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Thread: LS3 misfir

  1. #1

    LS3 misfir

    Hi all
    New here so I hope I've posted this in the right spot. My daughters 2012 Maloo developed a miss 2 months ago. It's been sitting idle ever since. It's got a cam, otr, 3" exhaust, headers, MAFless tune. We've known the car for 18 months and had no issues other than it was running a bit rich (or old school a bit smelly). Within about 3 days it went from ok to missing that bad you could smell from 30 meters and was getting worse. It was still able to accelerate but obviously not on all cylinders so I can't see how it could be limp mode. Prior to this it had hesitated on take off a couple of times on a cold start for a few weeks at random. Took it to a mechanic and it's still there.
    It's since had new plugs, new leads, coils checked and swapped around, injectors (and fuel line) checked and swapped around, new intake manifold gaskets, scanned tooled by several people, ECU checked by a mate with HP tuner. Checked valve lift and it seems ok. Compression test showed all cylinders within 5 psi. MAP sensor wotks.
    So far nothing has changed. Everyone agrees that 7 & 8 aren't firing. Everything seems to work when not attached. Turn the key it fires ok for about 3 seconds then runs rough and drops those two cylinders- and it appears only thos two cylinders.
    Haven't checked fuel filter or pump (just wayyyyy to hard to do at the moment) and no sensors other than MAP sensor.
    No fault codes are logging - at all!!!!!!
    At our wits end(as is the mechanic). TIA

  2. #2
    Join Date
    Jan 2004
    Perth, WA
    Replaced the O2 sensors?

  3. #3
    Join Date
    Feb 2014
    Pull the valve covers and check the valve springs, rockers, retainers and valve stem seals.
    Also when you have them off if possible rotate the engine by hand.

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  4. #4
    Thanks for the replies
    White_lie - We haven't replaced the O2 sensor yet but we will next week. We had decided to start replacing sensors and it seemed the most obvious one. If it fixes it then woo hoo but I'm not hopeful.
    GEnF-GTS - we've had the valve covers off - everything appeared intact, there was no metal lying around, and when rotated lift was the same across the cylinders. We were hoping for an issue there cause then we would have something to fix.
    To me it's like the timing is out on those two cylinders but that is highly improbable.
    Or there's no spark but the ECU is sending the request and it gets a response (not my words as I don't fully understand how the ECU operates). And it really does drive ok in the sense it can accelerate, like on 6 cylinders. Completely beats me.
    Thanks - any help or suggestions are extremely appreciated.

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