I recently replaced my IAC to address a suspected idle problem that resulted in a stall condition when braking, especially when the AC was on.

Well, I don't have a stall problem but now my idle runs high. The idle seems normal when cold started, about 1400 RPM. However, after the engine is warmed up it stays at ~1500-1600 RPM when driving (taking the car out of gear and coasting) and when coming to a stop until the car is completely stopped... then it slowly drops down to ~1000 RPM. If I tap the throttle it will drop down to ~700-800 RPM (but not always).

I didn't touch the TPS (well, I didn't move it and it's still securely fastened to the TB). I used Vader's IAC cleaning and TPS adjustment instructions to set the minimum idle, but mine's a manual and I don't have an instrument to accurately read RPM beyond the factory tach. It didn't affect the idle behavior anyway.

Any ideas?

 

it kinda sounds like mine and it is due to the fact that the TB is wearing out. The shaft that the butterflys are attached to in the TB is getting stuck and the thottles is part way open, my black car did this and not my white one which was a auto started doing it, i guess it is b/c of the added movement of the thottle from letting off and on it when you shift. mine does it and when i tap it is falls back off b/c the when it snaps back there is enough force to close it all the way but when you let off lightly it gets stuck, go under the hood and play with the thottle, see what happens if you open it and let is close slowly, see if it hangs up, if so you have a few options, one get a new TB, send yous out to a guy, forgot his name but someone is sure to know him on the boards, he makes your TB 52mm and machines in rollerbearings, or just get a spring and attach it to the braket that holds the thottle cable about half way back on the plenum and then to the throttle is self, that is what i did as a temp fix on the black car. it provided enough pull to keep the thottle all the way shut.

 

Thanks for the input, but it doesn't seem to be a TB butterfly shaft problem. It appears to be electronic, engine management related. The idle comes down but as long as you're coasting the idle stays up.

Any other ideas?

 

Well, spent a few hours doing a compression check on the motor. While I was at it I checked the plug wire resistance and checked the spark plugs, cap, and rotor (again). Compression ran 150-160 in all cylinders (what little variation there was could just as easily been attributed to the cheap gauge I have).

There is the slightest bit of play in the throttle blade but the return spring keeps everything in check, no sticking.

The TPS voltage is spot on at .54 volts (was at .53 but figured I’d get it perfect). Voltage increase is linear on up to about 4.42 volts. I set the minimum idle speed. No codes from the ECM.

Fired it back up and the idle still behaves the same way. Now that I’ve checked where do I start looking for the more complicated?

 

87LB9,

Did you lightly lubricate the pintle worm gear on the IAC motor?

Aside from that, since you "tap" the throttle and the RPM returns to normal, I would suspect either some misalignment of the throttle blades, a worn throttle shaft, or a throttle cable that is sticking in its housing. The next time that it idles high like that (and when it is convenient and safe to do so), you can open the hood and try to manually return the throttle to the stop. If that lowers the idle RPM, you have a sticking throttle issue somewhere.

Another possibility is a vacuum leak somewhere that is admitting too much idle air. Likely suspects are the PCV, EGR, or brake booster, but check all hoses and gaskets for leakage.

 

Nope, sure didn't. That's a bummer since I had to remove the TB to get the IAC out. I guess I'll try your other suggestions first.

Thank you.