Intermittent high idle problem

[86-ZZ3]

My '86 TPI has been having intermittent high idle problems. A few months ago, I reset the idle speed + adjusted the TPS. All was fine. In the past few months, the engine would start and idle high. Lately, it's been running really high. I've just cleaned the throttle body (on car and off), tried to clean the old IAC (no result) and replaced the IAC (no difference).

Once the engine starts, it will either stay at that high idle range (1900-2000 rpm - most engine starts) or goes down to about 900 (lately about 1 in 7-10 starts) and doesn't fluctuate so I don't think it's a vacuum leak issue. The engine pull when the idle is normal is quite pronounced so the car doesn't slow down much in idle (700R4) unless I brake when the idle will go down a notch.

I can't figure out what the problem is. I've also taken the car for a drive to warm it up and run it above 40 mph at steady throttle (supposed to
seat the IAC), and also reset the ECM by disconnecting the battery for a minute.

ANy hints/suggestions? I've done a search of previous problems posted and solved and kind of hope it isn't the EGR.

 

[86-ZZ3]

I did a test. Pull the IAC connector and leave the ignition on for about 90 secs. Plug back in connection, car starts at right idle. Turn engine off, start engine again. High idle.

 

[onedef92]

Bad MAF relay? Did you check your O2 sensor output?

 

[86-ZZ3]

The engine pull when the idle is normal is quite pronounced so the car doesn't slow down much in idle (700R4) unless I brake when the idle will go down a notch.

The above should have tipped me off.

I idled the car until warmed up today (15 minutes where I am! - 35F/2C this morning) and then took it for a road drive to reset the ECM. Idle seemed o.k. until I noticed the car wasn't coming off when I took my foot off the throttle (car was not really decelerating). I noticed that when I tapped the brake (or pressed more firmly) the throttle would go back to idle. Did this a few times, even shifted to N when at speed and noticed the revs increase to 1800, but die down once I tap the brake.

Figured it must be a leaky vacuum booster so I'll clamp off the booster vacuum next and see if the revs go up or stay low/normal. The high revs seem to occur in gear, N and probably P too.

 

[bczee]

I am also having a bit of a High Idle problem also.. I am interested in hearing or input as to what test or procedures I should be doing in order to find and resolve.

I had some Vacuum leaks, but I think I got all of them, had a lot of old and hard hoses.

I had some ECM codes but I resolved them by cleaning connetions, MAF and replacing the Relays for the MAF. Thought I had all running good until last night when I came back from a 30 highway trip and got home, Put it into Netural and it was racing, it would not come back down. Will look into it after work.

 

[bczee]

Well.. I resolved the high idle on my vette.. did the low idle adjustment for the IAC and it seem to be ok after driving it tonight.

 

[86-ZZ3]

Eventually solved the problem and it was a series of multiple issues. Had a remote start installed and the installer didn't screw on the kickboard tight so it was binding the throttle body a bit. I had also forgotten to power off the ECM after changing timing and the initial advance had been set for something very high (20+ degrees?) to reflect the 6,400' elevation the car used to live at.

Had a Scott Hansen chip installed and worked well after that though there was a lag in throttle response (and poor gas mileage) that seems to be alleviated each time I bump up the initial advance. Seems either my timing light is off or the ZZ3 combo I have likes a lot of initial advance.

 

[bczee]

I had a same type of problem that I informed someone else of .. my rug/mat was contacting the pedal and causing idle problem at times, they didn't beleive me or took that it can happen !LOL

Are you sure your using the correct timing cover and timing pointer ?..there are different ones! I forget how many degree it is off by.. but there are a few ones to use..

 

[86-ZZ3]

I
Are you sure your using the correct timing cover and timing pointer ?..there are different ones! I forget how many degree it is off by.. but there are a few ones to use..

Actually, I don't. Hope that it came with the ZZ3. Not sure if the harmonic balancer is the same or if it was taken off the original L98? That's explain a lot!