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Very Slight Miss Causing Major Problems...

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  • Very Slight Miss Causing Major Problems...

    Alright guys time to get serious with this problem ive been having...

    I acquired the car roughly 3 years ago, a 1984 300zx Turbo AE. Three years ago the first thing i did was replace the timing belt and attempted to start the motor. The engine ran rough, but it ran. not only was the belt replaced but the tensioner as well. Now roughly a year ago i discovered the belt had been off by one tooth for roughly one season of auto crossing, causing me to not be able to properly set the timing with a timing light.

    Once i had fixed the timing belt, the engine felt much stronger, but still had a miss to it. So i replaced the spark plug wires, spark plugs, rotor, and cap. The injector recall was also done when the engine first fired up leaving me with new injectors. The way the harness is still laid out though, i am led to believe that the dealership did not rewire the injectors to batch fire but left them as is.

    Back in September of this year, driving down the highway, the engine began bogging down really badly, and running like complete ass at low RPM's and would not idle.

    What had happened is the engine had run lean and caused a good portion of the edge of the piston to melt away, and leave 0 compression in that cylinder at lower RPM's and at idle.

    I have now rebuilt the engine using an NA block i have, and have left everything stock on the engine.

    While the injectors were out i eliminated them as a potential cause for the miss, and engine destruction by placing them on a pulse machine, on which i could set a certain injector pulse, and run a controlled amount of fuel through the injectors. Each injector delivered the identical amount of fuel with a perfect spray pattern.

    This past weekend i just got the engine re-installed in the car, and fired up ( Great easter weekend for me ) however that same stupid slight miss is still there.

    The miss is a simple lope noticed in the idle roughly every two cycles, which leads me to believe that it is in the injector wiring to the ECU, or something that can only effect one cylinder. The miss seems to dissapear at higher RPM's, but i do believe was the underlying cause of the engine destruction i saw back in september.

    Any insight you guys can provide for me as to the reasoning behind this miss would be greatly appreciated, as i am rather clueless as to what is causing it, and rebuilding engines is rather costly and time consuming to do more than once a year :?

    thanks guys !,
    take care.

  • #2
    You were also trying different ecu's right?
    You were just cruizing down the highway and it melted a piston???? not playing or anything :shock:

    You know for a FACT that the car is running lean when in driving range (higher rpms)? Do you use a wide band to determine this?

    I would advise you NOT to drive the car hard until you know for a fact that it has fuel when needed. Again its hard to understand why it would melt a piston just cruizing. You can run rediculously lean while cruizing down the highway. like 19-20:1 and I wouldnt think it would melt under vacume. Not even come close.

    Somthing doesnt seem to jive with your story :?
    85 Z31 6.0 LSX turbo 766whp/792wtq
    04 GTO, LS6, big cam, porting, N20... underway for summertime daily driver.


    • #3
      thanks for the response,

      i was not planning on driving the car hard AT ALL until i resolve this problem,

      i guess i failed to mention that i was hard on the gas at highway speeds when the deterioration of the piston happened. Nothing really happened i didn't even really notice a huge power loss until i pulled off the highway and noticed the car would not idle, and when i went to merge back onto the highway, the car was running like crap, and had a noticeable power loss.

      I am assuming the problem was caused by severe detonation, but it wasn't audible as it was only occurring in cylinder 2.

      I haven't tried another ECU, however i was going to try this next. Although i figured if the ECU was bad the car would run much worse, or not run at all. Guess i was wrong in assuming this.

      The problem with a wideband O2, is that if it is only the one cylinder detonating, or running lean would the wideband be able to pick this up accurately ?

      once again i greatly appreciate the help.
      And if you think it could very possibly be the ECU ill begin trying to source one immediately.


      • #4
        You are correct, the wide band will not pick up which cylinder is running the leanest however, it will pick up the "average" of all the cylinders. If one cylender is running lean enough to detonate and melt a piston, then they were ALL running dangerously lean.

        Ony any engine there will always be one or two cylinders that run a little leaner than the others. This is why it is imortant to tune for those cylinders. Meaning typically it is good to tune on the rich side of things.

        Say you tune your car to run an afr of 13:1 at 10psi boost this means that some cylinders are probably running 12.9:1 and those two that run lean are running somthing like 13.2 or 13.3:1.
        85 Z31 6.0 LSX turbo 766whp/792wtq
        04 GTO, LS6, big cam, porting, N20... underway for summertime daily driver.


        • #5
          Also, it could be the ECU but it is very rare that they go bad. If I were you I the next thing I bought to would be somthing to monitor my AFR's accurately. That is the most powerful tool you can have when figuring out why your car is running a certain way.

          Or you can just say " See that melted piston right there? this is how we know when we are running lean. Lets rebuild it and give another go round to see if things improved or not" :lol:
          85 Z31 6.0 LSX turbo 766whp/792wtq
          04 GTO, LS6, big cam, porting, N20... underway for summertime daily driver.


          • #6
            alright that makes perfect sense,
            thanks a lot,

            im gunna check into finding a turbo ECU to test, and figure out a way to monitor what kind of AFR my car is currently spewing out and go from there.

            oh and BTW the car is still stock minus the basics (intake/no-cat) as it was when it melted piston.

            heres a few pictures of what happened to the spark plug and piston in cylinder 2.

            thanks for the replies man 8)

            and even though your second theory would probably work ! i think its much easier and cheaper to go the first route :lol:


            • #7
              dam, chunked piston.

              Terrible idea putting those wheels on...