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Thread: Misfire Detection settings for a cammed engine?

  1. #1
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    Misfire Detection settings for a cammed engine?

    I'm trying to determine whether or not I have a real misfire or if it's just the cam overlap causing a slight surge. It occurs somewhat erratically between 1400 - 2000 rpm. It's not bucking, but more like a cylinder just isn't firing from time to time. There is no rhythm to it, or equal time between these little blips.

    I've checked plugs (TR6's) and they all look identical. I've tried gaps at .035 up to .042. No difference. I have tried new plug wires, no difference. I have also moved timing from a high of 50 deg to a low of 19 deg. Moving timing down to 24 deg seams to be the best, and that's where my tables are set to now. I have also changed injectors, but that did not make any change at all. I have VE (custom O/S) error under 3% as well as MAF error under 2% using LTFT's and verifying that with the WB.

    Nothing in the log is jumping out at me as the culpret.

    I figured I'd log misfires next to see if I have a bad coil pack. I bought this car used, and it was professionally tuned by someone else. I pulled up a stock tune as the misfire detection DTC was turned off. Well now I see where the tuner has changed all the misfire tables by a factor of four times. I can understand why they were changed, as he also installed a cam and probably did not want the customer complaining about nuisance misfires from the cam install.

    What should I set these table to? Stock, 2X, 3X or leave them? Is there a general rule of thumb for setting these with a positive overlap cam?

    I was also going to attempt a cylinder load test, but not having ever done this, was unsure how do it. I'm assuming the best method is on a load cell dyno, but I don't have one or access to one. I'll need to do this on a quiet stretch of road. Any advice on that would be appreciated too.
    When arguing with an idiot, make sure he isn't doing the same thing....

  2. #2
    Senior Tuner DSteck's Avatar
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    Run the car in open loop and see if it goes away.

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  3. #3
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    Still there Dave.

    I kept rpm pretty steady this log. I can hear the misfire in the exhaust tone up to 3000 rpm.

    I changed all the misfire tables back to stock, as well as the misfire DTCs. I can't make heads or tails out of the log though, all cylinders have misfire events. I can't really see if any one particular cylinder is misfiring more than the others, but I haven't build a histo for that - yet.

    I'm thinking I have a coil that's bad, or possibly a cracked intake.

    Any other tips up your sleeve?
    When arguing with an idiot, make sure he isn't doing the same thing....

  4. #4
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    I'm not convinced my histo is accurate, as when I select each cly misfire PID, then hit count, every one of them is the same. When I hit the max button, they are all different. Based on using max data, it looks like cly #3 is missing more than the rest.