Having a small issue that's nagging me and I could use some help. The car is an '88 IROC, L98. Stock except for a set of Edelbrock shorty headers. I am having a hard time with the idle. Cold start was ok but during a short drive I noticed that it was idling around 1000 RPM in gear. Got home, put it in park and the idle jumped up to 1500. I let it cool off and when I restarted it the idle was fine until it got hot again. I went through the IAC relearn process, set minimum air and TPS which helped at first but then the idle went up again. I checked for vacuum leaks and can't find any. Checked the PCV valves and they are working properly. I checked the fuel pressure and it's good. I also removed the throttle body and cleaned the IAC passages with no luck. I pulled the plugs and they're all clean. The IAC and MAF are new. There are no trouble codes.

It's very frustrating. I had it running great, it sat in the garage and idled at 900 RPM for 30 minutes then I drove it for a few miles and the idle crept up again.

Any help?

It would seem that the IAC is not doing its job. The trick is determining whether the ECM is not driving the IAC or the IAC is not reacting. A data scan might be revealing if you can monitor the target idle versus idle RPM.

I agree. I'm having a friend stop by with a scan tool to check the IAC counts. I had it running pretty good this afternoon then as soon as I went for a drive it started acting up again.

While you have the scan tool hooked up, take a look at what the ECM is seeing for coolant temperature (not what the gauge is reading) when the engine is at operating temperature.

Will do, thanks for the tip.

I worked on it a bit this morning and all seemed well, it was idling right at 750 RPM for about 10 minutes. I revved it up to 2000 RPM or so and then it wouldn't return to idle again. I think when I get the scan tool hooked up it's going to show me that the IAC isn't closing for some reason.

I was able to read live data on the car today. I used an ALDL cable to bluetooth adapter. Pretty cool. In any event, the car started up fine and once it got warm the idle went up to 1500 RPM. IAC counts went from 30 to 160. Something is telling the ECU to open the IAC. Coolant temp and RPM both read accurately. Unplugging the MAF results in a lower idle, but rich and rough as expected. If I put the car into gear the RPM will drop to 750 then the IAC counts go up to 150-160 resulting in 1000 RPM idle.

I had a good friend and professional mechanic stop by and he verified no vacuum leaks and agrees that the intermittent condition and related symptoms are unusual. He suspects a faulty ECU.

Update:

After chasing this issue for several weeks, I took it out and tore the whole engine down to find the problem. This ended up being the culprit.

Eww. I am fighting idling problems too. Mine is idling too low and dying though lol.

What’s the part in this picture? I’ve tried everything known to man to get this bogging to stop, it happens when in gear and when applying accelerator (and the motor shuts off at at stop lights).

someone mentioned the Engine Temperature sensor many? That’s the only one I haven’t been able to locate on a 1991 Firebird 305 tbi.

thanks for any help.

The OP hasn't been online here for 7 years so he probably isn't going to answer you.

The picture he posted is a fuel injector that is massively clogged up.

The coolant temperature sensor on a L03 5.0L TBI engine is on the intake manifold.