This may be a little long so bear with me.
My idle was gradually going up, it got to 1500 and I got the code35 for IAC not working. SES light only came on when idling and went out when driving. I decided to remove the IAC and clean it as described on some thread I found here. When I put it back in like a dump a^& I over tightened it and stripped the threads in the throttle body, my bad. I got another throttle body from a recently running car and put it on. It did not have an IAC so I purchased a new one and put it on along with the TPS from my original throttle body. Idled at almost 3000 rpm. I pulled the IAC and it was still fully depressed (like you are supposed to do.) I went to the shop manual it just confused me more. I disconnected the IAC harness and shorted the ALDL connector, got voltage on all four wires to ground. In the book this says bad IAC connector or IAC. I then took the old IAC and connected the harness to it, left it out of the throttle body and started the engine. It should have shot the guts out against the garage wall, don't ask how I know this. I can remove the air cleaner hose to the throttle and put mu finger over the air inlet for the IAC and idle goes down to about 1100 rpm. So in short the IAC is not working at all. My bid question is should I replace the connector to the IAC and/or the ECM to fix the problem? The idle speed did not increase suddenly at the beginning just gradually increased each time I drove the car. Hope that describes my dilemma, Any suggestions will be appreciated. Thanks in advance for your suggestions.
Mike

When you tested and got voltage on all wires was the iac motor connected?
The PCM looks at position then drives the motor in and out by reversing polarity on the motor windings.With the IAC connected you will get some voltage reading in either direction.
Normally if the PCM is bad it will try to drive the motor in both directions with solid on time voltage in both directions. The transistors are called quad drivers. They are called that due to them being 4/quad set of transistors sharing a common emitter or collector (source voltage or ground) with the control(base)separated. When they fail, they normally short(stay on) Later PCMs use more MOSFETs which normally fail open(off)
If the motor is connected, it would be normal to see voltage in both directions, but it would be pulsed. Think of it as someone with their finger over 2 switches. If you need more idle speed you tap the in button(1 pulse) if not enough you tap it again, till you get the right idle. If you went too far you would tap the out button. So you could see a time when the motor is near where you want it you tapping out,in,out,in an so on. When you first turn the key on, The PCM looks for the position. If it was a little out(normal) it will pull it back till it stops or reads where it wants it. If you have it out of the throttle body it will attempt to close it. Because there is no throttle body to stop it, it runs the pintle all the way out.
If the PCM is bad, It would be on steady because the quad driver is shorting. However before you condemn the PCM pull back the harness cover as far as possible. you need to see if all the wires stay separated as far as you can trace back. Have seen the IAC wires plastic shielding turn into goo making the driving circuit work both ways because the current is shorting to the other wire in the harness. Last time I had seen that was on a Cavalier. They were shorting together in the harness near the firewall right before it went into the car.
Also pull on each wire at the IAC connector. If any appear to stretch, the copper in the wire has broken right where the crimp is made. Very common.

[This message has been edited by cmechmann (edited 11-13-2013).]

I will get a new connector and install it first. I always solder these wire connections and cover them with double covering of shrink tugging. I think I will also replace the TPS connector at the same time as they both are in the same harness bundle at that point. I tested it by the book with ALDL grounded and switch on and harness unplugged from the IAC. I did not see pulsing just solid light at the IAC connector on all 4 wires (using a test light as the shop manual said. If that doesn't get it done I will check each wire back to the PCM for continuity. If that does not get it I will then replace the BOX.
Any suggestions on source for PCM? Thanks for the in depth reply. I think there has been a problem for some time and the idle just eased up until I decided there was a problem. Again Thanks
Mike

[This message has been edited by oldbikeracer (edited 11-13-2013).]

Most of the IAC quad driver errors in PCMS I saw were in 4 cylinder cars. Very few on V6s. The early 2.0 dribble/throttle body injected cars were good for that.
Most GM cars configured the same way near the same year. 86GT auto/ 86 Z24 auto. You would have to poke around, but the PCM numbers are floating around the net. Check other cars equipped the same way. You would have to swap in your prom chip. If I'm not mistaken, the GMtuners site has a list of PCM numbers. There is a few GM EFI sites that have PCM listings. If you collect a list of different cars with the same PCM, it will give you more options. If you just ask for a Fiero PCM you will have less options.
Get the service number off yours and do a Goggle search to see what other cars come up.
The PCM I'm going to use later for a 3800 is listed for a 3800 in 92 and 93. They wnet to a Pre/ODB1.5 in 94 for car 3800s. Found they also used it in some V6 vans and trucks till 95.

I forgot to tell what my car is. 87 GT V6. There that is taken care of. Thanks again guys for the replies.
Mike

[This message has been edited by oldbikeracer (edited 11-14-2013).]

You can fix the IAC by replacing it with a 3800. That was the last straw for me.

Finally got new connector for the IAC and installed it today, still not working. Checked continuity from IAC connector to ECM and all wires good and none shorted. Tested IAC in a friends car and it worked god. I guess next is ECM replacement. Any suggestion where I should look for a good on?? Will ECM from some other Pontiac plug and play with no rewiring or problems, mine is 1227170 ?? Again 1987 GT 2.8 V6 with cruise control and AC, automatic transmission.. Thanks for your replies.

Mike

[This message has been edited by oldbikeracer (edited 11-20-2013).]

PM sent.

Got my ECM today. Still no IAC operation. I have done the following. Checked continuity from EXM connector to IAC connector and all is good, replaced the ECM today, using tested IAC in another Fiero and it worked. Anyone have a suggestion??? I am at my wits end here, where should I go from here?? Thanks for any replies.
Mike

Glad to hear you got it figured out. Your EGR-D chip in the mail.