engine: 2.8mpfi
year: 1988
manual trans

when I start the motor the engine idles high (2000) then drops to (1500-1600) in about a minute or 2.
then it takes about 10 minutes or more to drop to 900, sometimes I have to actually drive it to get it to idle normal.

I recently replaced the IAC and adjusted the tps to spec. The engine codes that come out are only 45 (rich exhaust).
(the previous IAC went bad [I assume] and even after installing a new one the idle would just never come down, so I ran with the
IAC in almost full extended position and unplugged, which in turn got me an idle of 900, but with a very hard start and no check engine light.)

I have checked all vacuum lines and I believe there are no vacuum leaks. What else could cause this erratic idle?

 

did you un hook the battery for 30 seconds and hook it back up and do an idle relearn?

 

yes. I've tried several methods of 'resetting'.

-unhooking the battery and waiting (normal reset)
-driving the car at/over 40mph for well over a half hour (per part instructions)
-starting the car (after clearing memory) letting it idle for 10 minutes, waiting 5 and turning it back on. (per haynes manual)

None have solved the problem. I have installed 2 different IAC's and have had the same results, I do not believe the IAC is at fault.

 

hmm the code 45 for the rich exhaust is interesting... do you have a exhaust manifold leak? or leaky injector

did you clean the iac passage before you installed the new one. they tend to get alot of carbon build up on them.

mine was giving me a code 45 for a while when my muffler rotted out and had no backpressure. it went away soon as i replaced the muffler.

 

Thanks for the heads up on that. I don't know if it's leaking, I did have a leak when I had the pipes done, but they welded it up. The flowmaster on it is about 3+ yrs now... I suppose its possible. I did clean out the throttle body area where the IAC goes, but I will double check and make sure.

I have done some research, I compared 3 different manuals:
haynes
gm code book (came with scanner)
and online manuals,
code 45 can be caused by faulty o2 sensor, defective TPS, restricted air filter, or a faulty ECM.

I've had the ECM on a computer diagnostic recently and it checked out fine, air filters and passage ways are new/clear. So I'm down to a few possibilities I suppose.

Thanks for all the help/input guys.

 

Update:
I bought ANOTHER IAC from autozone, after a convo with a mechanic and the guy from there I installed the new one, step by step...

I did the reset process suggested from autozone's help site, this did NOT reset the IAC.
(connect negative power, start engine for 5 seconds with accelerator slightly depressed. turn off engine for 10 seconds, restart to check proper idle.)

So frustrated I drove off anyway, about 2 lights later driving about 20-35 mph (roughly a mile) the car seemed to start to adjust. I stopped about 3 miles up the road for gas, car was still hanging idle at this point.

After I filled up I started the car, it went to normal startup, no high idle or anything. I took off and for about 4 miles it worked as if nothing was wrong.
Lol, then after another 1/2 mile it went back to the same crap!
Hanging idle between shifts, (takes like 5-10 seconds to drop to normal idle). When I got home, I attempted to reset it again, to no avail. It starts at 2000 rpm, then slowly drops to 900.

I soap watered the throttle body and vacuum lines, I don't see a leak. I double checked the IAC tb port, its clean/free of debris. I think I have a short somewhere, but I don't know where to begin to check it.

question: would a faulty o2 sensor have anything to do with this?

I don't know what else to do, short of bringing it to a shop and getting charged up the butt for them to tell me they don't know what it is. (been there twice already).

thanks;

 

i seafoamed my car the other day. lets just say i found a few vaccum leaks in the intake i need to fix. my car is doing a simmilar thing when starting cold. when its warm it will idle just fine.
when i get around to getting new injectors i sapose ill pull the intake and replace the gaskets.

the leak is comming from the passenger side rear of the engine. didn't see the smoke to long since the car was running.
im ordering new injectors by the end of the month

 

First off, you have the 302 ECM. No idle relearn procedure required. That's only for TPI and 90-92 cars that have the speed density system and a MAP sensor (3.1).

Second, mine has been doing this since I had the engine replaced in '04. I have NO vacuum leaks, ANYWHERE, and haven't in years (and yes, I've had the engine smoke tested under pressure). I have replaced the upper plenum and throttle body, the IAC 3 times, the TPS (and adjusted it to factory spec of .55 VDC at idle), 2 sets of fuel injectors and soon to be 3, the wiring pigtails for the IAC and TPS, and I still have a code 45. Even though I KNOW the injectors don't leak as I've held fuel pressure overnight before, and the fuel pressure regulator doesn't leak, I still get the code 45. The only leak I have is post-02 and pre-cat (crossover/y-pipe to cat inlet), and I run a Thrush glasspack. Oh, and the IAC is as clean as the day it came out of the box.

So, now I'm going to be running a 730 system to go with my 3.1 and the IAC problem will be gone. If you can figure it out, please tell. I've just learned to live with it.

 

the code 45 in my car went away when i replaced the muffler. it was all rotted out. im thinking it has something to do with backpressure. i still have a passenger side manifold leak and not throwing a code.

 

Well, after a lot of frustration and a few parts...I solved the problem.
Turns out I have a short, the short is in the wire from the o2 sensor to the computer. I installed an air/fuel gauge a few years back and it seems that wire developed a short of some kind. After removing the air/fuel wire, the car actually went back to normal...go figure.

The IAC was actually fouling out, but it seems it was because of the short.
I still get the code 45 here and there, but I think it could be because I don't have a cat on it anymore, or my muffler needs to be changed.

So thanks for all the input and help guys, glad I can put this one to rest!