New member losing his mind with misfire going uphill

  • #1

    New member losing his mind with misfire going uphill

    Greeting everyone, I'm new here but not to the Z world. I was on Z31.com many years ago when the likes of Z-bum, Engloid, Al87t, Butter et al were around. So, I've been around too and this is not my first Z. It's actually my 12th. But I'm stumped, and I need some help.

    The back story:

    This is my first Z since 2009. It's an 87T and I got is as a non runner. There was much much butchery in the car. It didn't run because the "mechanic" that was trying to get it going cut the plug from the air diverter, tapped that into the #1 injector, plugged it into the #2 inject and ran the #2 injector lead to the air diverter. I was as baffled as you are right now. The ECU had two mystery diodes completely burned out, which I replaced. The car does run right now, mostly.

    The fuel pump has been replaced, it has a new CAS, I have replaced the coil and wire and the power transistor. Today I am changing the MAF and possibly doing the plugs as well. I have gone over the vacuum lines and replaced those. I am getting code 21 from the ECU.

    The car has a very slight miss at idle, and occasionally while driving around town. The problem really becomes apparent when I go up a hill. By the time I crest the hill the car is misfiring so badly it won't even move. I need to pull over, turn off, restart, and there is a 50/50 chance it will run "ok" after that. If I avoid hills I can drive it all day though this problem has cropped up once or twice while driving on a flat road at more than 80km/h. Also, if I do manage to get it up into boost it seems fine. Until I stop So the miss is under 3000 RPM roughly.

    I've been through a lot with these cars but this issue has me stumped. Has anyone encountered something similar? I was thinking the CHTS, but it's not tossing that code at me. Nor the MAF of course, but I had one blow once and do similar, but not the same, things.

    Thanks guys.
    Last edited by ShaikCorso; 07-25-2017, 08:47 AM.
  • #2
    So, the plugs were all new though not all of them were actually installed correctly. I discovered that cylinder 6 wasn't even plugged in. Interesting. I reconnected that plug and was greeted by much white smoke and a rough idle for a while. That has cleared, so I'm letting the car cool and then doing another cold start. I want to see if that rough idle makes a return. Interestingly, all the plugs showed that the car is running lean. This is making me lean (ha ha) towards the MAF again, though I noted that the pump is slowing down. I wonder if for whatever the reason the ECU is cutting the voltage when it isn't supposed to, so that makes me wonder if I should do the 12V constant mod to the pump, or if there is an issue with the TPS. No TPS code is being thrown, but one was stored in the computer when I got the car. This may have been from the constant cranking and no start condition that I had when I bought it.

    The idle is fairly smooth, but I have the miss at part throttle when stepping on the gas in neutral. Again, I haven't been out on the road to test it for the major uphill miss, but I can't see one dead cylinder causing that.
  • #3
    And now, I went for my test drive and was able to replicate the horrific misfire by just going on the highway in 3rd and 4th and keeping the RPMs high. The miss made it's appearance when exiting (close to home thankfully) so thinking about how lean the plugs looked I chug chugged my way back into the driveway and put the meter on the pump wiring before turning the car off. I got 8.5 volts at the pump, so then I shut down and restarted as normal and it went up to 11.5. Unfortunately no one was around to press on the gas with the 8.5 volts and chugging to see if it came back up with throttle. I know these things run the pump at lower voltage when cruising, so next step it to research those voltages to see what they should be at. 8.5 seems a little low to me so I might just as well do the pump mod. If the 8.5 is abnormal then I will be looking at the tps. Or, just do the mod and if that works I'll call it done. Sorry for all the updates, but they should help those that are looking to help me, and honestly typing helps me to think.
  • #4
    So, I did the fuel pump mod without any improvement. Oh well. I switched the MAF only to discover that the replacement was no good - car did not rev past 2500. I'll be needing another one of those for the parts/winter car now I guess. There was still a slight miss at idle even with the bad MAF or the original one unplugged. I went on to change out the ECU with one from an NA, thinking that the car would at least run. And it ran, but the misfire was horrible so the original repaired one went back in. Even though I had replaced the CAS in an effort to get the car running in the first place, before I found the mangled injector/diverter wiring, I tried another distributor last night, tried driving with the O2 and CHTS unplugged. No luck there either. I do note that when it has the misfire it totally clears above 3000 RPM. It makes me wonder if going to batch fir is the solution. With only two drivers, does anyone know how the computer does this? I know I can set up the wiring post recall style, but the injector lead that they use is the same one that was plugged into the diverter valve. Makes me wonder if there is something happening inside the ECU because of it, but I can't find an 87T ECU anywhere at all.

    Any ideas?
  • #5
    So, this lunch hour I pulled apart an NA ECU to harvest the funny diodes that were burned out of my turbo ECU. The ones I used initally were close, but not the same. Interesting a few things happened after this. 1: The fuel pump now primes constantly. This is not good, but with the relay mod back there I will just run an ignition to activate it so the ECU won't keep grounding that circuit at all times and address it later. 2: The idle has dropped to a steady 600 RPM, from between the 8 an 9 that it was. 3: The miss is still there, but I have more power when under boost.

    I didn't get it out long enough to see what happened after being under load for a while. I am still going set up the injectors for batch fire since the car runs perfectly over 3000 RPM regardless. I don't know what else the ECU does after 3000 RPM but I know it goes to batch fire. So, we will see.
  • #6
    OK, so batch fire rewire didn't work at all. In fact the misfire was horrible. So, I reverted and metered the injectors at the same time. All but one read 3.2-3.1 ohms, with the exception being a single one that the PO replaced and it was 2.6. I can't see that causing a major issue like I'm having, but I am going to look into that. Being that the ECU is now acting up with the priming circuit I went looking for a replacement again and found on from an 86T that I ordered, so now I will wait for that to arrive and switch it out.

    Again, any help or suggestions are appreciated.
  • #7
    So, today I drove around with the repaired ECU for a bit. Ran like a can of smashed assholes for the first bit, then just down to a bit of a misfire. By the way, did I mention that the PO burned out the EFI fusible link and replaced it with a 15 amp fuse? He did, and burned out 18" of wiring at the relays down into the fender. So, of course, the first time I repaired the ECU with not the right diodes I repaired that wiring too.

    Anyway, today, I touched the ECU and it was screaming hot. Great. I pulled that out and peeked inside, and whatever transistors are on the outside perimter were scorching hot. So I thought, interesting, and peeked under the hood. Sure enough, all the wiring at the EFI relays was hot. I haven't pulled them to check the condition, but I am hoping they didn't melt again. I tried an NA maxima ECU, and while I couldn't drive it very far it didn't heat up and the fuel pump prime actually worked. I remember that before I found the cooked wires in the fender that those relays would click on and off, so I'll be checking in there again.

    So now, with the orginal conditions replicated that led to me buying the car in the first place I am down to trying a new ECU. This one being repaired "right" this time and trying to burn out wiring, heating up, etc, and the maxima one not doing that makes me thing there is an ECU issue that I can't find based on visuals. The fuel pump still has a weird sound, and that is making me wonder too since the PO put in a replacement. A new replacement, but with unknown provenance. Until my ECU comes in the mail I can't do much but wait and look at wiring, or idle it with the Maxima ECU. So for now, it sits with the battery disconnected.

    As always, any help or suggestions are appreciated.