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Thread: L76 Pontiac G8 - WHY IS IT MISFIRING???

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    L76 Pontiac G8 - WHY IS IT MISFIRING???

    Hey everyone, just recently picked up a 2008 G8 GT. The car has 152k miles. Completely stock, never had anything done to it. But I'm chasing a misfire on two adjacent cylinders. #3 & #4. I've verified the coilpacks are good, put new plugs in it, checked the fuel pressure, and each individual injector. But it's still misfiring. It's not a "Dead-miss" but very close.
    I suspect this engine is suffering from collapsed DoD lifters. Hot at idle, it has very low oil pressure. 10-15psi, but no inherent lifter/rocker tapping.

    Just wanted to get a second opinion on this matter. Thanks! -Levi.

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    Tuning Addict 5FDP's Avatar
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    A completely collapsed lifter would be a dead miss all the time. The lifter is either stuck down or it isn't, there is no in between.

    I'd do a compression test and leak down test on the engine. Those engines are known for burning oil as well, so it could be a ring issue. Quiet possible you will have to pull the engine and clean it all up, hone it, new rings and delete the AFM/DoD system.
    2016 Silverado CCSB 5.3/6L80e, not as slow but still heavy.

    If you don't post your tune and logs when you have questions you aren't helping yourself.

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    Quote Originally Posted by 5FDP View Post
    A completely collapsed lifter would be a dead miss all the time. The lifter is either stuck down or it isn't, there is no in between.

    I'd do a compression test and leak down test on the engine. Those engines are known for burning oil as well, so it could be a ring issue. Quiet possible you will have to pull the engine and clean it all up, hone it, new rings and delete the AFM/DoD system.
    Well it looks like you were right. The lifters are not collapsing themselves.
    But, I did do a compression test on the offending cylinders (#3 & #4) both came up low (130-145psi).
    And on leak down test, both were producing a substantial amount of air through the exhaust.
    So either, some part of the lifter is keeping the valve open.
    Or some part of the head is preventing a complete valve seal.
    Though I imagine afm/dod is the suspect, mainly due to the super low oil pressure.

  4. #4
    You could take the rocker arms off on those 2 cylinders and do your leak down test again. Then you would know if you have a valve not sealing, or if its a cam/lifter/push rod issue.

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    Quote Originally Posted by WAK View Post
    You could take the rocker arms off on those 2 cylinders and do your leak down test again. Then you would know if you have a valve not sealing, or if its a cam/lifter/push rod issue.
    Gave it a shot, valve seals are good. Looks like the lifters are keeping the valves open all the time.
    I shouldve known, as the car fires through the exhaust on occasion.

  6. #6
    So the leak down test was OK with the rocker arms removed?

    If thats the case, sounds like you need to dig into the lifters.

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    Yeah the leakdown test was fine without the rocker arms on. Already working on tearing it down. But I've found something suspicious..
    The #4 (worst offending cylinder) is extremely clean on the intake port.

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    G8 owner here!!!

    I did a cam swap/DOD delete at about 120000 miles. DEAD miss. Cam lobe wiped.

    Its pretty much a REQUIREMENT to do a DOD delete on the G8 GT (L76) at ANY mileage. Especially over 100000 miles. Some failed at much lower mileage. Im the 4th owner (YIKES!!!) of mine AND THE LAST. Carfax says my car had a cam and lifter replacement at 50000 miles. Anyway...

    At MINIMUM do a DOD delete and stock LS3 cam swap. That will give a little extra power due to the increased duration of the LS3 cam. And peace of mind of not having to worry about the DOD crap.
    Last edited by jaydubb71; 11-15-2019 at 10:47 AM.
    Daily Driver= 2003 BMW 330xi
    Weekend Cruiser= 2009 Pontiac G8 GT (Vararam, TSP LS3 N/A Stage 1, OBX, CTS-V converter, MagnaFlows w/ J-Pipes, 160 t-stat)
    Project Car= 1991 Chevrolet Camaro RS (LQ4 w/ Gen 4 Rods, LS3 heads, turbo...)
    Truck= 2007 Chevy Silverado 1500 LT LY5 4x4

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    Here's what I've found after tearing it down.

    20191114_162603_2.jpg
    20191114_175733_2.jpg
    20191114_175658_2.jpg

    At first it seems like a blown headgasket. But, after checking everything over It all seems fine. Head mating surface is perfect, gasket isn't warped or blow out. Nothing strange.
    But, someone has done this before me. This car was sold at auction by a dealership. Looks like somebody traded it in with a collapsed lifter on this cylinder. They pulled it apart and attempted to patch it, by adding two brand new DoD lifters of the updated style. (the other cylinders still have the factory ones) Then threw it back together. But it seems like there's a problem with the DoD solenoid/oil passages or something. Since it's keeping the valves open during combustion.

    So, best I can discern the cleanliness of the cylinder is probably a result of fuel-washing due to incompletely combustion in that cylinder.
    I'm just gunna put a stock Lq9 cam i've got in it and put it back together.

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    Quote Originally Posted by jaydubb71 View Post
    G8 owner here!!!

    I did a cam swap/DOD delete at about 120000 miles. DEAD miss. Cam lobe wiped.

    Its pretty much a REQUIREMENT to do a DOD delete on the G8 GT (L76) at ANY mileage. Especially over 100000 miles. Some failed at much lower mileage. Im the 4th owner (YIKES!!!) of mine AND THE LAST. Carfax says my car had a cam and lifter replacement at 50000 miles. Anyway...

    At MINIMUM do a DOD delete and stock LS3 cam swap. That will give a little extra power due to the increased duration of the LS3 cam. And peace of mine of not having to worry about the DOD crap.
    Thanks for the reply! Sorry I didn't see it while i was composing that last message. I'm sure this car has had lot of owners. But Aside from the two new lifters they put in it, I'm surprised the original ones made it this long! 150k miles and the rest are still good.

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    Quote Originally Posted by gibygreen View Post
    Thanks for the reply! Sorry I didn't see it while i was composing that last message. I'm sure this car has had lot of owners. But Aside from the two new lifters they put in it, I'm surprised the original ones made it this long! 150k miles and the rest are still good.
    No problem, G8 brethren. LOL! Make sure you get the DOD delete kit for it. The LQ9 cam is weak compared to the LS3 cam. I would do the LS3 cam. Oh.... GM head/exhaust manifold gaskets, NEW GM or ARP head bolts (dont reuse the TTY bolts... Ive seen a blown a head gasket due to someone reusing old bolts... smh) and new lifter trays...

    LS3

    ls3

    Just a little subliminal coercion... LOL
    Daily Driver= 2003 BMW 330xi
    Weekend Cruiser= 2009 Pontiac G8 GT (Vararam, TSP LS3 N/A Stage 1, OBX, CTS-V converter, MagnaFlows w/ J-Pipes, 160 t-stat)
    Project Car= 1991 Chevrolet Camaro RS (LQ4 w/ Gen 4 Rods, LS3 heads, turbo...)
    Truck= 2007 Chevy Silverado 1500 LT LY5 4x4

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    Well I appreciate the advice, but this is for a friend of mine. Just trying to get it back on the road, and while LS3 stock cams may be cheap. It's not cheaper than a free LQ9.
    Thanks.

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    Quote Originally Posted by gibygreen View Post
    ...and while LS3 stock cams may be cheap. It's not cheaper than a free LQ9.
    Thanks.
    True that! LOL!
    Daily Driver= 2003 BMW 330xi
    Weekend Cruiser= 2009 Pontiac G8 GT (Vararam, TSP LS3 N/A Stage 1, OBX, CTS-V converter, MagnaFlows w/ J-Pipes, 160 t-stat)
    Project Car= 1991 Chevrolet Camaro RS (LQ4 w/ Gen 4 Rods, LS3 heads, turbo...)
    Truck= 2007 Chevy Silverado 1500 LT LY5 4x4

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    Getting the rest of the DoD delete parts is still required though even with that LQ9 camshaft.
    2016 Silverado CCSB 5.3/6L80e, not as slow but still heavy.

    If you don't post your tune and logs when you have questions you aren't helping yourself.

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    The DOD lifters can't fail by going taller than all the way up, they only fail by going shorter. Even if the internals broke and the plunger were free to go up beyond the normal height it would only be pushed there by oil pressure, and that isn't enough force to open a valve off the seat. Oil pressure is only enough to take out the lash.

    When you removed the rocker arms did the preload seem normal, or similar on all of them? Can you collapse the suspect lifter by pushing on the pushrod seat? Did any parts fall out the top of the lifter when you removed them? Did you absolutely 100% verify the DOD lifters were in the correct cylinders (a non-DOD lifter on one of the DOD lobes will open the valve waaaay early and cause a low speed misfire)?

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    Quote Originally Posted by 5FDP View Post
    Getting the rest of the DoD delete parts is still required though even with that LQ9 camshaft.
    I'm aware.

    Quote Originally Posted by blindsquirrel View Post
    The DOD lifters can't fail by going taller than all the way up, they only fail by going shorter. Even if the internals broke and the plunger were free to go up beyond the normal height it would only be pushed there by oil pressure, and that isn't enough force to open a valve off the seat. Oil pressure is only enough to take out the lash.

    When you removed the rocker arms did the preload seem normal, or similar on all of them? Can you collapse the suspect lifter by pushing on the pushrod seat? Did any parts fall out the top of the lifter when you removed them? Did you absolutely 100% verify the DOD lifters were in the correct cylinders (a non-DOD lifter on one of the DOD lobes will open the valve waaaay early and cause a low speed misfire)?
    1. Yes, the preload was normal on all the rocker arms ASIDE from the #4 (offending) cylinder. On TDC for that cylinder the valves stayed open. I checked 4 different times to ensure it was on TDC.
    2. No, I am unable to collapse the lifters.
    3. No, nothing fell out of the lifter.
    4. Yes, the DOD lifters were placed in the correct cylinders.

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    Is that one lifter just locked solid all the way at the top of the plunger's travel? That's all I could see causing it other than somebody putting in wrong parts (like from a different application).

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    Quote Originally Posted by blindsquirrel View Post
    Is that one lifter just locked solid all the way at the top of the plunger's travel? That's all I could see causing it other than somebody putting in wrong parts (like from a different application).
    I spoke with my machinist about it. He said it was very common to have locked lifters due to weird oil pressure.
    That seems to be the case here.
    Though I should mention those particular lifters are not the same.

    Though I've found more trouble. Got one smashed Non-DOD lifter and a wiped cam lobe.

    I'll upload pictures later

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    Same thing happened to me. Get the cam out and then try to get the lifter out from the bottom...
    Daily Driver= 2003 BMW 330xi
    Weekend Cruiser= 2009 Pontiac G8 GT (Vararam, TSP LS3 N/A Stage 1, OBX, CTS-V converter, MagnaFlows w/ J-Pipes, 160 t-stat)
    Project Car= 1991 Chevrolet Camaro RS (LQ4 w/ Gen 4 Rods, LS3 heads, turbo...)
    Truck= 2007 Chevy Silverado 1500 LT LY5 4x4

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    Hey everyone! Sorry it's been so long getting back to you guys.
    I finally got the car all torn apart and reassembled.
    Did a full DoD delete with a tune, and the car runs flawlessly.
    The issue ended up being the lifters for the #4 cylinder. They were locked in to the "Up" position and would not collapse, keeping the valves open constantly.

    I appreciate the help, couldnt be happier with the car.