This is gonna be long winded but I will try to go into as much detail as possible so you guys can give me an informed answer. I still am having problems with my idle and I cannot figure out what is causing it. I have a 91 Z28 with the L98 engine. With the IAC plugged in it runs a little rough but the RPMs usually stay within 50 RPM of 600. With the IAC unplugged it goes between 500 and 800 RPM at pretty steady intervals of about 1 second.

Alright, now for what I have already done or checked:

Fuel system- I have already replaced the pump, filter, and FPR and I got some used injectors. The guy I got them from said they were 19 lb/hr out of a GTA. I thought all GTAs were 350s with the 22 lb/hr injectors, but who knows. Fuel pressure was good last time I checked, which was a month ago and the same problem still existed.

Sensors- The O2 sensor is less than a year old but I had some severe fuel system issues (hence replacing the pump, filter and injectors) so its possible that it is fouled. I don't think this is the source of the problem though because the problem exists in open loop as well. The TPS is set correctly and does not stick, jump, or anything else abnormal so I assume it is good. I cleaned out the MAP but I have no idea how to check to see if it is good or not. I have no fuel related SES lights.

Emissions systems- The EGR and charcoal canister are both blocked off. I do get an EGR related SES light when cruising on the highway for like 10 minutes or more. The AIR system I have not checked or disabled in any way.

Other info- All vacuum hoses are in good shape but I don't know for sure if there is a vacuum leak at any of the gaskets. I have reused the plenum and runner gaskets twice in the past year so maybe thats an issue. I did a check for misfire by removing each plug wire and checking for a reduced RPM and all cylinders checked out good in that respect. The plugs, wires, cap, rotor, and coil are all less than a year old. Timing is set to 6 degrees above top dead center and is stable. There is no unusual color or smell to the coolant or oil so I believe the head gasket is good. The IAC seems to be good since when it is disconnected the idle gets far worse. My car has no mods except for the throttle body coolant bypass. I have the dual cat exhaust... could a single clogged cat cause my problem?

I plan on doing a compression check soon on all cylinders and while I'm at it I will replace the spark plugs again. If anyone has any other suggestions on what I should examine or replace please speak up. I am just stumped at this point.

 

Have you cleaned out the IAC? Could be buildup affecting the airflow. Since the ammount of airflow kind of controls the idle the airflow needs to be precise.

 

Yeah, I cleaned the IAC and most of the intake. Sorry I didn't mention that before but there was so much other stuff I was bound to forget something.

Preliminary results on the compression test (cylinders 1-5) showed all cylinders between 190-200. I'll check the other 3 when I get a chance but I don't think this is the problem. The new spark plugs on these cylinders showed no change as well.

 

I know this kind of thing is tough to pin down. It feels to me like the MAP sensor is not reading the appropriate value, either because it is bad or because of an air leak somewhere below the plenum. How can I determine a bad MAP sensor without having a known good one to check against and how can I positively identify and locate a vacuum leak?

 

I have the same problem with my car except its an LO3.

My car idles really crappy at 500rpm, and I mean the car really shakes and its bumpy.

However when the car sits at like 800rpm I believe (the tick mark between 500 and 1000RPM) it is so smooth. Like I can't feel a damn thing.

It seems to have a consistency of running a thousand times better when the car is cold. I have no idea why, how, or when, but I can definitely feel a difference. It only stays at like 800 rpm for about a mile or two then it drops down again to 500 rpm when the car is fully warm.


I am trying to figure out a way to adjust my idle or increase it so it stays at 800rpm but the idle key I believe is computer controlled and I have no means.

I would also like to know if anyone would be of any help.

I changed distributor cap and rotor not too long ago.

 

I'm starting to think that my problem lies in a crankcase air leak. I think this because I get no idle change when I remove the PCV valve from the valve cover but I do get a change when I plug the PCV valve with my finger. Also, I am less than confident that I adequately sealed the intake manifold to the block when I replaced the intake manifold gasket. I'm guessing this added air is causing me to run lean (The spark plugs were all a little white also.) Is there another way to test this before I reseal the intake manifold to the block?

 

The idle should not change when the PCV valve is lifted from the valve cover. Plugging the PCV valve should make a slight RPM drop. Make sure the PCV valve is the correct one for the application. Remove PCV and shake it. It should rattle. If there's no rattle, it's gummed-up internally.

Once the PCV valve issues are resolved, check for vacuum leaks.
There's a few different ways to check for vacuum leaks. I like to use a squirt bottle with water when I can. I'll also use a propane torch (unlit) to blow propane around the suspected areas. The propane method works best when there is NO wind.

I've seen others use various chemicals in a spray can. I don't use that method anymore after seeing a friend catch a customers car on fire after he accidentally sprayed the exh manifold with B-12.

A piece of tubing will work too sometimes. Place some hose over one end of the tubing and stick the hose in your ear. Place the other end of the tubing near the suspected area and listen.

After you have checked the vacuum hoses for dry rot, cracking, etc, you can pinch them closed (I use smooth-jawed vise-grips) and see if the idle speed changes. That will tell you if what ever is connected to the other end of the vac hose is leaking or not. Use this method to check the power brake hose, the A-C/heater control vac hoses, and most other vacuum hoses except the MAP sensor hose. Pinching the MAP sensor hose will cause problems for the ECM.

 

PCV valve is good. Most of the vacuum lines are new. I tried soapy water to detect a vacuum leak at the manifold/head mating and plenum to runner and then used the hose to ear method to check anywhere else I could think of with no success. I have not tried the test you told me to check for inner head to intake leaks yet.

The IAC actually seems to keep the idle between 650 and 675 RPMs but with the IAC closed and disconnected, the idle is crazy (between 550-900 RPM with stalling often). I can feel that the engine is running unevenly even with the IAC working.

 

ZZ28ZZ gave you a really good method with the propane torch. Don't ignite it, just let the gas run and go up and down your intake manifold, runners and so forth with the car running. You'll know if the propane gas is getting in where it isn't supposed to and maybe lock down that leak in a hurry. This is excellent advice.

 

I'll see if I can locate my old torch then.

 

Let us know how it goes.

 

I used propane, carb cleaner, and soapy water (all at different instances of course) and none turned up any vacuum leaks. None of the hoses seem to be leaking. I did the "palms over the pcv holes" test to check for leaks into the crankcase and there was no building vacuum. I guess I have no leaks. Either that or they are being very elusive.

That still leaves me with a rough idle and no explanation. The plugs are all new and compression reads between 190 and 200 PSI on all cylinders. Wires, cap, rotor, and coil are about a year old. FPR, fuel filter, and pump are all new and fuel pressure is good. Fuel injectors are used but they all have similar resistance. O2 is new. The entire upper intake, throttle body, MAP and IAC have been cleaned. Timing is set to 6 BTDC. The TPS is set to 0.54 V and doesn't seem to stick, jump, or anything else to lead me to believe its bad. EGR is blocked and the charcoal canister is disconnected (vacuum port plugged). My only mod is the throttle body coolant bypass. PCV valve is new. There are no SES lights except one due to the blocked EGR at highway speeds.

Idle is kept fairly steady, yet rough, at around 650 RPM with the IAC plugged in. With the IAC unplugged and closed it varies from 500-900 RPM with frequent stalling.

My next moves will be to try to check all my ground wires (where should I look for important ones?) Possibly get a vacuum gauge (what reading should I look for?) Check timing chain for being loose and then maybe the cam for being worn. I may get new injectors because I am unsure of their quality (Far better than my old ones though) Could this problem possibly be a partially failing ignition switch or VATS system? Got any more suggestions?

 

TRY USING A CHEAP STETHOSCOPE AND LISTEN AT EACH INJECTOR
YOU WILL HEAR A CLACKING NOISE. IF YOU HEAR A WEAK OR NO
CLACKING NOISE, YOUR WIRING TO THAT INJECTOR MAY BE
OPEN OR CORRODED, OR THE INJECTOR MAY BE BAD.

 

Not to be mundane here, but check all your spark plug wires to see if any of them have been burnt. If that still turns up nothing, you might want to pop the valve covers off and make sure all the rockers are working properly and you don't have a bent pushrod or two. Keep us updated and we'll continue to help you troubleshoot this.

 

When I replaced the plugs I made sure that none of the wires were sitting on anything hot both before and after I was done. I'll do a more careful visual inspection for burns though soon. One of the wires I saw has a small cut in it, but I put my hand on it while the engine was running and didn't get zapped so I don't think I'm losing any spark there.

How do I test for bent pushrods without removing them or running the engine and getting oil everywhere?

I really appreciate the help. Thank you guys.

 

I have a couple of questions before we go any further.
1. Is the motor a 350 T.P.I?
2. Are you experiencing a sever power loss while driving?
If the answer to this question is no, it's not a bent pushrod.
3. Are you experiencing a miss at idle and while driving?
If the answer to both of these questions is no, then it's not a
problem related to your ignition components.
4. Is your engine loping or idling erratically?
5. Did you replace the E.G.R. valve with a block off plate?
If the answer is no, then you could have a leaking E.G.R. valve.
6. Have you checked your air filter to make sure it isn't dirty or
clogged? Either case could be causing the problem. Especially
in a T.P.I. motor.
Check for codes in your computer again and if the E.G.R. code still comes up, try replacing the E.G.R. valve with a block off plate and see if that fixes the problem. A leaking E.G.R. valve could be the cause of your problem. Let me know how it goes.

 

Yes, its a 350 TPI. There is probably some power loss when driving, but I don't think its severe. The car is running better than it ever has, but thats not nessecarily saying much since I have had some severe fuel problems, ie fuel pump and two bad injectors. It does have a hell of a lot more kick to it than my 3.0 V6 Pathfinder does though.

EGR is blocked off as well. Air filter is about a year old. Even a clogged filter wouldn't cause unstable idle though I wouldn't think.

With the IAC plugged in the idle stays pretty close to 650 RPM but feels rough. With the IAC closed and unplugged, idle varies greatly from between 500-900 RPM with frequent stalling.

 

Also, there is no miss at any time.

 

Believe it or not, an air filter can cause a loss of power as well as idle problems. It's definately worth taken the few minutes to check it out. Let me know how it goes. You're not throwing any troble codes, correct?

 

Obviously, the IAC is trying to maintain the correct idle RPM. Disconnecting it reveals the true nature of what's happening. There is probably nothing wrong with the IAC system itself.

Upon reading your posts, my first guess was an EGR control problem (leaking solenoid) but you replied that the EGR is blocked.

My next guess would be the PCV valve and system. You mentioned that the PCV valve itself is "good" but I'm not sure how you may have tested it. A perfectly good PCV (clean, moving freely) will cause lots of trouble in the wrong application, especially on a speed/density application. If you have an aftermarket PCV valve, clean your original AC valve and reinstall it, or acquire a new AC PCV valve. These are flow calibrated for each engine design, and are not generic. I have a long, boring story about several days of fighting a wandering and high idle on a S/D engine with a brand new aftermarket replacement PCV valve, and solving it by finally simply changing back to thje AC/Delco valve. At least it got a full treatment of lots of other new parts. Closely inspect the PCV hoses as well.

Connect a vacuum gauge and get a reading at idle, even while the RPM varies. Temporarily disconnect and plug the brake booster vacuum line. Temporarily disconnect and plug the FPR vacuum line. Once you have eliminated all external vacuum connections, you can concentrate on the problem - If it still exists.

 

There are no codes except the occasional code 32 since the EGR is blocked. The PCV hose to the valve is new. the hose to the other side is good. I'll try to get a new AC Delco PCV valve since they probably only cost like $3. I just bought a vacuum pump/gauge so I'll be testing to see if every vacuum actuated system holds vacuum.

 

I've had a rough idle problem and it got pretty much cleared up with a new ECM..

I know, I know... but thats what helped me!

 

ECM is an expensive thing to replace without anything specifically pointing to it.

I ran some vacuum tests and it looks like the MAP, FPR, and vent controlls all hold vacuum. The EGR line slowly bled off however, so I plugged the port. It didn't help the idle.

The over all vacuum at idle is about 19.5 in/hg. I hear 20 is normal, so I don't know how bad 19.5 is. I would guess that .5 in of lost vacuum could be caused by the rough idle.

 

Quick question. How exactly is the E.G.R. valve blocked off? I was originally headed where Vader was, a leaking E.G.R. valve. You said that it was blocked off. Since I mentioned an E.G.R. valve block off plate and you said it was blocked off, I assumed you were using one. Now you're posting that you have a leak in that area. Is the E.G.R. valve still on there? If it is, then it's probably leaking and causing your problem.

 

EGR is still there, with a plate blocking the intake side of the valve, sealed with RTV black. There is no way anything is getting into the intake from the valve itself, or now though the vacuum port on the plenum.

 

Use the propane torch method I described earlier and just see if running it around the E.G.R. valve has any effect. The computer is setting a code for the E.G.R. system. It's the only code you've got. It's worth checking out.

 

I did that the first time I did the propane technique. Worth a shot, but didn't reveal anything.

 

your map sensor should be ~1.2v at idle check for its operation KO/EO with the vacuum gauge 4.7 or so with no vacuum, down smoothly in relation to vacuum applied

 

Not to sound stupid or anything, but whats KO/EO mean?

 

Key on/Engine off - at least, that's what I know it to be.

In the context, it probably means Key on/Engine on.

 

Wow, quick reply. I figured it out also but you realize it could also mean key on engine on?

 

key on/engine off... the other is KO/ER key on engine running sorry shoulda just said it

 

Disconnect the O² sensor and the EST bypass connector to force the ECM into open loop mode. My WAG is that the problem is not with the engine control, but what the control is attempting to do to react to a problem with the engine.
Forcing open loop and backup fuel and spark should help reveal the problem. The engine will be running as if it had a carburetor set a little rich, and no timing advance. You can then concetrate on finding streaming injectors, vacuum leaks, ignition problems, or whatever else is creating the wandering idle situation.

 

agree with vader 100%

also BTW a problem that i am trying ti deal with right now can cause the same symptoms worn throttle shaft seals

 

The MAP is moving steadily, but it is 1.70v at 19.5 in/hg (my idle vacuum) and 5.6v or so at zero. Reading a bit high according to you guys. Recommend replacing?

Also, how would I go about checking for streaming injectors and the other stuff you mentioned? And how can I check for bad throttle shaft seals?

 

Yeah that seems a bit off, I have a map that i'll sell ya for 15plus shipping have to test it first though, as for the throttle shaft twist it open maybe ten percent then try to move the shaft radialy in its bore(side2side or up and down) <not in and out which is axially>. i dont have any exact numbers for maximum allowable play but i would guess .001-.003in would be a factory tolerance for this, if you can feel more than a C_hairs movement then it is worn beyond this, I dont believe GM ever used any bearings bushngs or seals at the ends of the shaft so 99% are bad in this way, I'm still looking for a bushing or bearing that i can use to correct this. Keep getting sidetracked.... ...The problem that I noticed was my throttle sticking slighty open. I cleaned my throttle bore and plates to temprarily remedy this. I cleaned them 6 months ago so it doesn't last longer.

 

I'll test the throttle shaft soon. After reading up some on the MAP sensor, I found that the ECM is supposed to send a 5v reference signal to it. If thats so, how could I get a reading of higher than 5v? I find that odd.

 

I was thinking I told you to check that already musta been someone else O'well good catch

 

I'll check the reference signal itself before I jump to any conclusions, but if the signal is indeed high, what do I do about it?

 

It would probably be a leaking transistor or voltage regulator inside the pcm check battery voltage \, grounds and check for a/c voltage across your battery terminals with the engine running~2grand If you have any replace your alt

 

an old wives tale i heard is to put the ecm in the freezer,

 

I checked the voltage with the engine off. I don't think the alt is the cause.

 

Indeed, the reference signal to the MAP was 5.80v and rising at ~0.01v per second. Also, I noted that the TPS reference signal was 6.11v and should also be 5v.

 

I would say disconnect the ecm and turn the ignition on if the high voltage goes away it is probably in the ecm, maybe a temporary fix would be install your own voltage regulator on the reference wire... I really wouldn't try that though. I'll attach a scan of a bussiness card for c-plus computer repair, I have spoken with him about repairing OBD-1 computer and he stated that he was capable, not sure about pricing though.

 

Since the ECM is responsible for the 5v reference, I would assume unplugging the ECM would cause the voltage to drop to zero. I still have a few grounds to check, but it really looks like its the ECM voltage regulator. I believe a new ECM from Autozone is $80 so though a pain in the ***, it could be worse.

I already know the answer to this, but a high reference voltage and a subsequently high signal voltage from the MAP, TPS, and probably other things will cause a rough idle correct?

 

absolutely

 

Joe,

A few points/observations:

1. Be certain the meter you are using is calibrated. It would be a shame to replace an ECM because the meter was not reading correctly.

2. High DC reference voltage would certainly be a problem. It would not only affect the sensor inputs, btu much of the other ECM functions that use the 5V supply internally (processor, VRAM for I/O tables, logic gates, etc.);

3. The reference voltage is derived internally from the ECM via a regulator. If you truly have more than 5.05VDC, the regulator may be kaput;

4. If you checked the voltage with the key on/engine off, instead of with the engine running, that should eliminate any potential AC contingent in the power supply (being generated by the alternator and not being filtered by a sorted diode in the rectifier bridge).

In short, if you know the meter is corect, the ECM has problems.

 

Darnit Vader you never say anything I can argue with

 

It appears that my digital volt meter that is less than 6 months old is not properly calibrated. Good call on that one Vader. Thus the search continues for my idle problem...

Anyone know how to calibrate a digital volt meter?

 

I have the same problem. Read my post from last june. https://www.thirdgen.org/techbb2/sho...hreadid=303955

The only difference is that I blow occasional black smoke out and I have no cats.

Not trying to hijack thread, just showing you what I did so that you don't go spending lots of cash without solving idle issue