Strange Idle on Cold Start

you're idle valve maybe failing somewhere. there are checks per the FSM to do with a multi-meter. has it been doing this for long or did it just suddenly appear? I am having a similar issue on my NA2T conversion, but I believe the problem is with my turbo idle valve as it idles quite high after warm up. anyways, do the checks on your valve and see if its malfunctioning.

Bon

It sounds like we may have similar symptoms, mine is worse when cold and gradually returns to normal once the engine completely warms.
It seemed like it got worse over the last month or two, where as now it seems pretty consistent.

I went around with a Multimeter and checked the IACV, air regulator, TPS, and all associated relays, all of which seemed to be operating fine :/
I'm thinking this may be a vacuum leak somewhere, injectors, or possibly timing (I had the timing belt replaced a month or two before this issue occurred).

hmmm, yes possibly. mine didn't begin until we did the turbo conversion. when it was n/a I never had any idle issues, none whatsoever. did you change anything or remove anything that could be causing this? on mine, it barely starts (separate issue) and when it does the idle is high, around 2000. then once it warms up it stays steady, however the idle is much higher still than it should be. have you tried removing the oil cap while its running? supposedly on a vacuum sealed system, when the cap is removed, the engine should start to die. if not and it is steady, then you have a leak somewhere else. my 280zx does that… works like a charm. but my 280z na2t doesn't die at all when the cap is removed. haven't tried it with my Z31. so, have you messed with the timing? you should at least check it and ensure it is at the proper mark (15 for 87T and 20 for 84-85T and 87n/a) check your manual and hood sticker. another thing you can try, which going to be the next step on my own car, is remove the valve, block it off along with the valve cover hose, thus eliminating its use completely, and see how it runs. you will need to revert the idle screw on the TB and use it for adjustment. some guys have had no problem in running their cars this way.
let me what you find out, as I am currently troubleshooting my own car. also, run your ECU codes and see if its throwing any.

cheers,

Bon

My two suggestions are to check the CHTS, there's two different chts sensors, ones for the gauges on the dash and the other is for the ecm.

The other is to check the fuel temp sensor.

As for the motor dieing with oil cap off. I've never had one die on me doing that, just made a big mess with all that oil splattering around and spewing out of the hole.

Not having much luck tracking down the cause of the strange idle :/
I tested the CHTS which seems to be working. I was under the impression that the fuel temp sensor is a small player in the ECUs engine management, but it tested fine too. I'm suspecting a rogue vacuum leak that slowly closes as the engine warms and the material expands. I sprayed around the manifold and intake with carby cleaner but the idle didn't react so if it's a leak it isn't obvious.

I did notice, while I poked around after shutting off the engine, a very faint hiss coming from the intake side of the plenum and it seems to be coming from my middle injector (the fun one under the IACV). Not sure if this could be the cause but I'm gathering parts to do a well needed injector overhaul.

Maybe leaking injector?

Definitely a leaking fuel injector somewhere along the rail but is that enough to cause the high idle on cold start symptoms?

If it is somehow sucking the excess fuel until leak stops.