My IAC has been a piece of crap for as long as I can remember. I've replaced the ECM twice, tested the pins in diagnostic mode more than I can count, replaced the IAC and replaced the throttle body. It still sucks. When I start the car the IAC counts are relatively high, and gradually work their way down to 0 where they stay. Unplugging the IAC doesn't have any effect on how the car runs. Once it stabilizes at 0, I intentionally turn the idle screw to drop the idle below the desired idle. The IAC doesn't change. It doesn't react to the lower idle to try to bring it up at all.

Well anyway... at the track tonight I had a hard time launching, and when I shifted (5 speed), the car was pretty non-responsive after the shifts, then gradually recovered. My TPI vette (6 speed) throws me HARD into the seat when I shift. Hell, even my LB9 GTA (5 speed) lunges forward fairly hard when I shift. Not the IROC. I don't even think I chirped the 1-2 shift tonight. Is this a result of my IAC issue, or am I barking up the wrong tree with that?

I ran a best of a 14.3 @ 97 tonight, so if I can get rid of this problem, I think I'll be solidly into the 13s.

 

I don't see how it would affect your WOT performance.
What motor do you have and what mods? Is it running worse than before?
It may be tranny/clutch related. A low 14 second car will easily chirp 2nd without much hassle.

 

On a slight tangent;
my 14.3 GTA doesn't chirp second in the slightest yet my 14.7 second Formula barks it pretty good. In my case I think it may be tires.

 

I understand that it won't effect WOT performance, but I'm not speed shifting. I'm getting off the gas, shifting, then back into the gas. When I do this, the car is very slow to recover, and my understanding is that the IAC has a dramatic effect on throttle response.

Anyway, it's a 10:1 350, 461 heads, accel base, slp runners, 1 5/8 headers, etc. Transmission and clutch are fine.

The car isn't running "worse than before". I've had non-stop problems since I built this motor. Last year I converted to the 165 ECM and swapped an 87 TPI harness in rather than modify the stock harness. I've had IAC issues since then (although I don't think it was ever right even when I had the old 85 computer). I'm about ready to hook the plug to a different IAC out of the car just to see if the damn thing even moves at all.

 

What about the TPS? Has that been thoroughly checked or replaced?

 

I've checked the TPS a hundred times, and set my minimum idle a hundred times too, but since plugging the IAC back in doesn't bring the idle back up to where it needs to be, I've got to turn the idle screw just to get it to idle at a reasonable rpm.

 

Nah, that IAC doesn't affect WOT.
You can actually run without an IAC if you want.

 

I'm aware of its uselessness at WOT, but I'm not shifting at WOT, I'm lifting off the gas, and getting back into the gas is when the car is a pig.

 

Do you find the car to be running rich or lean at all Jim?? I can't recall are you MAF or SD? MAF right? If so you may even have a contaminated MAF or dirty which will effect alot of drivablity issues or even the CTS maybe???

 

Technically you are not shifting at WOT but if you are driving it hard enough (i.e. at the track) then the IAC should be a non-issue.
That doesn't mean your IAC is working properly but you may have other problems.
Do you have a scan tool?

 

The IAC acts as a throttle follower. WRT that function it ABSOLUTELY effects the transient performance of your car, i.e. while you're shifting, in 2 ways:

1 - you lift. IAC is dead, so you have no dashpot. RPMs thus drop further between shifts

2 - you get back into it. Your PROM's AE is calibrated for the extra air from the throttle follower. You don't have that extra air. Thus your pump shot is rich.

Even at WOT, an open IAC does help let extra air into the engine. Not a lot, but every bit counts. Then again, with a big TB car that makes your kind of power, this shouldn't be a factor. The stuff above is for sure though.

 

My BLMs are decent, so if I am rich or lean, it's not by a great deal. My MAF isn't the issue... I've got a couple of them that I've interchanged, and it has no effect.

As for the scan tool question... yeah, I've got one.

 

Something I've read is that people like to adjust their IDLE IAC steps to like 20-30. Rather than setting minimum air.

Lower the steps, the less air the IAC is letting through, the farther out the idle screw is. I had kind of a problem with the IAC so I adjusted it so the IAC wasn't thie big deal.

When you jump the A-B to extend the IAC, you should be able to hear it running.

Clean the **** out of it or replace it maybe?

Try adding 2-3deg timing between 400-800rpm through the load range. That helps it idle more stable and give some more throttle response.

As for it it's working at WOT like ed says I have no clue.

Hope my .02 helps.

 

I've tried that. Once the IAC counts go to zero, they stay there. Like I said above, I set the idle all the way down to 500rpm (desired idle is 850) and the IAC counts don't change.
IAC is brand new, as is the throttle body.
Once the car warms up, the idle is perfectly stable... but all of my idle air seems to be coming through the throttle blades and none through the IAC.

 

Jim,im getting the same exact thing with mine,no matter what i do itll always go to zero, and idle off the throttle blades.im putting that cap for the cold start on tonight ill let you know how it works.

 

Nitro... if you wind up figuring out your IAC issue before me, make sure to let me know what it is! I'll do the same if/when I find out. It's nice to finally find somebody else with the same damned problem as me.

 

I have a 1987 Monte Carlo SS with a 406 and a SD TPI and it seems like mine does the same thing. When I am at idle (800 rpm) without the air cleaner the sound of the air feeding the big 406 past the throttle plates is nearly deafening!

 

Will do Jim.

 

Tonight I tested the IAC circuit (again), and as usual, all of the wiring & ECM performance seems spot-on. Tomorrow I'm buying yet another IAC to see if it causes any changes.

 

What kind of test equipment do you have that you can read the 'steps' in the motor?
I have a funky scanner but would like to be able to read the serial data to a laptop.

 

I've got an Auto x-ray. Someday I'll get off my lazy *** and build/buy an ALDL cable and start running Moates software.

 

Auto X-ray tells what the steps are in live data.

 

I threw another new IAC on there (the one I took off isn't even dirty yet), and surprise surprise, it didn't change anything.

I started the car and it wouldn't maintain the desired idle. Once it warmed up, it would idle ok. IAC counts were steady at 151. I raised the idle screw a little bit and the iac counts slowly went down.... and down.... and down until finally it parked at 0. I brought the idle back down a little bit, and the IAC counts began to go up. At around 35 the fan kicked on and the IAC counts went up to around 50, but seemed to keep rising... then the POS scan tool lost communication with my ALDL and wouldn't work the rest of the night.

Anybody have any ideas at all here?

 

Your IAC is supposed to go up when the fan kicks on. In the chip there is an option to put in how much it goes up.

Don't peg the IAC counts to 0. When they get to 0, that means all idle air for sure is coming through the throttle blades and the IAC can't slow or restrict any more air if it wants to.

With your bad idle, have you tried some more timing? That helped my problem too. It may not be an IAC problem. If the counts move, and the pintle moves then you got good parts and you say they are clean, and if it does move your wiring is right....

 

I'm not pegging the counts on purpose.

The car barely struggles to idle at 500 rpm with IAC counts of 160 when it's cold, then after it warms up, it idles at whatever RPM I put the screw at, with counts of 0. I can't get it to find a happy medium in the middle, and no matter what I do, it seems that the progression from 160 to 0 is very, very slow.

 

my car has a weird idle problem. it will seek between 500-800 sometimes,sometimes it wont even do it, ive repalced the entire ignition system,done the timing a few times, relaced the egr valve. hooked it to a scanner at school and a osciscope or whatever the f**k its called.whats the tps voltage supposed to be at idle and what should t be at WOT?oh yeah,wanna gimme 1 of ur old IAC valves? lol

 

Kool! I will do a little research as I need to read the info on the software/hardware
Thanks

 

Jim,

I noticed that you said the TB was brand new. Were you having the same problem with the old one?

At the risk of sounding incredibly stupid, it seems to me that the the ECM regulates voltage to the IAC, causing the mechanism to move in and out regulating the amount of air coming in at idle right? I'm still learning so maybe I'm totally out in left field.

That said, IF it really works that way, what if there were some obstruction that slowly moved in one way or another within the air passages that eventually caused a total blockage within the IAC passage? Feasible? You seem stumped, so I am just throwing another idea out there. Good luck with it.

 

I've checked the throttle body for obstructions a million times. No dice.

I'm starting to think this might need to be solved with chip tuning, but considering it should start and idle with a stock friggin chip, I really think chip tuning will just be a band-aid.