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Engine falls on its face with throttle body closed, has absolutely no power when cold

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  • Engine falls on its face with throttle body closed, has absolutely no power when cold

    Howdy y'all. I'm sorry if this is a lot to read but I've laid out all the details i can to try to find some answers. After having hard cold starts and high idling in my 85 Z31 for awhile, I replaced the tube running from the MAF to the throttle body because mine had a tear in it, and now I have more issues, haha.

    Before I replaced it the car would be hard to start when cold and the engine would noticeably shake for a second when it started (IE not firing right for a second or two.) The car would idle fast, then drop lower usually to 500-600 and act llike stalling, slowly warming up to around 800. If I tried to drive the car like this it would act like it has absolutely no power, revving way higher and out of proportion to what you'd expect the car to be moving. I would have to wait until it "warmed up" and the idle picked up to 900-1000 and would drive OK. It would keep 900-1000 no matter if the lights/AC was on/off.

    At first, after replacing the big air tube and the smaller tubes (PCV, etc) that run off it, and pulled the ECM fuse, the car took a few seconds to start but started right up to 900, then quickly idled down to 700, just like it's supposed to. I couldn't believe that so I tried to drive it immediately and it did the same thing, drove with high revs putting out no power. If you keep driving like this eventually the engine hits that "warm up" point and it's like the engine gets "unblocked" and it'll drive normal, but idle around the 900-1000 or even 1100-1200.

    It's always done the no power when cold thing, but after fixing the leak in the tube it's doing new things. With the throttle body closed and engine braking, the engine acts the same way, like it's "blocked" or something.If you let off the accelerator and push the clutch in, the engine acts like it's laboring and then with the clutch in it falls on its face roughly down to 1000-1200rpm. If you keep the throttle even a little bit open while engine braking or pushing the clutch in, it's fine and smooth, but with the throttle fully closed it acts up. Sometimes if you engine brake with the throttle fully closed, it'll brake smooth and you can push the clutch in and it'll drop the rpms smooth, sometimes you'll engine brake and it'll be smooth, cut into that rough labored braking, then cut back into smooth, etc. Some kind of fuel cut off issue?

    The only other new thing is sometimes at stops it'll idle hunt, usually jumping around 900-1000 no matter lights/ac on (idle up solenoid on/off). Could the solenoid be defective and could that explain the other stuff?
    I had tested the MAF before with a multimeter and it met the benchmarks OK. I have replaced CHTS, plugs/wires, all vacuum hoses repaired and leaks inspected. Any insight is appreciated guys.

  • #2
    Like Z_Karma said, 84-85 cold start stuff (hard start, low idle until warm, no power) is an ECU problem. You should have no problem finding an NA ECU from 87 up, that should fix your cold start stuff. I want to do the same but everyone holds on tight to their 87+ turbo ECUs. If you had your vac leaks smoke tested then you can eliminate that possibility entirely for your other issues but if not then I suggest you do so. High revs no power? sounds like a driveline issue. My logic is, how can the engine be spinning so fast with the clutch engaged if the car isn't moving? For my cold start issues I have to baby the throttle because I couldn't get the car to rev past 1500 or so, so it sounds like you have a different problem if you can get the car to rev up when in gear. Did you ever replace your clutch?
    '85 2+0 Turbo


    • #3
      Yes the clutch was replaced, it can't be the clutch, if it was a slipping clutch it wouldn't suddenly gain power after the engine warmed up, especially on a brand new clutch?

      I found a remanufactured ECU and tried it in the car, it made no difference but is showing more trouble codes than the previous ECU did. It's still not a turn key car, the engine eventually starts and will idle at 800-900 900-1000. if it creeps to 1000 it'll suddenly drop to 900, then creep back up over and over. same thing with 800-900 mark. Unplugging idle up solenoid makes no difference.

      Codes the new ECU gives:
      It also flashed these codes, although for a 1987-1989 model. I guess this is a newer ECU:

      I haven't replaced the fuel temp sensor or the throttle position sensor, but I had replaced the CHTS and made sure the harness is in good shape, so it's weird that's coming up. I did check the TPS with a multimeter before, and it looked like it was functioning and showing when the throttle was closed. Unless the temp sensor is shot, I'm lost, the other codes should be meaningless


      • #4
        have you compression tested the engine?
        1986 Z31 2+0 Turbo GLL ---- spearco IC ,3" exhaust, Poly everything with solid subframe and diff mounts, HX35/40 Holset Turbo, CLSD, Nistune with 1220cc FIC injectors on E85.
        1993 Jeep Cherokee 4X4
        1995 Z32 2+0 NA Black
        1985 Z31 2+0 turbo Black
        1986 Z31 2+0 NA Red
        1988 Z31 2+2 NA Auto
        1988 Z31 2+0 Turbo White ---- Stripped for all of its goodies.
        1984 Z31 2+0 turbo Brown
        1985 Z31 2+0 NA Black
        2003 Nissan frontier D22 supercharded vg33e 4x4


        • #5
          The fuel injectors are different for an 87-89 than a 84-86. If you're using the later model ECU with early injectors, that could create some issues.
          Legal stuff:
          **Nissan Employee**
          Any information shared is simply my own opinion and NOT the opinion of Nissan

          not legal stuff:
          '88 300ZX 2+2-- Under the knife
          '86 3000ZX 2+2-- the knife victim
          '20 Leaf SV-- Because I hate burning money
          '19 NV passenger-- Because my wife likes burning money


          • #6
            Originally posted by mwolvinm View Post
            The fuel injectors are different for an 87-89 than a 84-86. If you're using the later model ECU with early injectors, that could create some issues.
            Wrong 88-89 is high impedance, 84-87 is low impedance.
            Cha iro

            enjoy building it yourself.
            if it fails, fuck it.
            at least you gave it a whirl.


            • #7
              Originally posted by Wilsonthesphere View Post
              have you compression tested the engine?
              ^do this