1985 Z31 NA - High Idle

  • #1

    1985 Z31 NA - High Idle

    Hi guys, I've been thru so many threads trying to nail down the high idle problem on my 1985 300zx NA, I'd figured I'd come here and hope you guys can help me out!!

    I'm sorry if this is going to be a lot of text, I just want to give out every detail I can and hope I can finally get this car running like 1985 again- It's 5 speed and digital package if that matters-

    I've had the car for many months now, and it's always had a high idle (Currently around 1100 RPM/ 1300 with lights on) and has been hard to start cold (Would always crank and crank and finally sputter into life). I'd have to let the car warm up before trying to drive it or else it feels like it's firing on half it's cylinders and missing. After the engine actually starts it'll run, then the idle drops to about 900, sometimes the normal 700 or 800, but like I said you can't drive it while it does this. Eventually the idle will jump back up to the high 1100 or up to 1400 at times and then it'll finally drive, but with definitely less power than it should have.

    Yesterday was the first time in about 3-4 days I started the car up, and it actually fired right up- but then went into a fit with the idle dropping as low as 500/600 and the engine struggling for life. Revving it you could hear missing. After 10-20 minutes it finally picked the idle back up and would drive, with not much power but it'd drive.

    Since owning the car I've done just about everything all the forums say- CHTS and harness was replaced and reads within 2k-3k ohms like it should, every vacuum line (even way back above the fuel tank for the EVAP) has been inspected and/or replaced. I did spark plugs and wires (Gapped to 0.40). I spent today making sure all the plug wires are firing, disassembled and cleaned the Idle Air Control, and replaced any even REMOTELY sketchy-looking vacuum lines. Still 1100 RPM.

    I have checked the ECM for codes before and they were 23 (Throttle Valve Switch (IDLE SWITCH) circuit) and 31 Load signal circuit (Think it only throws that because the A/C needs recharge).

    I'm only seeing so many options left, but I'd really appreciate any input you guys can give me. All I know left to do is replace the IAC or fault the ECM. Looking back on code 23 I'm leaning towards the IAC is dead, but it's weird how it still raises the idle 200 RPM just like it should when you flip the lights on? Has anybody else had high idle trouble and what was the cause?

    Thank you so much for reading / any ideas guys!!
  • #2
    IAC would be where my mind goes to first with your issue. All except the lights deal.... only thing I can think of would be to check grounds. Possibly the lighting circuit is causing some sort of extra resistance in the IAC circuit causing the ecu to register the difference at the IAC. Maybe this can be of assistance?
  • #3
    I appreciate the assistance, I might have to rule out the IAC tho- With it disconnected, the car started right up but idled at 900, creeping up to 11 after hitting the throttle to check for missing- it would definitely misfire with too much throttle at once. It did the same thing it always does- the high idle at startup, then the engine kicked into a rough idle (went back down to 900 but would still misfire with gas) then jumped back up to 1100 and ran smooth- punch the gas and no misfire. After this I plugged the IAC back in- started right back up at 1500 and crept down to 1200-1300. No misfiring.

    I don't think the IAC is malfunctioning, even without it the computer is still somehow controlling the idle. What's probably happening is the car starts in high idle, kicks down like it's supposed to, but runs rough because a sensor is toast. It'll only drive when it kicks over to a higher idle/fail safe mode, but that's just what I think. I guess I'm out to check MAF, O2, grounds, anything else we can think of lol
  • #4
    Do you have good fuel pressure? And have you check to see if maybe you jumped time? And yes, MAF signal, O2, or possible fuel pressure. Is the TB clean and vac ports clear? I wonder if we are seeing weird intake vac causing something weird.
  • #5
    Well in short I spent the whole day fighting with the O2 sensor to replace it, and all I have to show from it is smoke from all the WD40 burning off- no improvement in idle and startup, but car still runs with little to no miss when it's warm. It'll do it's best to get moving when it hits 4th gear. Just in case the ECM didnt reset when i pulled the fuse I'm gonna let the fuse sit out overnight. At least that's ruled out, maybe some Z owner in the future can learn from my mistake lmao

    The TB looked clean and vac ports should be clear, I went over everything. Haven't checked fuel pressure and I understand why that could cause the hard to start, but the misfiring and high idle? It's usually easier to start if you just hold the key cranking until it sputters into life, if you crank and stop and try to crank again it'll usually start up for just a second and die back out, then you have to do this over and over until it sputters into life.

    I dont think time jump would make sense, the high idle is always moving around (1100rpm to 1700 or below) like something is controlling it in response to something, not like it's just straight up idling at a certain rpm because the timing is jumped to that point.\

    Weird vac might be a problem for all I know, something I thought was weird was starting up the car after work, you could hear the coolant lines gurgling in the dashboard, like a fish tank or something. Low Coolant light comes on every once in awhile.

    Again, thanks for the input
    Last edited by ComeAlive!; 02-06-2019, 12:42 AM. Reason: I just want a good running car!!
  • #6
    Well the mystery continues and things get even stranger-

    When decelerating (engine braking) or pushing the clutch in, the car will, at random and for random amounts of time (fraction of a second to seconds) act like it's misfiring, like it's in too low of a gear. Like just for that fraction of a section it's losing power bad, actually taking power away from the engine braking. Sometimes it happened once for a few seconds, other times once for a few seconds then 2 fraction of a second burts, etc. The best way I can describe the sound is picture a turbo blow off valve sound, but instead it sounds like the engine is dying. When coasting in neutral or driving in gear, it seems fine. There's not a RPM drop associated with it. When just idling, stomping on the throttle you can hear misfiring, and it sounds like there's a hole in the muffler.

    Another weird thing was when I started it after work, the car didn't do the regular high idle / drop to rough idle (900 rpm) / kick up to a higher idle and actually be drivable (1100 rpm) thing, It did the high idle and kicked down to 1100. Kicking the lights on no longer raises the RPMs either.

    So I have these ideas-

    1: IAC has died after I spray cleaned it out- it no longer raises the idle for lights, and with the throttle closed during clutch shifting and engine braking, it isn't opening enough to keep the engine from stalling.
    2: MAF has died- Engine still runs restricted, is misfiring due to incorrect readings
    3: Exhaust system is blocked, engine runs as if it is restricted, and for all I know maybe the muffler did blow a hole.

    I bought MAF cleaner for tonight and I'll do the autopsy in the morning, what do you guys think?
    Last edited by ComeAlive!; 02-07-2019, 10:12 PM. Reason: I still just really want a decent running Z car lmao
  • #7
    After reading that, catalytic converter is stopped up? If you can unbolt it and bust the middle out and see what happens.
  • #8
    I haven't dived into the catalytic converter yet, but here's some more digging I did-

    -Blocked off accessory vacuum lines (HVAC, cruise control, vacuum ball, etc) to eliminate it as a vacuum leak- no difference.
    -Took off and cleaned Idle Up Solenoid (passenger side on intake), started engine with it blocked off to eliminate it as a vacuum leak- no difference.,
    -Blocked off Idle Air Control (driver side on intake), car started but idled at a ready-to-stall 400rpm, so I cut it back off. Connected back the car still has the high idle, unplugging the electrical connector makes no difference in idle.

    This make me think the IAC is either dead and open, or is way out of adjustment. I was thinking it would be weird if the catalytic was plugged, because then the engine would be trying to stall out from the restricted exhaust. But what if somebody adjusted the IAC to idle high enough to keep the car from stalling? And now the catalytic is failing and that explains the restricted kickback noise from the exhaust? Hmm

    Or maybe the computer itself is adjusting the idle enough to avoid stalling because of a blocked catalytic?

    I'm gonna pick up some offset screwdrivers and try to adjust the IAC down to a normal idle, if it acts like it's gonna stall from restriction then I'm ripping the catalytic off and exploring
    Last edited by ComeAlive!; 02-10-2019, 01:33 PM.
  • #9
    Small update, I ohmed the MAF and it pretty much ohms within spec, I don't think it's the issue (car still revs above 2k no problem) and TPS is good. I took the catalytic off and inspected it, the material is still intact and air passes thru it no problem, so I'm ruling that out too.

    The car still has that weird "kick back" kinda like a turbo blow off valve when you let off the throttle, and you can tell it's trying it's best to stay alive and running with an occasional miss here and there. I checked the codes again and it's the same load circuit and idle air control circuit codes, but there's also a fuel temp sensor code (but that wouldn't affect running).

    Now that I think tho it's funny, the TPS just tells the engine when the throttle's closed and it can return to idle, and that's when the kick back blow off happens. Maybe the IAC is so toast it isn't responding right to the throttle switch's demands. Who knows, guess I'm gonna find out
  • #10
    Have you checked the voltage going to the IAC? Just a thought.
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  • #11
    It might be worth checking- I ended up replacing the IAC with one off a junkyard car and it actually runs like it should- almost-

    The car will do a high idle for 30 seconds, then drop to the 700 or 800 range where it should be. BUT you can't drive it like this, feels like it isn't running on all cylinders. The car then kicks into a higher idle when warm (1000rpm), kicking the lights on drops this to the correct 900rpm with lights. Over the course of driving tho the idle with lights will eventually get up to 1100, and actually raises 100rpm to 1200rpm when you kick the lights off. Funny, but the car drives and doesn't act like it's having an anemic throw-up session when engine braking or anytime the throttle is closed

    So now it looks like for some reason the car won't run at the correct idle, so computer (or something) raises idle up to 1000-1200 so the car actually drives. Hmmm....
  • #12
    check your timing. if its off the ecu likes to play high idle games with you. sometimes just nudging it 2-3 degrees and it will drop back to normal.

    is the engine popping and spitting when engine braking? the fuel cutoff can be funky if the rpms are close to 1600-1700 when engine braking. the logic switches the injectors off if engine speed is about 1600 with the throttle switch closed. and switches them back on when it drops below that. and can cause an interesting issue when it keeps switching on and off.