IAC testing

[Dark Knight]

Hey everyone. My truck idles pretty high, right at 1100 RPM when started. The temp here is about 95*. I've replaced the CTS and no change. It won't lower the idle until the engine temp gets just below 210 and even then it's still high in park. When I put it in drive, the RPM drops to 550 when warm and a little over that when cold. I can watch my tach when the A/C compressor kicks on and see the RPMs stay the same. When I read the IAC on Ohm's scale, my resistance was 53 ohms for both coils. When I jumped the diagnostic port on the ALDC, (key on, engine off) I can hear the IAC moving, but it never seems to turn off. Should it? Any other testing I can do before I drop $35 on a new IAC?

[ChevyTech]

First thing you should do is check for vacuum leaks. Extra air getting in the engine will speed up the idle. Check all the vacuum hoses.

On TBI engines it is common for the gasket under the TBI unit to leak vacuum.

Many people spray carb cleaner or WD40 at suspect areas to see if the engine speeds up. I prefer to use a 3 foot long piece of heater hose to listen as I move it around the engine.

Vacuum can also leak through components such as the brake booster, bad PCV valve, or charcoal canister purge valve. I use pliers to pinch off vacuum hoses and listen to see if it slows down.

[Dark Knight]

OK. Checked for vacuum leaks and couldn't find any. I used propane (terrified of using the stuff near an engine, but it's all I have at the time.) Went all around the TBI unit, vacuum hoses and the PCV valve. The engine speed never increased. I had the booster hose plugged while I was doing this, put the hose back on and no change in RPM, so the booster is ruled out. It idled at 1000 RPM for a couple of minutes, then dropped to 900 RPM as it started to warm up. Timing is set at 0* with wire unplugged. I'll post an update after I get home from college today, but it almost seems to be working properly now. I would ask what the cold idle speed is supposed to be, but I assume that changes with the ambient air temperature, correct? On the bright side, I found out that NAPA has the correct rag joint for me, so I can get rid of that pesky slack in the steering. Yay!

[ChevyTech]

I would ask what the cold idle speed is supposed to be, but I assume that changes with the ambient air temperature, correct?

Close,
If this is your 1989 that you are working on, the TBI system does not measure the air temp, but is very dependant on the coolant temperature sensor.

You will not find a fast idle specification, but they do publish what the IAC count should be. The IAC count can be monitored with a scan tool or WINALDL.

Specification:
For a TBI truck, 50 is the max allowable IAC postion, at warm engine idle with all accessories off.
20 is ideal.
You do not want the position to hit zero.

Usually the IAC does not go bad electrically, but it is common for them to stick and stop moving.

The IAC is a stepper motor and uses short pulses of electricity on the wires to move its position.

If the IAC sticks as does not move as it should for each pulse, the IAC count will get off because the computer does not know the actual position of the IAC, but the ECM tracks the commands it has sent to the IAC and assumes it has moved every time it has been sent a pulse.

[scotts62]

Kinda sounds like the cool/temp sensor but maybe not.

[Dark Knight]

It's most likely the IAC. When I jumped the ALDL connector and turned the key on, it just didn't sound right. Almost as if something was stripped out internally. It made a lot of clicking noises. I had the engine temp just below 210 yesterday, it idled at 550 RPM in drive and 1000 RPM in park. I'll probably just replace it. My buddy has a scan tool. Maybe I can scan it when I get some free time and see what the counts are.

[Boog]

Clean down in the port while you have the old iac out. Carbon builds up in there and the pintel may not be seating every time. Be sure to follow the instructions with the new iac also. You don't want to screw the pintel out too far and install it. It gives you the measurement in the instructions.
Heck if that throttle body hasn't been off in quite a while why not rebuild it while you're at it?

[Dark Knight]

It's never been off to my knowledge. I've got some carb dip that will make it spotless in 30 minutes. I'll have to do it one day when I don't need my truck. Probably a couple weeks from now. I had thought about re-building it just to do it.

[Boog]

Well there's sure nothing to it. In the rebuild kit you'll get a new fuel pressure regulator diaphram and spring and new seals for the injectors. It's simple.

[Dark Knight]

Great news everyone! I finally got my truck fixed. I borrowed a scan tool from my buddy and the IAC counts were reading 60 something. So, I threw a new one in and it idles at 575 RPM in D and ~750 RPM in P. IAC counts are now down to a healthy 26. I'm very pleased right now.

P.S.: I know this is not a Pontiac forum and totally the wrong spot, but I figure since it's my thread I can hi-jack it. Hehe. My girlfriend needs a new computer in her 2004 Sunfire. I read that the computer is behind the passenger side kick panel. Can anyone confirm this for me? Please...

[ChevyTech]

Thanks for letting us know what you found on your truck.

As for the Sunfire computer, my first GUESS would be that the computer is under the hood somewhere.

[Dark Knight]

You're quite welcome. Nothing tees me off more than reading a post that never gets resolved. Someone else might need the information later. As for the Sunfire, I have read that the computer can be behind the bumper, on the strut tower by the inner fender, or by the pass. kick panel. All depending on the year model. At least I have a place to look. I have to get this fixed though, b/c she rear-ended someone recently because the ABS wasn't functioning properly (faulty computer). I'm so darn excited! I'm 21, she's 23, and this is my first ever girlfriend. The kicker is, she enjoys working on vehicles with me. She can even put freon in the A/C system. I've never had anyone care about me in this way before. I swear I'm happier today than when I was 15 years old and my grandpa dropped the keys to my '67 in my hand. I could go on and on, but I'd probably start to bore everyone. My word of advice to everyone: "Patience pays off."