I have searched and searched this site and a few others for a problem I am having but have come up with absolutely nothing. My car is a '89 L03, 5-speed. The previous owner had given me a Jet Stage II chip with the car as well as the stock 5-speed chip. Turns out, the upgrade chip is for an automatic. Now to my current issue.

I started getting this idle issue a while ago where my idle would stick. I was thinking it might have something to with the timing being slightly advanced and possibly, for some reason, the stock chip. So I retarded the timing back to 0 degrees and installed the upgrade chip again (I say again because I have gone back and forth between the two chips a handful of times in the 4+ years of owning the car).

Unfortunately, I can't remember any details of what happened after installing the upgrade chip, I can only express what has been happening the past few months.

Whenever I start the car up cold, she will idle up to 1800, then slowly drop to 1500ish within probably 10-15 seconds. Once she drops down to 1500ish, I start driving because I don't have the time to sit around and wait for her to MAYBE drop down to 700ish. I will accelerate through first up to 1500-2000 rpms, then shift to second and proceed to slowly accelerate (no reason to romp on her early in the morning), and as soon as I hit around 1500 or so in second, the car will begin to accelerate on it's own. When I clutch in (either to stop or upshift) the idle will be at 2000 rpms. If I stop and sit for 15-20 seconds, the idle will drop down to 700ish and I will continue to drive. If I don't stay still for the stated time, then the idle will remain to stick at 2000 (or slightly lower when warm) until I do stop for a few seconds and give her the chance to stop. Sometimes after getting the idle to drop right away, she will again rev up in second gear.

After having this issue for several months and doing some reading and some thinking, I came to the conclusion that my VSS might be causing these issues. I had received a Code 24 previously and replaced the VSS with another one I had laying around from a spare cluster. However, I was still having idle issues, and it felt like the power was sluggish in certain rpms. Almost as if the computer wasn't able to determine how fast I was going and properly adjust A/F. Also, I was getting really crappy gas mileage. Well, I finally went out and bought a new VSS after months of not wanting to spend that kind of money. I managed to get a brand new one for $100. I installed it this past Thursday night. In the process, I re-installed the stock 5-speed chip into the ECM. I was very certain the VSS would fix the issue since my IAC, O2, fan switch, t-stat, CTS, knock sensor (I know some of those have nothing to do with idle), and possibly some other sensors I forgot were all new and/or replaced within the year or two (the CTS is the oldest replaced sensor).

After starting the car up, the idle dropped right down to 700. I took her out for a drive and got on her. The engine was still cold. As soon as I clutched in, the idle was hovering at 1500. The idle would not drop until I got the car until 5-10 mph and came to a stop and let the idle drop. As long as I stayed stationary and would blip the throttle, it will return back to 700 fairly quickly. However, as soon as I start moving, the idle will stick until I stop. I only drove the car for maybe 10 minutes until I returned home and slammed the door and walked away mumbling stuff under my breath (it has been a long 4 years for me just getting this car back to stock running before I start upgrading). Now, maybe with a few more minutes driving the idle will drop when shifting and not be an issue...but I doubt it.

The only sensor left to replace are the TPS, ESC module, and ECM. I don't see how it could be the TPS since the throttle linkage does not stick. Not to mention, the throttle drops back down to idle when not moving. The ESC module is fine too and that only has to do with retarding/advancing the timing when knock is present (or something along those lines). I want to find a donor ECM to try first and see if that is messed up. I really don't want to just go out and buy a new ECM and still have these same issues.

Do you guy's think the ECM could be causing these issues? Sorry for the long read, but I don't see how one could properly diagnose something without background information. For all I know, the ECM could be bad. The car has at least 200,000 on her, if not in excess of 300,000. Ideas?

 

It' not the ECM, they either work or they flash the SES light eradically to signal that you are running limp home mode.....which acts like it sounds. Runing an auto chip on a manual could cause a high idle, as the park/neutral switch functions differently causing the ecm to add IAC steps when you don't need them.

The hanging idle you describe now sounds like how the throttle follower routine works only exagerated. This routine is supposed to let the throttle "hang" for second between shifts to allow easier shifting. It sorta sounds like your IAC is just acting sluggish, without hooking the car up to a scanner and checking the IAC to see if what the scanner shows is actually happening the only thing you can do is swap in a new one and hope it works.

The other things that could be happening is that you have a vaccume leak, which on these cars won't cause any running problems because the MAP sensor doesn't care where the air comes from as long as it's in the intake. What happens is that the IAC will close completely but the vaccume leak still provides too much air to get the idle back down to where it needs to be.

I would start by doing a search for a vaccume leak and then try swapping in a new IAC.

 

I agree, sounds like a classic IAC problem.

 

I'll check for vacuum leaks. Unfortunately, the IAC is less then a year old, but it's not the first time components have failed right away.

 

Anothing thing to add is that if I am idling at a stop and then start rolling forwards down a hill, at a certain speed, my idle will start to rise like I am applying gas. However, I am still in neutral/clutch in and there is no power being transmitted to or from the rear wheels which would cause the idle to go up.