Car: 89 Z31 n/a
Problem: Had a recent fender bender, but nothing more than a bump and scrape across the nose fender. Everything appears fine except for the idle, which gets rough once the engine reaches operating temperature. I've checked every line and connector I could find. The FPR was replaced and functioning about two months prior. Acceleration is fine and the engine maintains a steady purr when holding the throttle. Release the throttle and the engine drops to 500rpm then bounces roughly to around 700rpm.

It seems as though a sensor is bad, but the ECM diagnostics indicate everything is fine. Also, unplugging the AAC did not affect the rough idle. Any ideas?

vacuum leak perhaps? when's the last time you replaced all of the vacuum lines and other hoses in the engine bay? that slight jolt might have been just enough to send those 18 yr old vacuum hoses over the edge and crack...

it does this at all temperatures after it warms up?

I replaced the three main lines from the canister about two years ago, and some of the others around the engine that looked like they needed it. It was one of the first things I looked at when this started.

The idle is normal until operating temp, then rough. If I drive it a few miles, it gets worse and almost stalled on me last night while at a traffic light. It seems as though there's not enough fuel getting through and the sensors are correcting this by giving it a shot in the ass every 10-15 seconds. I'm going to try and give it a good dose of injector cleaner and see if perhaps some carbon deposits or other crud was knocked loose and blocking some ports.

yeah try running some of that seafoam stuff through it to clean it up a little as well...

also, check out 88hybrid.com for some really knowledgeable z31 guys

Well, injector cleaner did absolutely nothing. Oh, thanks for that link to 88hybrid... that's a great site and a smokin' project car! Gives me inspiration for mine, now if only I had the deep pockets to fund it...

One thing I noticed yesterday is that when I have a load on the engine, in this case it was the economy air ventilation, the car corrects for whatever is troubling her. There is a slight twitch in RPM, but continues to run in exactly the band it's supposed to.

I keep seeing posts of people hooking up laptops to their cars and running diagnostic software. Anyone know where I can get some more info on doing this with a Z31?

Thanks.

i've never seen a program that runs a z31 diagnostic, but some shops might have one

anyway when you're putting load on the engine it trips the idle up switch and raises the idle slightly to correct for the extra load... i think it's the aac/fcid that does this...

Well, I still haven't solved the problem yet. I've been going through the troubleshooting checklists in the FSM, but still nothing. To make things even crazier, I was just transferred to El Paso, TX and had to drive my car 11.5 hours. Everything still ran like a charm, except for the rough idle.

What is that SeaFoam stuff you were talking about DeleriousZ?

basically just an engine cleaner, takes off all of the old carbon buildups and stuff...