idle issue cold start

[tt90z]

Hey all, i bought a 1990 ttz and i have been having some problems with it that id like to get ironed out so i can enjoy it. When its cold, the car will start then die. i can help it idle for a bit then it will start to warm up and will idle but it idles a little high at about 900-1000rpm. Also when its cold, its so rich it will puff out black smoke while you are helping it idle. I also noticed that it doesnt idle up when you turn the wheel or when you use the air conditioner.

Things i have checked: ecu and there are no codes, tps adjusted properly, did a boost leak test and fixed all leaks, compression check: all good, ohmed all the injectors: all good, ohmed coils: all good, replaced plugs with ngk's. Im really banging my head up against the wall with this one

[DCaff300ZX]

Welcome!
Most likely you are having an issue with corrosion of the IACV connectors, a frozen FICD (part of the IACV) or both, or possibly also just that the always-feisty IACV is slowly dying.
The FSM has lots of tests and such for the IACV, and if yours is failing buying a new one versus used parts is recommended.

[tt90z]

Today i cleaned the maf before i started it for the day and that seemed to make the rich condition considerably better and it made the idle around 800rpm once it was warm. i still had to help it run but it was much better. ill have to look into the idle valves. i suspect one of the o2 sensors might be junk and i also suspect ill be pulling the idle system. at least today it didnt start like a diesel

[DCaff300ZX]

That's a good start, you definitely want to check everything related. O2's will cause different issues than yours, but they will go bad or intermittent so a check is worthwhile.
Another area to look while checking the IACV is the PVC's and hoses for cracked/loose ones, or anything that can be a vac/boost leak. The PTU harness can also be a corrosion problem, same with CTS and injectors and CAS. Any corrosion issue (VERY common, bad Nissan connectors) can make these type issues come on, and usually only after awhile and several instances spread about which makes it hard to diagnose and fix.

[peterpilot9]

Just finished a plenum pull and injector replacement and thought I'd mention one of my experiences here. Even before my injector died, I had some idle issues that resemble what you describe. Kind of a different car when it was warm in terms of idle quality. While my plenum was off, I pulled the IACV assembly and cleaned all connections as well as the AAC spring and chamber. My connectors were really gunked up and there was a fair amount of carbon build up on the AAC spring and in that chamber that connects to the plenum.

My idle now is as near to perfect as it has ever been with the car in my experience. Not sure if this would be your solution or if it is feasible to pull that assembly with the plenum in place but just thought I'd pass this along. Good luck.

[brian300zx]

I’m with dcaff with the vac leak. I’m having a similar problem but quickly found out it’s my Plenum gasket. Spray some carb cleaner wherever there might be a leak if your idle goes up you have a vac leak. I sprayed some on all my hoses as well as egr tubes where there was also a leak which I replaced just don’t want to replace the Plenum gasket right now because just went through about 3 weeks without the car so I can live with helping it idle on a cold start. Good luck man just be paitient!

[tt90z]

so i spoke too soon on the rich condition being fixed. fired her up this morning and it was like starting a diesel, chugged black smoke and idled rough till it warmed up. i did a boost leak test again and came up with no leaks and my z1 idle air hoses showed up and i threw those on and did another test to verify and it pressurizes the system with no leaks. if not o2 sensors then maybe leaking or bad injectors even though they all ohm out good?? im really at a loss here, its driving me nuts and honestly i dont want to drive her till i get my issues ironed out

[tt90z]

when the ecu commands the idle valves to be open for high/cold idle does it also richen up the afr to compensate or does it rely on the maf and other pre closed loop parameters? Because that would definitely explain my issue. Fat fuel table for cold start with high idle but idle air valve compensates by opening. My condition (in theory going off more modern efi) is it triggers the high idle (which is faulty) and runs the cold start fuel table and since the idle air valve isnt working its not compensating for the rich condition and equates to an excessively rich idle.

[DCaff300ZX]

That's a reasonable thought process, an a problem I had early on with my NA before a WD-40 shot luckily fixed that one's sticking FICD, plus a new rear hose between the FICD and balance tube which was split on the bottom where you couldn't see it, from prying it upward while removing it.
Generally though when the IACV's get funky, replacement with new is the best choice should a cleaning and WD-lube not fix it...that's how my IACV went with my TT, no fix-ee so I replaced it with used/good which didn't work, so then a new one with upgraded harness and no problems.

[Ace2cool]

I think it's all based off metered air at all times, but I can't be 100% sure. There are people that know a lot more than me though.

[tt90z]

ok, replaced idle valves with new ones, replaced the injectors and checked over and over and over for boost and vac leaks, tested wiring meticulously with a dvom. tried a different ecu, re checked timing and tps and did another compression test.

think i need to just count my losses and sell this and walk away.

[DCaff300ZX]

ok, replaced idle valves with new ones, replaced the injectors and checked over and over and over for boost and vac leaks, tested wiring meticulously with a dvom. tried a different ecu, re checked timing and tps and did another compression test.

think i need to just count my losses and sell this and walk away.

OK, next step before bailing is to check the PTU harness connectors, all four of them (both halves, two connectors) for corrosion. The PTU can be an issue and was with mine, replacing the complete harness FINALLY fixed the similar issues I had as yours, and after doing as you- replacing damn near everything under the hood electrical- and idle-related.
Can't hurt to check, one side (original non-shielded connection) most likely will be VERY green with crud while the other, newer shielded end will have no corrosion whatsoever. Damn Nissan used the old crappy connectors to connect the PTU but then used new ones for the PTU service call replacement harness connections- awful nice of them, eh?