Cylinder 4 misfire at idle when warm
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  1. #1
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    Cylinder 4 misfire at idle when warm

    I've done quite a bit of searching and have found several causes for similar situations but feel I've ruled them out. Unsure where to go from here, whether I should take it to the dealer or find a good LS shop.

    Like the title says I have a misfire only in cylinder 4, only at idle and only when the car is fully warmed up. It's not a dead miss, I'll get anywhere from a 4 to 24 misfire count before it resets. If I unplug the coil to cause a dead miss I get over a 100 count. It threw a P0304 code once when I idled the care for a long time the other day. No other codes.

    I have a Dashboss and began using it around the same time I did a pulley and tune. The car has 65k miles and I've had it for two months. I did a ZL1 lid, Airaid intake, Varimax pump, FIC 1000 injectors, TR7ix plugs and a 2.5 upper pulley. I was tuned at CSP in PA. I can't say for sure whether the miss was there before I was tuned or not.

    Here's what I've tried so far:

    Swapped plugs
    Swapped plug wires
    Swapped coil
    Swapped injectors
    Compression Test - 150psi (same as cyl 2)
    Leakdown test - 10% (same as cyl 2)
    Plug looks normal, no fuel on plug when pulled
    Checked for collapsed lifter
    Pulled pushrods and checked, OK
    Checked for broken valve springs

    I also ran a cheap borescope in the cylinder. Couldn't see anything alarming but honestly it was a POs and resolution sucked. I'm not really burning oil, no smoke from exhaust. One thing I noticed when I changed plugs before
    I was tuned was that the plug from cyl 4 had no electrode left. The ground strap was intact but electrode was gone. No idea how old the plugs were but they were OEM. All other plugs looked ok. I also have a moroso catch can fwiw.

    Any ideas or help would be greatly appreciated, it's frustrating to say the least.

  2. #2
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    From what I've read before, burnt-out electrodes would indicate a lean condition. So even after you've swapped injectors, the misfires were still present?
    2009 CTS V Black Raven Sedan, M6, Recaro's, Ultraview
    Stage 0.6 Airaid CAI, Kooks 1 7/8" LT's, Kooks catted xpipe, stock mufflers, Mamofied LS7, NGK TR6, SM Dual CC, 160F stat, Dynojet tuned by Dave @dasilvamotorsport, SAE 526hp/508tq/91oct

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    Quote Originally Posted by Blown376 View Post
    From what I've read before, burnt-out electrodes would indicate a lean condition. So even after you've swapped injectors, the misfires were still present?
    Yup, misfire stayed in cylinder 4. The plugs were replaced before I did any mods or got tuned.

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    You could have a weak spring, or the beginning of a crack in the ringland or even within the sleeve of #4. After searching similar threads, other possibilities also pointed to bent valves or improperly seating valves, but the consensus was towards a valvetrain issue.
    2009 CTS V Black Raven Sedan, M6, Recaro's, Ultraview
    Stage 0.6 Airaid CAI, Kooks 1 7/8" LT's, Kooks catted xpipe, stock mufflers, Mamofied LS7, NGK TR6, SM Dual CC, 160F stat, Dynojet tuned by Dave @dasilvamotorsport, SAE 526hp/508tq/91oct

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    Misfire in that cylinder is pretty common, I've seen alot of threads on it. I have it there and tried all your remedies you listed to no avail. I monitor with an aeroforce gauge but other than that I run it on the road course and daily drive it without issues.

    I'm running the same plugs as you and injectors. I'd stay away from the dealer since you're modded.
    Last edited by Barryg; 12-24-2016 at 01:21 PM.
    2009 6MT - 708hp E63CORN-634hp/637tq 93 octane - stock cam-- 2.5/91 - ported blower/snout/90mm LS7 Mamo TB - Track Attack HX - Norcal Ice box - FIC 1000 - JMS BAP- Dsteck FF - MM catch can - ZL1 lid - AirAid Green Filter - pocket protector/side swipe-Kooks 1 7/8 Headers - 22in resonators - SW catback


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