Jim85IROC

iTrader: (2)

I've had ongoing IAC issues for almost 2 years now and still haven't been able to fix it, and so far nobody here has either. It's been a while since I posted, so I thought I'd try again.

Basically, the IAC doesn't do a damned thing on my car. I can unplug it at any time and not effect how the car runs. After I start the car the iac counts slowly drift down from their maximum value until it settles in at 0 about 5 minutes later. I've reset the IAC and minimum idle a million times with no help. I've changed the IAC a couple times. I've verified that the iac passages in the throttle body are clear and unblocked. I've changed the computer. I've tested the wires. I've replaced the wire ends and the plug that connects to the IAC.

What in the hell else do I do to make it work?

DENN_SHAH

iTrader: (1)

something that i think you need is a GM-dedicated IAC tester,
with this you can check to see if the ecm is commanding the iac properly.

autozone may have these as a loner tool, or you may have to buy 1. normaly they come in a kit with other noid lights.

here is what they look like.

contactpatch

Is the IAC being 'commanded' to close down?
(slightly confused on that point)
Is there anything else obviously wrong with the car?


Wild guess number one...
For some reason, you ecm is
interpreting something as a falsely
high engine rpm.

#2 , the car rewired itself,
there is a short somewhere, and the IAC changes without
being commanded

contact

ULTM8Z

What happens if, when the car is running, you start turning the idle stop screw down to close the throttle blades? Does the IAC start opening up? Or does it remain at 0?

Jim85IROC

iTrader: (2)

UTLM8Z: If I turn the idle screw while it's running, the IAC counts will very slowly begin to change, but I can't get them to stabilize. They either continue to slowly fall, or continue to slowly rise.


Contact: My auto x-ray is displaying the correct rpm through the ALDL, and it's behaved the same way with 2 computers, so I would think the computer is unlikely. I don't believe there is anything else wrong with the car.. at least nothing related to the IAC circuit. The car has terrible throttle response and a huge lack of low rpm power, but I think that's a combination of my heads and poor chip tuning.

DENN: do you observe this noid light as the vehicle is running, or do you simply put the car into diagnostic mode to verify that all of the bulbs light? The GM service manual has a test sequence for the IAC wiring that requires a test light on each of the 4 terminals. I've done that test and it tests ok.

ULTM8Z

These may be (really) dumb questions, but I'm unfamiliar with everything you've done thus far and wanted to eliminate the basics...

1.) Tried the stock chip?

2.) Eliminated all potential sources of vacuum leaks? If there was a vacuum leak, the ECM would close the IAC in an attempt regulate air flow down.

3.) Tried a different throttle body? Perhaps you have a vacuum leak in yours- maybe through the throttle shaft bearings.

Man, I can sympathize with these seemingly-impossible-to-solve problems. My idle misfire drove me nuts for about a year and a half.

mikobu3

i also got a bad idle , when i first start my car up it starts up than idles like at 100 rpm and dies, i have to keep the gas pedal down for 3 minutes to get the idle to finally stay semi decent , i also noticed that when i am driving for a while and i go from park to reverse the car idles really low and dies

i will let the mechanic fix this problem ,

Mark_ZZ3

I recently worked on an 87 Z28. HAd a big idle problem. Could not get it to run unless the set timing connector was disconnected. Turned out the pick up coil was gone. Swapped dist. and it ran to the point that I could set the min air flow, TPS and so on.

If you can't set min air flow, something is gone. Try to disc the set-timing connector. Also try to disc MAF (if you have it) and so on.

Mark.

contactpatch

>Basically, the IAC doesn't do a damned thing on my car<
Please elaborate, the IAC is a stepper motor, if you pull
the electric wires off, the motor will just hold its current position.
suggestion...
make sure the EGR is completely disconnected.
\
is the VSS working correctly?
\
Reoor throttle response, does it bog on tip-in,or?
is the engine, coughing, bucking, or just seems lazy?
is spark advance correct?, is the tps working?
is the Maf working?
is cruise fuel economy resonable?
\
so, sometimes the count goes to zero,and the engine then dies,
or close to it,,,other times, the count floats up,
and idle is very fast?
\
is crankcase ventilation, ok?

Jim85IROC

iTrader: (2)

contact:

I don't have EGR on the car. I'm using a block-off plate.

VSS is working fine.

throttle response is lazy. If I blip the throttle very fast it'll stumble.

MAF works. Well... I've got issues with that, but that's another story. When it drops below around 45* the car will stumble then throw a 33. When it's warm out the MAF works flawlessly. I've tried my other 2 or 3 MAFs but it doesn't effect the IAC situation.

Fuel economy is somewhat reasonable. I'm in the mid to high teens for gas milage. Not great, but not much different than my Corvette and my GTA.

The counts float to zero and stay there.

I assume crankcase ventilation is ok. I've restored the stock-type ventilation system with the feed from the throttle body going to the passenger valve cover, and the pcv valve on the driver side going to the vacuum port on the intake base. Most of the parts only have a few miles on them.

My car also seems to make around 16" of vacuum, and the system holds vacuum for quite a long time after I shut the car off, so I don't think I'm dealing with a vacuum issue, although at this point I'm willing to entertain almost any idea.

mikobu3

my car idles really low and i loose my breaks , or if i turn the steering wheel it idles really low , i should of left the car alone , stupid me

contactpatch

Jim, is the throttle position sensor working correctly?,
IS TPS ok when looking with the ALDL
(if tps was goofed up, would explain a lot)

After conversion to 355, did it ever work right??

Is there any chance of mistaken wiring to the IAC

When you write that the IAC count goes to zero,
is that by looking at ALDL?

Perhaps you can post the proms
'IAC count vs coolant temp' table.
'target idle rpm v coolant temp' table

Trivia question, for GM v-8, how does the computer 'know' the position of the IAC.

With a timing light, does distributor advance look
normal, when you open the throttle?(with normal dist. hookup)

Do you know, for sure, that the prom coding is what you intend it to be?
Computer is '165?
Memcal is ???
Car is a LB9 to 355 conversion?
Maf is stock?, for 305 or 350?
Stock prom?
What are your prom AE settings?

Jim85IROC

iTrader: (2)

TPS is functioning perfectly according to the ALDL. .52 or .54 (can't remember which) at idle, and a smooth progression to WOT.

no, it hasn't worked right since the conversion. That includes upgrading from the original 85 computer to a 165 ECM & wiring harness.

IAC wiring is correct.

Yes, counts at zero are observed on the auto X-ray hooked to the ALDL.

IAC steps vs. coolant:
Deg C......Steps
-40..........125
-16..........100
8.............75
32............50
56............25
80............10
104..........5
128..........5
152..........5

Idle speed vs. coolant temp:
deg C........RPM
-40............1050
-28............1050
-16............1050
-4..............1050
8................1050
20..............950
32..............900
44..............850
56..............800
68..............750
80..............750
92..............750
104............750
116............750
128............750
140............750
152............750

I'm not sure how the computer knows the position of the IAC. I assume it doesn't know, and that's why it just displays the steps on the ALDL. The steps are just the command telling it what position to take. I don't believe it knows what position it's actually in. It just tells it to open or closed based on the differential between actual idle and desired idle.

Hmm... I never checked advance with more throttle.

For my PROM, I'm running a modified APYP prom. APYP is the 89 corvette 6-speed prom.
The computer is a 165.
Memcal is from an 87 L98 camaro.
MAF is a Wells 165, but I've used a stock Bosch unit too.

AE vs. async. pulse is .5 in all 8 entries.

AE vs. Delta LV8:
LV8...........Multiplier
0............... .105
64............. .105
128........... .133
192........... .172
256........... .172

AE vs. coolant temp:
Deg c............factor
-40................ 1.0
-16................ 1.0
8................... 1.0
32................. .75
56.................. .5
80.................. .5
104................ .5
128................ .5
152................ .5

AE decay rate vs. coolant temp
Deg c........... % change
-40.............. 25
-16.............. 25
8................. 17.2
32............... 17.2
56............... 25
80............... 39.8
104............. 50
128............. 59.8
152............. 59.8

contactpatch

>and it's behaved the same way with 2 computers,<
Jim, could you elaborate.
Was anything common, between the two.
When it was a 305, did it work good enough
for you to tell if it had these two problems?
Are you absolutely sure that everything computer related,
is set to eight cylinders? Pehaps the tach and something else
was monkeyed with, before you got the car.

You should summarize this post, and post it in DIY Prom.
Those guys know lots about tests you can do.
Write clearly, what tests you've done, and how you did them.
Make clear that the computer is commanding the
IAC to close to zero. Tell that you have another problem,
poor throttle reponse, these problems may be related, or not.

Jim85IROC

iTrader: (2)

I don't remember having any idle issues with the 305, but it's been a long time since I had that motor. Since I built the 350, I started with the original 85 ECM with the stock 85 305 PROM but I can't remember exactly how it idled. I think it behaved better than it does now.

A couple years ago I swapped to a 165 ECM. Since then is when I've been aware of the IAC issues. I've swapped the 165 for another one with no change. I've burned numerous chips and tried a bunch of stock ones, but none of them have had any effect on my IAC issues, although some of them have had an effect on the idle condition when the car is cold. But... I think that's just because some of the chips have had a "band aid" built into them to help compensate for my lack of a functional IAC. My motor is mild enough that it should have no problems starting and idling with a stock 350 chip.

contactpatch

Is it possible to go back to the 85 computer?
That test might narrow the list of possible troublemakers.
.
When you say you tried two different '165s,
was any part such as memcal , had to be re-used.
Related q.
When you swapped, was it straight swap from
a '86/7/8/9 v8 Camaro with manual T, or what?

DENN_SHAH

iTrader: (1)

"The GM service manual has a test sequence for the IAC wiring that requires a test light on each of the 4 terminals. I've done that test and it tests ok."
thats what the noid light does, only easier to hook up.

what rpm does it idle at?

try this, make sure the oil cap is on right & is tight, then unhook the pcv valve & plug the opening where it goes in the valve cover, do the same with the beather hose, pull the dip stick out & hook a vacuum gage up the the dip stick tube & start it up, does the vac gage show it pulling a vacuum?

the computer knows the position of the IAC by seating the IAC everytime you turn the key off, it then opens it back up a certain amount.

Jim85IROC

iTrader: (2)

Contact:
There's no way I'm going back to the 85 computer. It requires a complete wiring harness change.

When I swapped 165s, I reused the same memcal. It was just the computer I swapped. I have tried different memcals though, and that didn't change anything. I believe the memcal is transparent until you find yourself in "limp home" mode.

When I swapped, I used a wiring harness out of an 87 automatic car. I made the necessary distributer wiring changes to my chassis harness so that I could run a small cap distributer. One of the 165s came out of an 86 or 87 Corvette. I'm not sure what the other one came out of. Since they're all the same, that shouldn't matter. My PROM code that I'm running is a variation of the APYP, which is a 350 TPI manual transmission code out of a 6-speed 89 Vette.

DENN: Right now the car idles at around 750 or 800 once it's warmed up. I had to set the idle with the idle screw on the throttle body instead of setting it with the minimum idle procedure that you're supposed to. When I set it to 500 or 550 like the minimum idle procedure calls for, it stays at 500 after I plug the IAC back in.

When I get a chance I'll check my PCV system again, but since everything is brand new, I would be surprised to find any leaks.

contactpatch

Jim, perhaps you have a friend with a similiar
car...try your computer on on other car, see what happens,
look for IAC problem or poor throttle response.

Jim85IROC

iTrader: (2)

I have 3 tpi cars, 2 of them use the 165 ecm. I've already swapped computers. I've got a total of 3 165 ECMs, and I've tried all 3 in both cars. The problem is independant of the computer, and seems to be independant of my PROM changes.

ULTM8Z

Jim, just wondering if you tried a completely different throttle body? Like I said in my last post, maybe you have a vacuum leak at the throttle shaft bearings? I've heard about that happening before.

Jim85IROC

iTrader: (2)

I *think* had this problem with the stock throttle body too, but swapping back to that one is on my to-do list.

I'm skeptical though. Any air leak around the throttle shaft is going to be constant and very small compared to the amount that's got to get around the butterflies for minimum idle anyway. It shouldn't matter as long as you set the minimum idle properly.

VetNutJim

Here's a long shot.
I don't know exactly what effect this may have but it's something to check....

1) Is your 'park/neutral' (P/N) switch wired up and functional?
Is the ECM getting the P/N signal form the P/N switch?

2) Are the little bolts that hold the water bypass and IAC valve TIGHT on the bottom of the throttle body?
Gasket good?
A leak here could drive the IAC nutz.

3) Same as above for the top screws that hold the TPI throttle body plate on. They good and tight. Gasket good?
A leak here could drive the IAC nutz.

I had a throttle body someone put back together and left the gaskets off. They has just used RTV.... cost me some time finding the leaks.

4) Another thought: Is your PCV system connected and working?
That's a 'calibrated' leak to your TPI intake system.

Hope this helps.
Good luck with it.

Slow89Iroc-Z

does the scanner report the correct RPM to what your tach says?

Jim85IROC

iTrader: (2)

hmm... rechecking the seal on the coolant body to throttle body sounds like a good idea. After waiting a week for my gasket to come in, I had a TBI gasket kit waiting for me, so I made my own out of cork & RTV. Maybe it's time to pull that puppy off and inspect it.

P/N switch is jumpered because I've got a 5-speed now. I don't think the ECM ever sees that though.

PCV is working.

My auto x-ray rpms are consistant with my tach.

I think this weekend it's time to pull the throttle body off.