pulled the long vacuum line out that goes from the back of the intake manifold, wraps around the back side of the engine, comes down the side of the driver's fender and hits a T-joint. I turned the air compressor on, turned the regulator down low and filled a bucket up with water. I plugged one end with a finger, and forced air into the other end while submerging the entire line. No bubbles.

I did this with the vacuum line from the back of the TB to the MAP sensor, and did the same thing with the vacuum line from the TB to the EGR solenoid and from the EGR solenoid to the EGR. No leaks.

There is a T-joint off of the back of the intake manifold. One way goes to the other T-joint I mentioned a few paragraphs up, and the other end goes to the heating controls inside the cabin (I assume so, it goes into the bulkhead). I pulled this T-joint out since it had two small rubber hoses attached to it. I tested it under water and I had bubbles appearing. So I replaced the two small hoses and had no more bubbles.

I re-installed everything, started her up and it sounded like she didn't hang as bad, but still hung up. I put my tools away then drove her around the block a few times. The idle seems to hang at 1000 now instead of 1500. And it still sticks at 1000 until I stop. I still don't think this is normal. I swear I remember a time where the idle dropped down to where it is when sitting at a stop between gears or when coasting in neutral.

Heck, my girl's '95 Mazda 626 drops right down to idle in neutral or in between gears. I don't see why the idle would need to hang.

I really hope there aren't any vacuum leaks in the lines that go to the heating controls. However, I do remember now that some of my vents don't work unless I have the A/C on, but that could be a separate problem.

My next step is to see if my girl's Dad will let me borrow sensors off of his Suburban which has a 350 TBI in it. That thing just passed smog extremely clean, and I don't think he has issues w/how it runs. So hopefully this helps me out.

RFmaster

'Vette

That was one extensive troubleshooting session! Regarding idle speed hang between gears. IIRC manual transmission cars hang idle speed between gears to keep CAT fired up to meet emissions requirements. The way to verify this is get ALDL cable and record IAC position during driving. If IAC stays well above 4 to 10 counts idle speed will be high. This IAC behavior can be dealt with by altering EPROM - which ECM service number and Broadcast code do you have???
I am still trying to recall a thread dealing with this issue - Perhaps Rbob can shed a light on this subject.
Before swapping sensors you can record sensor operating voltages and see if they are in range. It is very easy - you just need a DVM.

//RF

VetteC5RFrEak

Well, the idle stills hangs at 1500. Dunno why yesterday she decided to be a little nicer to me, but she is back to normal now.

I am currently in the process of switching sensors from an '89 Suburban which has the L05 in it. So far I have tried the Map sensor, EST module, and the ECU, no difference. Well, there was a difference with the ECU, but it was because the ECU wouldn't cooperate with the engine. Why is it that the trucks that received TBI have an extra prom chip that can be removed, when in mine, that extra prom chip in there is embedded into the board on mine? I've tried two different trucks that have TBI, one being a 305 and the other a 350. Both do the same thing when I install my PROM chip into those computers and run them in my car. The engine will stick at 1500 and no drop at all while the SES light constantly flickers. I do find it odd though how the idle sticks at 1500 with those ECU's and sticks at 1500 w/mine when driving around. I have yet been able to find a ECU from the same car as mine to just try without purchasing.

I know the sensors are reporting correctly since I had the car hooked up to a GM Tech II scanner in class and was watching the data and everything was within spec according to my Shop Manual I have.

However, I just did notice something when removing the TPS (which is the last sensor I haven't tried or replaced on my engine). The 350's have bigger injectors compared to the 305's right? And there is a way to tell which is which right? By the color of the injector I am assuming? The connectors on the 350 injectors are orange, so are the ones in my car. I am going to look up some information on that after I finish this reply.

If you could find that thread, that would be awesome. As far as ECM service number, I know where to find that, but what is the broadcast code? Is that the 4 letters that is on the ECM and on the PROM chip?

VetteC5RFrEak

Well, I just took a closer look at the injectors after reading up on some information. The ones in the 350 have black and orange in the center, mine as far as I can tell only have green, as the other color has been wiped away. So the only injectors that had green on them were the 5.0's.

Just curious, could a injector leak be causing my issues? If so, is it possible for the injector to only leak when opened up? By that I mean anything more then idle?

Wait, that contradicts another problem I have though. I can be sitting on a decline at a stop. Keep the car in neutral, idle is at 600-700, where it is supposed to be. Then let off the brake and let the car start rolling and picking up speed. I can get up to 10 or so MPH and the idle will kick up to 1000-1500. I had forgotten about that since there aren't too many hills I find myself stopped on.

VetteC5RFrEak

I tried a different TPS, no difference. I am really leaning towards an injector leaking at this point. Whenever I shut the car off I can always smell fuel initially. Also, it has been sucking gas like crazy, not to mention it does smell awful.

I have a rebuild kit I had been waiting to use until I finished the Ultimate TBI mods on the other TB, but I might just have to use it now.

RFmaster

1) VSS provides vehicle speed and once over 3 MPH so that idle speed strategy is different between standing still and rolling.

2) I think that you have incorrect IAC. If you look at IAC pentacle it is of a certain shape, which should much the TB orifice shape. When engine is idling, while standing still IAC is commanded a handful of steps 4 to 6, which allows very little additional bypass air through idle air passage. When ECM detects that vehicle speed is rolling it opens IAC to raise idle speed. I think there is a lower step limit for IAC for moving speed. If the shape of the IAC pentacle does not match TB orifice excess air is allowed to enter engine - raising idle RPM, but ECM can not command IAC to lower step value due to vehicle movement! What is needed is the correct IAC which has corresponding pentacle shape. GM used about dozen or so different IAC in TB applications which can be screwed in your Camaro TB (according to one source it should be GM#17111288). If someone swapped TB the IAC has to match TB. So, post TB part number stamped on the front left vertical TB edge.

3) Truck ECM most likely has a service number 1227747 with broadcast code (BCC) ASDU, ASDZ or similar. Your camaro has a different ECM service number with BCC tag attached to EPROM - it should a four letter code. The two ECM's are not drop-in replacements for each other - they are very similar, but there are several differences.

4) I've had a chance to look for that link. Here is the link
https://www.thirdgen.org/forums/diy-...-follower.html

//RF

VetteC5RFrEak

All right, so you are saying the ECU kicks up idle when above 3 mph? However, the ECU should not be kicking it up that high.

As far as the IAC, I could very well have the wrong one. Yet, that should only affect idle speed when the pentacle is almost fully extended. I have looked into the TB and watched the pentacle in action. I just moves slowly to the extended position. And this is the case with another IAC I used. I know the IAC can move faster then what it does because when you tap the throttle, the pentacle moves rather quickly inward; but inward I mean opposite of extended.

Truck ECM's being different, I have found out the hard way haha. I believe the BCC for mine is APTX, and the SN is similar to the one you posted because the truck one had that SN you posted and I remember comparing them and them being off by 2 numbers.

As far as that link, holy hell bro, there is a lot of information there that does not make sense to me. In all honesty, I don't even see why I would need to edit the parameters for my prom chip. The chip in my car is the stock chip, and I know these cars didn't come stock from the factory with an idle stuck like mine is. I know this because mine never used to do this, and others haven't reported having similar problems.

I have an idea on what it might be, and I mentioned it in my previous post. My car has somewhere near or over 300,000 miles on it. The engine is not the original engine. The heads, intake, and TB are the only original parts of the engine itself. The block is a '77 305 block because the previous owner ran the engine with pretty much no oil. Anyways, that means if the TB is the original one from the factory, which I have no doubts why it wouldn't be, then the TB is probably due for a freshen up. I doubt that TB was ever serviced. I was watching the spray pattern from the injectors and pretty much it is a mist. However, if you look closely, it appears there are small driplets from the center of the injector dropping into the bore. Now this is where I have come up with a speculation, and you can let me know how close or far off this speculation might be:

If the injector is dripping extra fuel into the engine, the ECU would have to compensate for that by opening up the IAC so the engine isn't running rich, this in turn would raise the idle. The injectors might only leak extra fuel when opened more then what they are at idle. So when I tap the throttle, it opens the injector up and squirts the fuel in to raise the rpm's, but when I let off the throttle, the injector has now been opened and extra fuel can leak out which would cause the IAC to stay open so the engine doesn't run real rich. You mentioned that when the car is up 3 mph the ECU kicks the idle up to help with shifting, well, if the scenario I just described is true, it would exaggerate that kicked up idle since the ECU is opening up the injector.

Like I stated, I have a rebuild kit, I can throw it on and see if it helps or not. But I just want to see if my logic (1) makes sense, and (2) is correct.

RFmaster

Hey

Sorry about information overload!

1) Well, based on information you've provided your Camaro is equipped with 1228746 ECM with BCC APTX which was used in 1992 305 TBI 5spd in California emissions car. It is not that unusual to find later BCC in earlier cars. It is possible that someone replaced original ECM with a later unit. Like you said she had tough 300k on the clock so anything is possible.

2) Take a look at TB body for part number identification it should start with 17xxxxxx - here is the photo of one of my JY pulled units 17093030:



This unit I pulled of early 90's C2500 PU with 350 CID at a local pick your part. Once you have TB part number you can find a matching IAC. Mismatched IAC maybe causing excessive by pass air - high idle speed. An all too common condition with IAC that’s fully extended (closed). This is often a symptom of an air leak downstream of the throttle, such as a leaky throttle-body base gasket, intake manifold gasket, vacuum control circuits, injector O-rings, etc. ECM forces IAC to close bypass air circuit in an attempt to compensate for the unmetered air leak that is affecting idle speed. High idle speed can also be caused by a shorted A/C compressor clutch wire or defective power steering pressure-sensor circuit. Having a WinALDL or Tech2 scanner is invaluable.



3) Injectors should spray a nice cone of fuel - there should be no fuel drips. In most instances external injector rubber o-rings are history.

4) Which thermostat (if any at all?) do you have in your Camaro? If you run 160F it may be causing rich condition and high idle speed - ECM sees colder CTS and commands higher idle speed, richer mixture.

//RF

VetteC5RFrEak

I have a 195 T-stat in her. I was running a 160 for the longest time but about 6-8 months ago finally went back to 195. CTS is functioning properly I believe. My gauge in the car rarely reads above 120F, but I have had it hooked up to a Tech 2 and it showes 190F+ etc.

I understand what you are saying about the IAC pentacle. If the pentacle was a triangle shape the TBI had a circular curve for the pentacle, they would not match and air would get by. However, wouldn't that only be an issue with the pentacle fully extended, 0 counts? Because otherwise, the IAC could compensate for it by instead of being at 10 counts, it could go to 9 or 8? I only mention that becuase I am getting IAC counts when she is at idle, 700ish rpms. I believe something like 8-12 is what I am getting. But I will pull the TB number, try to find a P/N for the IAC on there, and double check the ECU S/N, although I do believe you nailed it with the S/N you posted up.

I also understand the bit about if the pentacle is fully extended, the ECU is trying to compensate for an air leak somewhere because air is obviously getting in. Initially when hooked up to the scanner I was showing 0 counts for the IAC. After properly adjusting the idle described in my Shop Manual, the IAC counts were as described in the previous paragraph, 8-12. So I don't know if I have a leak, or not. Also with what you were saying about incorrect IAC could've been the reason she kept running when the pentacle was fully closed. As far as I can tell, everything between the intake gasket to the TB gasket is fine, I wasn't able to find any leaks. I need to check the EGR gasket, see if it still looks good, then if I still have a vacuum leak, which god I hope not, then I gotta do the intake gasket.

What do you think about my analysis of the fuel injector leak though? Does the logic make sense, and would the computer actually do that?

RFmaster

Yes, I think that you are correct in your assessment. Why all this fuss about deceleration, well a lean fuel mixture is required to reduce emission of hydrocarbons (H) and carbon monoxide (CO). To accomplish this, fuel delivery is either reduced by the DE (Deceleration Enleanment) portion of the ECM programing or if certain conditions are met fuel is shut off completely during a fuel mode called DFCO (Deceleration Fuel Cut Off.) Consider operating engine conditions while in neutral, coasting down to a stop. When you begin to coast ECM will detect a sudden drop in TPS value and decrease in MAP voltage which indicates low engine load. ECM reduces the fuel while continuing to monitor engine load, RPM and vehicle speed. That would be fine but injectors are still (dripping) dumping fuel. Under DE mode integrator is held to the value of 128 and fuel correction is not performed. Mean while ECM commands IAC to allow additional air to bypass throttle plates, since TPS is low, and with extra fuel present engine speed increases! Under DFCO ECM cuts out fuel, and O2 sensor should read very low mV (lean condition). To keep engine from stalling ECM commands IAC to allow additional air in to keep idle speed up. Extra leaking fuel will increase idle speed. Once you come to a stop (or below speed threshold) or if there is a change in TPS/MAP value ECM exits DE -DFCO mode back into normal idle mode, close loop operation. Integrator is unlocked from 128 and O2 readings are taken into account - ECM will force PW to achieve O2 cross counts. Interestingly enough DE -DFCO are not a big issue with automatics - trany is always coupled to engine. I think you should pose this question in DIY EPROM sub forum Also this is a classic article worthy of a read: http://www.diy-efi.org/gmecm/papers/747iac.txt //RF

VetteC5RFrEak

Well, time for an update, yet I still haven't gotten anywhere.

I use the rebuild kit on the TB. It was fun doing that; it was nice having everything go together and come apart like it is SUPPOSED to haha. I installed the TB back into the car, no fuel leaks from any of the gaskets; that made me happy, I did it correctly. Unfortunately, no difference with the idle issue. It did feel like it had a little more oomph in the higher rpms, but no change in idle like I stated.

A few weeks ago in class I realized we had a hand powere vacuum pump with gauage. So I figured, wth, I'll recheck the vacuum lines I tested before. All of them held their vacuum except for the one that goes into the passenger compartment for the vent controls. It wouldn't hold any vacuum. I was thinking, okay, my teacher was right; I had a vacuum leak and here it is. So as a temporary fix, I disconnected it from the T-joint off of the back of the intake manifold and plugged it up with a plug. I rechecked that line and it held vacuum. Started the car up, drove it home, no change in idle, it still hangs.

Eh, I can't entirely say there is no difference. As time has gone on, the idle seems to hang around 1000 about half the time, and the other half around 1500. But, it is possible I wasn't paying that close enough of attention to the tach before. so I dunno.

I haven't entirely ruled out the idea of a vacuum leak, but the only place it could be leaking would be either the intake to head gaskets, or through one of the valves inside the head. After thinking about it, if one of the valves was leaking air into the combustion chamber, wouldn't that also mean that there would be a compression problem? That compression would be lower on that cylinder then the others? Is it possible that air can still get into the intake tract of the heads through the valves?

BTW, I did get the number off of the TB and IAC finally. The numbers are as follows:

Throttle Body
17089063
0469 KAH

IAC
90513
J1256

Then on one side it says "Assembled in Mexico" and has an F below the Mexico. On the opposite side of this statement is the number "21". I am not sure if those are important or not, but figured I would put 'em out there.

I am still not ruling out the fact that the injector themselves leak when opened up beyond what idle would be. However, I haven't been able to get my hands on a spare set of 305 injectors for cheap.

Sorry for all the questions, but I am throwing out anything that could possibly spark an idea to fix this issue. I am getting something like 13mpg on average because of this. My car smells like fuel when you shut it off. But it is strongest in the rear, which is weird. I don't have any fuel leaks as far as I know, I would see the puddle.

I Seafoamed the engine last week, didn't change anything. I ran a full can of seaform through a full tank of gas, nothing changed.

Is it possible an O2 sensor could cause all this? I pulled mine out today to look at it and it is white. But I have no codes. I tried doing a search on this site seeing if when an O2 fails if it will throw a code or not, but I can't find anything. And I don't want to just throw down $20+ for something that I already have you know?

I think I have pretty much exhausted everything going through my head at the moment, let me know what ya'll think.

slow_90firebird

I am not really smart about all this hi-tech talking, but maybe I can help you feel better (or worse) about the situation...

My car is a 1990 LO3 TBI with the T5. So should be almost identical motor wise.

The high-idle contidions you are describing, seem 100% normal to me. I bought the car last year, with 47,100 miles on it. Nothing had been touched. Even had the origional (now black) air filter.

When I slow for a red light, usually decel in 2nd gear, if I push the clutch in at 500 RPM it will immediately shoot up to ~1200 or more. Then the minute I stop, it will come back down to 5-600.

If I am stopped and idling at 500, and I start rolling, the computer automatically increases the idle again for me. It kinda sucks, but whatever.

If I am on the highway in 5th, or in town in 4th, and start coasting in neutral, it will keep idling right along at ~1500.

----------------------

I had a small chat with RBob on this forum, and he admitted that GM did a really crappy job programing the idle conditions for the TBI motors. He said that when he wrote the EBL, it will drastically change the way the car runs and especially the idle routines, and make the T5 cars more "drivable". How true that is, I cant say. It is just one of the reasons I want to get the EBL flash soon though.

VetteC5RFrEak

Wow, that is the exact same problem I have, and you are the first person I have come across with the same exact problem. You are right haha, it does make me feel better, but at the same time even more so frustrated because it might actually be a "normal" thing.

However, I have one problem with that, the idle in my used to drop when in neutral. I used to be able to compression brake (that is what I always did when come to a stop, drop to a lower gear then use the brakes, I feel it is a safety thing) and it would dramatically slow the car down. But now, I can't even try and save gas by upshifting soon because the idle is too high. In fact, I used to be able to throw the car into 5th at 35mph (on my speedo, my tires are a little taller then stock, so I am sure it's more like 37-38mph) and be at 1000 rpms and I wouldn't get that little jerk forward I get now because the rpms are sticking. Now, I am lucky if I can get into 5th at 45mph without feeling a jerk, forget even trying 35.

So yea, GM may have done a crappy job on our engines, but mine was running better then this. Not to mention, if I jump on it in, lets say 2nd at around 2000rpms, she will scoot along, but the power is not consistent.

Another situation, I will be doing lets say 50mph and drop into 4th to begin to slow down. It will get to around the 1500 rpm mark and all of a sudden kind of jerk forward for a second then slow down a little more, but then kind of level off because the idle is sticking. I just can't accept the fact that this is "normal."

Then again, I used to run a Jet Stage II chip for an automatic car in my car (I dind't know it was for an automatic, the previous owner gave it to me with the car). But there was a reason I pulled that chip out, and it might have been because I was starting to get this high idle issue.

I really want to datalog my car. I have a laptop (which I actually bought to datalog), but I don't have the ALDL to USB cable. And I just can't justify spending $60 on a damn cable

RFmaster

Vette

A while back I was reading a post regarding high idle in neutral and I also run across technical Q&A in CHP magazine for a similar problem afflicting RAM-JET 350 with a new GM computer. In both discussions that followed on pointed toward EPA mandated fuel enrichment routine which is used to keep CAT 'fired up' during neutral drops or deceleration. With RAM JET it was so bad that it became de-facto cruise control as engine maintained 1500 RPM at low delta TPS values. CHP guru recommended contacting GM for an updated EPROM since it was used in non CAT application.
In your case, if I may suggest, post this question in DIY-EPROM forum - ask Rbob if he knows of this problem. Be prepared to supply all ECM, engine information.

//RF

VetteC5RFrEak

Haha all right. The reason I laugh is because in order to ask the question, I feel like I have to provide all the background information (otherwise, how is one supposed to properly diagnose something, you know?). I think I'll create a thread, post this link and hope like hell people will take the time to read this because otherwise it'll be an essay.

Racer X79

I have a 91 RS 305/5 speed with 87K and the exact same problem, and it drives me nuts, that's why I was looking for help here. It is bone stock-but not for long, if I can't get it to idle at 600-800 rpm when it's warm(it will do it all day long but as soon as I move-whammo--1200-1500 rpm all the time) I am trashing the whole TBI and switching to a carb. I have tried all of the exact same steps as listed here and nothing has worked. I feel your pain C5.

VetteC5RFrEak

I am about to just throw down the $60 for an USB ALDL cable so I can datalog this car and figure out exactly what is going on. Before I do that, does anyone know where I can get one cheaper?

I am glad to see I am not the only one in this situation. Yet, at the same time it sucks because no one has answers really haha.

89IROCZ28

iTrader: (14)

Vette,

What happens if you extend the IAC to close it off (there's a proceedure for this somewhere in these forums), then disconnect it and adjust the idle manually to 600-700 rpm? If it acts more 'normal' then you'll know it's the ECM controlling the IAC. If it still idles high, the problem is with something else - like a vacuum leak.

I'm having an issue w/ the injectors metering too much fuel and I can say w/ some certainty, too much fuel won't cause an increase in idle w/o additional air. It will only cause it to idle very poorly due to the rich mixture.

Racer X79

I decided to try one last thing yesterday. I took out the little steel plug over the idle adjustment screw and turned down the idle to 650 rpm before I drove the car over 3 mph. I let it sit and warm up to the point that the fan kicked in the idle stayed at 650 rpm. I then took it for a 30 minute ride with several starts and stops, and several wide open through 1st/2nd/3rd gear runs. Every time I came to a stop, it idled from 800-900 rpm. I know it probably isn't the "right" way to fix it, but I can live with 900-1000 rpm at idle....not 1500. Performance is the same, idle quality is good, and everything seems fine. Now, the TPS and the IAC are most likely a little off-but I can live with that. Good Luck.

Racer X79

500+ miles now since idle adjustment......it's still running flawless. It seems to work!

VetteC5RFrEak

All right, well I got my USB-ALDL cable from Moates yesterday, and did some logging, but was having trouble sorting out the information in notepad. So I did some more logging today, and converted it to Excel. Here is the spreadsheet. From what little I know on reading the information, it looks like the IAC is fully extended (0 steps) when the car is idling. Which, if that is the case, doesn't that mean air is being sucked in someplace else? I did re-adjust this months ago in class using a Gm Tech 2 scanner and was getting 10 steps at idle, but now I am back to 0.

Am I correct in my assumptions, or does the data show something else?

RFmaster

I took a look at your excel datalog late last evening. You have a combination of idling and actual driving data. This is fine overall, but I wonder what your engine does starting from cold start (cold, over night, CTS 25C) and just idling up to fully warm condition (195F or 92C CTS). This should remove a lot of variables. IAC is behaving as if you have a large vacuum leak somewhere - this probably explains high idle.



//RF

VetteC5RFrEak

I will see what I can do about getting a cold start datalog. I work at 4:30 in the morning and I usually get up with just enough time to throw on some clothes, brush the teeth and head on out making it to work with a few minutes to spare. So getting up earlier is a no no haha. I'll see if I can get one this weekend though.

I was messing around with the datalogger again last night and when I first start up the car, it was showing like 6 counts for IAC. I was confused by this because I had just posted the datalog above and in there it shows 0 counts. I tapped the throttle, rpms went up then dropped slowly and then my IAC was reading 0 counts again. So it appears it will get some counts upon initial startup, but then after any throttle it drops to 0? The car was at like 120F or less when I first started it up, so maybe it was still warm, I dunno.

VetteC5RFrEak

Well, I have still yet to get a chance to get a cold start reading since I work 3:30am to 12:15pm, I just don't get up early enough to go sit in my car and let it fully warm up.

However, I was tossing around the idea of an internal vaccum leak, in my head. Like on the underside of the intake manifold where the manifold meets the head. I did some searching on google and came across this site (http://www.therangerstation.com/Maga...acuumLeaks.htm). I borrowed a vacuum gauge from a friend and initially just disconnected the vacuum line that hooks right up to the hot air valve on the intake duct. I realized it wasn't a straight shot from the TB. So I disconnected the hose that goes from the air cleaner to the TB, and attached the vaccum gauge there. I started up the car and was getting excited as I was coming around the hood because I figured I was going to find out something was wrong. Unfortunately, the needle was in the green labled "Engine is okay". I opened the throttle and released it, the needle would drop then come right back to the green. The needle barely moves when in the green too.

So, I decided I would try and reset the idle speed again by following the procedure in my service manual. Turn key on, extend IAC pintle using scan too (I used a paperclip between A and B and verified pintle was extended by looking down into the TB), disconnect EST and IAC, remove paperclip, hook ALDL/USB cable up to car and laptop, then start the car. Initially, my car surges. This is something I have noticed before when disconnecting the EST to set the timing in the past. It makes it really hard for the car to idle, but eventually the idle mellows out and becomes stable. I adjusted the idle to 475 plus or minus 25. However, when I had the idle at 650 and was backing the idle screw off, the idle would not drop unless I manually put some pressure on the throttle linkage, then eventually it would drop. I got the idle set correct, turned the engine off, reconnected the IAC and EST, then reset the IAC by starting the engine with the pedal slightly depressed for 5 seconds, turn the engine off, and start it back up.

At this point I hooked the laptop back up to the car and was seeing that there was IAC counts, like 14 I think. But if I open the throttle and let off, it will slowly drop and once it finally reaches around 700 rpms, the IAC counts are 0.

I am confused as what to do now. The laptop shows I have a vacuum leak, but I have tested all the lines coming from the TB and manifold, they are fine. The Vacuum gauge shows that the internals are fine. Whats going on here?

Side note, as I was typing this, I realized something. A few paragraphs up when I was talking about setting the idle, the throttle blades wouldn't close past 650 rpms on their own. This means that the blades are completely shutting, and thus air is getting by. In effect, the computer sees this and closes the IAC thinking the air is getting by there, but then it completely closes the IAC and air is still getting by. Do you think if I were to hook the laptop back up, start the car, blip the throttle so the IAC counts are 0, then go under the hood and apply a little pressre to the throttle blades to close completley like they are supposed so, the IAC counts would go back to around 10? So could the whole culprit here be my TB?

RFmaster

Possibly bent throttle shaft or out of round shaft through hole - very typical of high millage TBI!!! Worn out shaft / though hole is a prime source of vacuum leaks!!!

//RF

VetteC5RFrEak

Yea, I know the shafts are a problem, especially with higher mileage. The blades sticking weren't as prominent before as it is now, which is why I kept dismissing the thought of the TB being bad. Time to source one out and buy it.


A question though, is the engine supposed to surge when you disconnect the EST? Eventually the idle will mellow out, but then once in a while it will start surging again, then mellow out.

VetteC5RFrEak

Well, after going to the dealership to see if they could get me a new TB, they told me it was discontinued. I had a buddy check prices on one from a L31. GM wants $375+ tax just for the TB alone, no injectors. Screw that, the L03 ain't worth that much money.

I noticed there was some good play in the shaft when the blades were closed. So I pulled out the TB that I had done the Ultimate TBI mods on and put it back together to check the play in the shaft on it. The play was significantly less then my stocker. So I swapped over the injector setup, TPS, IAC, and other stuff. The second TB I have is off of a astrovan I believe, so it is missing the vacuum line for the hot air valve in the intact duct. That isn't a big problem IMHO, so I just left it unhooked. Unfortunately, I did wind up breaking the hard vacuum line that runs to the EGR solenoid. So I had to plug up that vacuum port on the TB. Hooked everything up and she started. I then set correct idle using WINALDL.

I started the car back up, and it had a IAC of 9 counts and was idling at 650rpms. I blipped the throttle, counts went back to 9, and idle was still 650ish. I did it again, and this time the IAC counts slowly dropped to 0. After them being at 0 for a few seconds, the counts went up to 4 and stayed there. Everytime I hit the throttle after that and let it back down to idle, the IAC counts would be 1-4, and sometimes 0.

I took the car up and down the street while datalogging, there was no differences in idle when I would push the clutch pedal in. So now I am really stumped. Could it still be the TB, even though this one has a lot less play to it? When setting the idle, the engine wasn't surging like it did with the previous TB. So there is already a noticeable difference when it does idle.

Somethign is causing the IAC to slowly extend after letting off the throttle. When you open the throttle, the pentle retracts really quick, but it extends slowly. What would cause such a scenario?

My datalog is attached below:

RFmaster

Hey C5Freak

Looks like you got most of you problem solved. I looked at your data log and it appears that IAC is doing its thing. With car stationary IAC drops down very low - to about 6 to 8 counts when you blip throttle. With car in motion it looks like IAC drops to about 20 counts, but slowly. During AE IAC opens up to about 80 counts. Your TPS reads negative values - I am sure that it due to software glitch.

IIRC the reason for slow IAC response during cost down is partially due to emission calibrations. Also, during deceleration ECM may enter DFCO mode, which causes very lean mixtures. A very lean mixture tends to produce higher NOX levels which is a big problem. Since I am not an emission calibration guru I would refrain from giving any further explanations at this time.

I get all of my TB's from a local pull your JY. About every couple of month they have 50% off sale. Get there early and pay about $30 for a complete TB (injectors, TPS, IAC still attached). Lucky for you 4.3 L Astro van TB is same size as 5.0 or 5.7 L TB. There are minor differences - such as vacuum ports, or throttle arms for TV cable. Next big 50% is around labor day.

//RF

VetteC5RFrEak

Well, the TB I installed last night was from a local pick n pull. I actually got the TB for $14. Funny story how. My Dad goes to pick n pull to get tires for mainly my mom's car since she has a bad strut rod and it tears up tires. I went with him one of the days to look for a TB. I paid for both his tire and the TB at the same time to make things easier. The cashier apparently didn't know much about cars. He called the TB a Ford Bracket and I was only charged $14 for it. So I got lucky there.

Now since I have the "new" TB on, and you still think it is my TB, I will just find a machine shop and have the shaft bore reemed and new bushings installed to tighten things up.

You state that when I let off the throttle, the ECM enters DFCO mode. I vaguely remember seeing what that stands for in another thread. Isn't that something to do with fuel cut off? If so, I thought fuel cut off only happened under heavy braking? Carrying on, having a lean condition like that, wouldn't the ECM want to close the IAC quicker to cut back on the amount of air entering the engine if the fuel is really being cut off, or cut back?

A side note, my car does have periods here and there where she runs pretty good, and the idle drops instantly. For example, if I start her up in the middle of the day, if I keep the rpms under 1500 when accelerating, the rpms drop back down to 650 or so. But as soon as I get above 1500 rpms or so, that is when the idle seems to hang a bit more.

RFmaster

Yes, DFCO stands for deceleration fuel cut off. In automatic transmission applications (when in D4) automatic trany present a limited load to engine (no transmission braking, unless if trany is set to D1 or D2). ECM has no idea what gear your transmission is in, but it knows vehicle speed and MAP and TPS. Based on the last three ECM may chose to enter DFCO.

Entry into the DFCO mode is customarily initiated when the engine is decelerating,
with the engine control element (typically the throttle valve or
accelerator pedal) positioned for engine idling The purpose of this mode
of operation is to reduce fuel consumption and maximize engine braking
induced by the drag, or negative torque applied by the engine load
Recovery from the DFCO mode is generally initiated, when either the engine
rotational speed drops below a predetermined minimum speed near idle, or
the engine control element is moved from the idling position to accelerate
engine rotation and increase output torque.

If an engine is quickly transferred from normal operation, to the
deceleration fuel cut-off mode, or vice versa, the sudden transition in
engine output torque from positive to negative, or negative to positive,
results in undesirable ringing or jerking in the engine driveline. These
torque transients are particularly noticeable, when a manual transmission
is used in coupling the driveline to the engine.

According to conventional practice, the output torque transients are
smoothed by gradually adjusting engine spark timing or the quantity of
fuel injected into an engine, during entry into and recovery from the DFCO
operating mode. Both of these traditional approaches have a negative
impact on engine exhaust emissions, because they substantially increase
the amount of hydrocarbons present in the engine exhaust.

The last three paragraphs are exerpts from US patent 5,119,781.

http://www.patentstorm.us/patents/51...scription.html

//RF

87transGTA

how would you go about cleaning the IAC>??

nimrod_sixty9

https://www.thirdgen.org/forums/tbi/...check-out.html
Post number 6

https://www.thirdgen.org/forums/tbi/...tml?highlight=
Theres another that may help you...

DanScram

Sorry to dredge up an old post but I am having a similar problem on a 4.3L TBI and was wondering if anybody ever really pinpointed the cause and a fix. On mine, it happens with the original IAC installed and with 2 other junkyard sourced IACs.

Don't hold any of this against me but I swapped the engine from an Astro van into a Jeep and this condition is very unsatisfactory when crawling.

RFmaster

Welcome to TGO
The best way to get an answer is to post a new thread by detailing your set up. For example:
1990 4.3L TBI with 7747ECM running (list here BCC, custom?)
TB - list here (stock SBC or modified)
injectors - list here
Fuel pressure - list here
transmission - list here
rear end ratio - list here
etc
List a problem that you are experiencing, conditions, etc

//RF

VetteC5RFrEak

Good timing actually. I did just solve this issues just yesterday. My girl's brother is donating his 1990 silverado to the state, so I decided I would swap the junkyard TB I had on my car for the one from his truck. Before I even got to installing his TB onto my motor, just a quick wiggle of the shaft and I realized it was in A LOT better shape then the one I had gotten from the junk yard, and WAY better then my stocker. I swapped over the fuel stuff, installed it in the camaro, and I could not stop grinning. Everytime I let off the throttle it drops down to the maximum of 900 rpms compared to 1500+ before. It still sticks just slightly, but it definitely confirms that the TB was the issue. Now I am confident in spending money to have my stock TB reamed and have bushings installed.

One last note, with the idle issue, apparantly I was missing A LOT of power. Now I can snap my head back when I jump on the throttle. Even 5th gear has some oomph to it, and the only thing done to this motor is exhaust.