Hi, I'm new to here and fairly new to Z's too. Bought my first one month ago.

To the issue My Z has. It randomly runs lean on idle and part throttle (17-20 afr) and misfires. No check engine light or nothing. Doesn't die either.

Previous owner who built the car and owned it for ten years tried to solve the problem with no luck. He also tried to tune it richer by nistune, and some times it indeed runs near stoich (14.7) without a single misfire for a while.


For my experience, it sounds like a leak by the symptoms but I havn't found any. Could be iacv too? but who knows. Don't want to replace expensive parts and notice that the issue still presists...

Welcome!
Tough one there to be sure, as these types of issues are the most difficult to find and fix.
I agree that a boost/vacuum leak seems the best starting point, but surely the PO had the same thought given his actions. I'd definitely inspect and test anything possible in that area. Another thought is corrosion of the various connectors (common), so clean them and or replace as necessary. The PTU harness can cause a myriad of odd issues, usually corrosion related but I had to replace mine as it would consistently cause issues even after several cleanings. The IACV generally is only involved with things during warmup, and only a leak in the unit itself most likely a problem other than the harness and connectors, and with the best solution should yours be gunked up or otherwise is new replacement as the used units commonly have the same or ready to have the same issues as your old one.
The fsm has a lot of tests and such to help boost leak testing, and testing of the other areas in the car that can be an issue...that's my best advise for help moving forward other than a Z32 specialist.
Good Luck!

Thank you!

Harnesses are relatively new and seems to be in great condition. Looks like new. The IACV didn't look bad either. I think a boost leak test is next on the "to do list".

On the positive note, at least I got plenty of time to fix the issue Car is sitting in a garage over the winter

Sorry for the double post. Don't know how to edit..

Today I checked that the throttle body valves are shut and just to make sure, I re-adjusted them. After that I found a leak under the drivers side (jdm) TB. It was compeletely missing the vacuum line. I blocked that but it was so small leak that it didn't affect on anything. I also did the boost leak test and found nothing.

However, after all of that I started the car and first it was running near stoich (~15afr), but soon after it started to run rich, between 12-13afr and that is on a warm engine. Before it was mostly running like crap at 15-20 afr.

Maybe the throttle bodies were tiny bit open. I hope it's fixed now. And I think it's running rich now because the PO tried to compensate the lean idle by adding more fuel.