I have done the clean the IAC thing, and although it appeared to help, I still have a problem. My 305 TBI idles in park around 1000rpm. When I shift to drive, it drops to around 500rpm and sometimes kicks up to 900rpms. Other times it stays around 500rpm, idles really rough and stalls.

Also, when I engage the Air Conditioner the car sometimes stalls. I have checked for codes, and I don't have any.

I have changed the IAC valve with a new one from the dealer, and the problem is still there.

Any ideas?

 

I have also had the car to a mechanic and he had it hooked up to his machines and didn't detect any spark in any of the cylinders to be "missing".

 

As part of the mechanic's troubleshooting, did he/she check the IAC counts and compare that to idle RPM? That might tell whether the IAC was active. You can do the same thing by removing it, putting it in a plastic bag, and jumpering the ALDL for servive mode. With the ignition on, the IAC pintle should step outward until the pintle falls out.

 

When I removed the old IAC to clean it, the pintle did fall out. When I re-installed it the problem was still there...actually it was worse for a little while. The engine wouldn't idle at all...I had to keep it running by pressing the accelerator. After a minute or two, it held an idle in park, but not in gear.

The new IAC acted the same way as the old one.

What do you mean by "checking the IAC counts and compare that to idle RPM?" Is that what you meant by having the pintle falling out, or should I be looking for something else?

 

Anybody have any ideas?

 

I checked the TPS voltage today and it was 4.91 without moving the throttle. The way that i checked it was to turn the ignition on, unplug the connector, and insert the probes into the connector. Was this the correct way to do it? If so, seems like my TPS may be bad?

 

The TPS cable needs to be plugged in to get a correct reading. Should be .54 volts at idle and should exceed 4 volts at WOT. They make jumper cables or you can make you own. That's the right way. The other way is to get meter leads with sharp tips and just press through the isulation on the wire.

 

check the impedance of the fuel injectors. 12,16 ohm ok, (low 12 is bad replace injector).

 

I'm having a very similar problem, if not that EXACT problem, with my GTA. I've done the same thing with the IAC that you have. I've replaced it and I get the same problems. Runs a little better, but still having an idling problem where it want's to stall unless I give it some gas. It really only does this when the engine is cold. After some research and talking with friends, I think it may be my injectors. (not to mention that they've never been replaced since installed in the factory back in 91) It seems like a fuel problem... As for my TPS, I have a new one of those too, but never got an accurate reading having trouble getting the jumper leads to stay, but thats a different story. I'm gonna try another method when I get back (i'm away for a while) as well as install new injectors. I also gotta check my fuel pressure. Should be about 48 psi right?

 

Sorry to hijack but where can i find a write up on the cleaning of the IAC?

 

The stepper motor that controls the IAC pintle position consists of a pair of rack gears that rest against springs. The rack gears are slightly staggered in their positions. The pintle has a mating set of teeth along its length. The two rack gears are actuated outward by the solenoid coils in the motor, and returned by the springs. If the coils operate alternately, the pintle is moved in a direction depending upon which coil/rack gear is operated first. To reverse the direction, the coils are operated in the opposite order. This is what the ECM does to control the IAC pintle position, and therefore, the idle RPM.

You can move the pintle by simulating the alternate operation of the stepper coils. If you are trying to extend the pintle, pull very lightly while you move the pintle side to side. This will alternately engage and disengage the rack gears and allow the pintle to "walk" out over them. It takes about 3-4 minutes to remove a completely retracted pintle that way.

Clean everything with lint-free cloths and a mild solvent. Harsh solvents can affect the insulation of the stepper motor coils. It's generally the dirt and buildup on this worm shaft that causes sluggish IAC operation. Set the pintle aside and allow it to dry while you clean the spring. Make sure the spring is at least 110mm long (about 2¼”. If not, you can usually carefully stretch the spring to the correct length.

When the worm gear on the pintle shaft is clean and dry, apply one or two drops only of clean light oil to the pintle shaft, then work the pintle back into the rack gears of the motor by the same rocking motion. It takes a while to get the pintle back into the worm gears, but you'll get it. It is important to get the pintle fully retracted into the housing so that the pintle is not forced against the gears when reinstalling the IAC unit in the throttle body.

While the IAC is still out, clean the air passages in the throttle body. The orifice in the TB where the IAC resides is the seat that the IAC valve closes against, and it can accumulate a lot of carbon, dirt, and debris. The easy way to do this is with carburetor spray cleaner and a small stiff brush or cotton swabs.

When everything is clean and dry, replace the gasket if it is damaged, apply a little anti-seize to the threads, and torque the IAC to the proper specs. (13 ft/lb for '85-'89 , 30 in/lb for 1990-on.)