I have a 91 NA with ~150k on the original motor. Everything is bone stock with the exception of an aftermarket intake and a Bonez high flow cat. Compression is strong both hot and cold. We have checked the TPS, adjust idle screws, checked for vacuum leaks, and confirmed that the BAC is working (well, it revs to 3000rpm when I start it up cold, so that's my BAC confirmation). I have been having a strange idle issue since I bought the car which I have narrowed down now very slightly.

The idle when cold and warm is great, sits around 750-800rpm, but when the engine crosses a certain temperature point the idle becomes awful, and usually drops below 750 and often just dies. If it doesn't die, then the idle bounces between about 300-1000rpm continuously.

What I've discovered is that there is a definite temperature point which is causing all this to happen; if I'm sitting in traffic and the engine heats up, then the idle issue starts happening. Then, if I cruise for a while and cool the engine down, it idles fine again until it heats back up at which point the idle issue reoccurs.

Has anyone experienced anything like this or have any idea what could be causing it? Help appreciated!

 

the Bac isn't what causes the high rev at startup, its the AWS. To check the BAC connect the terminals of a Multimeter and check for resistance. it should be between 10-20 ohms if i remember correctly. go throught the checklist here http://www.teamfc3s.org/info/articles/idle.html
hope it helps

 

Interesting, thank you. I was misinformed regarding the BAC. Will double check all that this weekend and report back.