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Misfire at idle driving me nuts!

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  • Misfire at idle driving me nuts!

    So I have ran out of ideas on what to test and the problem has only gotten worse. I have an s14 with a KA-T at 10psi and haven't had any problems for the last year until this miss at idle,light load and decel near idle (farts and pops) have occurred. So basically anytime its at or near full vacuum. It has progressively gotten more noticeable it then past few months and now its just embarrassing and annoying. Its definitely worse when the engine is cold but is still there when at normal temp. I had very similar symptoms when my intake gasket was leaking but I have smoke checked it and have tried spraying carb cleaner around all the vacuum lines and gaskets and have found nothing. I have only been able to scope the primary side and saw the firing line drop down way low when it misses. which lead me to believe it was the coil in the distributor but I swapped in 2 others and still had the miss. the plugs and wires are new and test good. I have compression tested it and did a leak down test which both were fine and I even swapped stock ECU(without Nistune) which I also had to switch back to the stock injectors to rule those out and it still missed. It seems like it is a lean misfire but my vacuum gauge is steady at 24in/Hg and the wide band confirm it doesn't allways show lean when it does it. I have tried pulling a plug wire off while its running but the miss is random and not on one specific cylinder. Sorry for such a long first post any ideas would be very appreciated. Thanks

  • #2
    It would help to know a little more about your setup. I.E.:
    Bov open or recirculated?
    What size injectors?
    What do the plugs look like?
    ...the more info the better.

    Are you getting any backfring on decel? I was getting some misfiring some time ago and it turned out to be a torn injector o-ring. I pulled the injector plugs individually and the one that did not cause the engine to change note was leaky one.
    "To an engineer, the glass is neither half empty nor half full. It is twice as big as it needs to be."
    '96 Nissan 240SX KA-T
    '05 Grand Cherokee

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    • #3
      It seems to happen to many many cars(240). I had it for a while and after an upgrade to better plug wires it went away. I believe its a grounding issue and others I know have resolved it by either wires, cap, rotor,etc. I would focus on grounds wires and ignition overall....and yes it is annoying. Also even with the aem it still was prevalent, so it leads me to believe that the problem can happen to our cars over time. I also logged to see missfires under load and it only was at idle. Hope any of that helps

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      • #4
        ignition timing off?

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        • #5
          correct gap spark plugs?

          When Being faster its all that matters

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          • #6
            My set-up is a s15 t-28,370cc injectors,3in exhaust,recirculated. The plugs all look the same(light brown/tan). It does happen on decel or any other situation with high vacuum(off throttle). My o-rings are ok and I changed them when I switched to the stock injectors and have two sets. I have played with ignition timing and plug gap,both are set to factory specs now. SM240- the wires are new along with trying a few different caps and rotors but I guess switching wires might be something to try, but they ohm test good. I have added additional large diameter grounds to the block and chassis but I will go back and make sure everything is good. I did have an issue with my car breaking up in boost a while back due to a ****ty temporary ground terminal that had a bad connection but have soldered one since. Thanks for your replies

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            • #7
              So its my understanding that when a brake booster leaks vacuum the brake pedal would gets hard,right? well today on my way home as my car was missing as usual I pumped the brake pedal rapidly like 5 or 6 time and my car goes lean to 17.5, but dosen't get hard. I have never tried this before the miss and dont think its normal but when trying to eliminating the booster and vacuum block as a cause I disconnected it from the manifold and capped the other end (of course) and the miss was still there. So whats up with it going so lean? Can anyone with a wide band try it and confirm that it shouldn't do that? maybe this is my problem.

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              • #8
                Anytime that you decelerate with the engine in gear the ecu shuts off the injectors. So it is quite normal to see very lean readings on the wideband...hell I have heard of the possilbility of recallibrating the wideband on a long decel since it is pretty much just air getting pumped though the engine at that point.

                Actually the fact that you are not seeing readings in the 20's leads me to believe the injectors are still putting some fuel through. That could be where your "farting" is coming from: A small air leak in the hot exhaust igniting the leftover fuel.

                I am curious to see your nistune bin. I would like to compare your k-value and settings with mine when I had 370s.
                I had a bin from one of the US reps and after using it was getting a slight backfire on a long decel with the engine good and hot. There were some wierd readings in ignition timing. I asked the rep about it and he stated "It's all about the fireballs" (or something to that effect...there was a "yo" in there too somewhere) Anyway when I copied the stock values back in to that setting and the pops went away.
                "To an engineer, the glass is neither half empty nor half full. It is twice as big as it needs to be."
                '96 Nissan 240SX KA-T
                '05 Grand Cherokee

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                • #9
                  My A/F ratios are in the twenties on decel, it's at idle when I see it go a little lean when it misfires but not always. I also tried a stock ECU and still had the same problem. My tune has been perfect for the last year or so and I didn't have this problem until a few months ago. That is funny that some one would want it to backfire(that is so ghetto)

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                  • #10
                    I would say gap the plugs down (mine are at around 0.30" for 12lbs) but it sounds like your not getting blowout under boost. The fact that it happens with the stock ecu would lead ya to believe it is something electrical (plugs, dist, etc) or mechanical (injector o-rings). Otherwise I'm stumped.
                    "To an engineer, the glass is neither half empty nor half full. It is twice as big as it needs to be."
                    '96 Nissan 240SX KA-T
                    '05 Grand Cherokee

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                    • #11
                      yes, I am stumped too. Cuz I've have gone through many different sets of plugs, 2 sets of wires(recent set NGK's),and 2 used distributors and a "new" one but looked like it had been peviously installed. So I recently replaced the battery and replaced the grounds that the body shop had done work around and it seemed to be definetly not as noticeable but now it is totally still there, just not as bad.

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