I've been having an intermittent idle problem with my 88 IROC. It all started after I reset the timing to 6* BTDC. I have to do this because engine vibes work the distributor down to a few degrees ATDC every few weeks.

My normal startup routine used to go like this before the problem:
I'd turn the key to ignition, wait for the fuel pump to prime for a second, then start the motor up. Upon startup, RPMs would shoot up to somewhere between 1800 and 1500 depending upon how the motor felt then, and slowly settle back down between 1000 and 800 on a cold engine and 600 at operating temperature.

I was really sick of the eternity the motor would take to settle down to a reasonable idle speed. I had to wait for like 30 seconds before driving off, otherwise the idle would stay pegged at 1500 - 2000 rpm until I came to a complete stop, allowing it to resume its "settling down sequence". As you can imagine, it's a total pain in the rear to wait for the motor to sort out this BS every time .

I thought that maybe my IAC, TPS and timing adjustments might be off enough to cause this high idling nightmare. Sure enough, everything was off by quite a bit. I set the timing to 6* BTDC (EST bypassed of course), set the IAC to 450 rpm with the IAC motor disconnected, 600 rpm otherwise, and set the base TPS voltage at 0.58 volts, where I got the quickest throttle response.

A few days later, I started the car up while it was still hot from a previous drive, and the high idle somehow magically disappeared. The motor revved upto only 1000 rpm and quickly settled back to a rather low 550 rpm. Upon driving the car I also found out that the motor no longer seemed as responsive as before and that rpms dropped very quickly between shifts, making for jerky balky shifts, unlike before when the rpms seemed to drop at just the right rate allowing smooth shifting. The motor also sounded a lot quieter between shifts as I let up on the gas while changing gears

Since then, this problem has been popping its ugly head up every other time I start the car up. The motor just refuses to idle if it happens when it's cold. In fact, now the idle speed drops down to 500 - 450 rpm whenever this problem occurs.

I have no idea what's causing this problem. The only things I can think of are a bad IAC motor that may be dying, or something with the EST circuit. The IAC motor did make funny clicking noises the first time this happened. Before that, it would click a few times then go silent. Now I don't hear it click at all whenever the problem occurs. I think it could be the EST circuit too, because when the problem occurs the motor initially revs upto only 1000 rpm, exactly like when the EST is bypassed for base timing. Maybe I should try hook up the timing light next time this happens.

Sorry for making this so long but I'd rather give all the potential problem solvers out there all the information they need right away . In the meanwhile, could any of y'all throw me some ideas as to what may be causing this. Thanks in advance :hail: :lala:

 

88,

Your initial description of the idle speed sounds like the system was working the way it is supposed to. Target idle speed is mostly a function of coolant temperature. Starting a cold engine will result in higher idle speed until the CTS starts to get above 104°F. At that point, idle speed should begin to drop until it reaches the final target idle speed at 176°F. Beyond that, a 30 second warmup is not excessive, unless you don't happen to like your bearings. I'll normally wait at least a full minute, or until the temperature gauge starts to rise off the "100" mark.

Your current problem sounds like something has changed, either in ignition timing of fuel delivery. It's possible that while yout IAC was working correctly before, it is not now. You may want to remove and check it again, and clean the passages more thoroughly. Checking the EST bypass and timing is also a good idea. You may want to meter the CTS sensor or check the reported temperature with a data logging scanner.