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Thread: Random Misfire

  1. #1

    Default Random Misfire
    Engine: 572 BBC ProCharger, 93 pump gas in a jet propelled boat.

    Attached above is a link to a datalog file and Global File. At 69 seconds through 71 is an example of these random misfires. It occurs randomly while under steady state load anywhere between 2800-3800 RPM. This is a pump driven jet ski/jet boat setup so the load at any given RPM is always the same. And the load always increases exponentially as RPM goes up. At about 4000 RPM is when I cross over 100 kPa and my engine starts to see boost. Again this always happens between 2800-3800 RPM my kPa in that area starts at about 75 and goes to about 85 as RPM increases. I originally thought it was too lean so I went as rich as 12.6 in that area, but it always still randomly happens. One thing to note is that in this datalog, I don't have the right side bank WBO2 running. But every time this misfire happens, BOTH sides have the same looking lean spike at the same time. Any thoughts what could be causing this at just this RPM range? Thank you in advance!

  2. #2


    That's a good looking tune for your application. Is there any possibility that water is getting into the exhaust and causing wideband O2 sensor errors? Andrew
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  3. #3


    Thank you! Very Unlikely as water is introduced approximately four feet downstream. I should also add that if I ran the tune Open Loop it happens as well. Overall it runs really well. The misfire is very random and you just hear RPM drop and come right back up when it happens, as you can see in the datalogs. When I had a less aggressive pitch impeller (meaning load would be a little bit less through entire RPM range) it seemed not to do it at all. Now 2800-3800 RPM, the load isn't very large as it's not requiring boost to run it, but it my no means is that small either. MAP kPa isn't that low and there's still a significant amount of fuel flow, so it's definitely a lot more load that what a 'cruise' load is on a car. I'd still compare it to going up a few percent grade on a highway. It's just wierd that it seems flawless below and above that RPM.

  4. #4


    In that case, I'd carefully inspect all parts of your ignition system and related wiring. Andrew

  5. #5


    If my cam sensor was intermittently missing/failing is this something that would be happening? What throws me off is that when these random misses happen, both sides have the lean spike. RPM signal in the datalog never hits error, so crank signal seems like it's stable. If I had a random miss the lean spike should only be on one side?

  6. #6
    Join Date
    Dec 2009


    Quote Originally Posted by Kkuzmicki14 View Post
    If I had a random miss, the lean spike should only be on one side?
    What does your ignition system consist of? If Coil-Near-Plug ignition coils, check the brown cylinder head ground wires and the black battery ground wires.
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS T/W 11R 205 heads, 232°-244° duration/.623" lift/114° LSA camshaft, 12:1 C/R, TFS R-Series FTI ported intake, BBK 80mm T/B, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 200A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, -6AN fuel system plumbing, Walbro 255 LPH pump, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/3000 RPM converter, B&M Hammer shifter, FPP aluminum driveshaft, FPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 M/T Street Comp tires.

  7. #7


    Yes, IGN-1A smart coils. And yes, checked those and all related wires already.

  8. #8


    Perhaps it's not as random as it seems. If there's a loss of the cam signal, I think all spark stops until the crank & cam sync up again, which could be a couple of revolutions. In which case both banks don't fire and you get the lean spikes from incomplete combustion.

    I like this theory. I guess ideally you'd take the distributor out and spin it up and look at the output on a scope. There are inexpensive PC base scopes that can be purchased very reasonably. Or just replace the cam sensor and see what happens. Andrew

  9. #9


    That is my thought as well Andrew. Here's another sample datalog.This datalog I was running significantly leaner (13.5 rather than 12.7) in the area (and both WBO2 banks recording).
    I just read about the option of setting up Ignition Parameters to "Ignore Cam Signal After Start" This seems like a good way to rule out the cam sensor.

    If I was losing crank signal intermittently than I assume my RPM reading would read Error in the datalogs at that point and that has never happened. So I'm feeling good trying this. In this datalog I have both banks reading, so you can see the spikes on both. If anyone has any other food for thought or if anything jumps out feel free to let me know. Thank you in advance!

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