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Thread: Misfire issue on stock 4.8 with external FP setup.

  1. #1
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    Misfire issue on stock 4.8 with external FP setup.

    Alright, I have been fighting random misfire issues for the last 1.5yr since I got this truck. So I replaced the entire fuel system that was in the truck to rule out any issues. System is now setup in the following manner:

    20g Summit fuel cell
    100 micron pre filter
    DW350iL pump
    10 micron post filter
    Aeromotive FPR 12" after the above post filter mounted on bed frame (Vacuum reference hooked up from intake manifold)
    -8an return to cell from FPR at bed rail
    -8an feed to billet rails
    Stock injectors
    E-fans programmed

    Everything else on the truck is stock, down to the air filter. Below is a scan after I installed the new FP, which fixed my previous fuel pressure issues. It is now dialed in at 58psi idle. Then here is a scan after I took the truck for a 1mile drive down the road. This was a slow cruise, I did not beat on it at all in that mile. I am lost as to what would basically net 2 dead cylinders after a mile drive. Truck ran great until I took it for a short drive, now its acting like it doesnt want to run... I was charging my laptop when I took the truck on maiden voyage, so I do not have a log of the drive


    2003 Sauce Truck stock e-fan 122116.hpt
    03 Sierra 01082017 after drive.hpl
    03 Sierra 01062017 new FP.hpl

  2. #2
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    I will add, I have done plugs, wires, coil swaps, compression test on all cylinders, PCM swapped, Racetronix FP hotwire, pretty much anything I could think of to try to rectify the issue. Just makes no sense that it ran good in the driveway yesterday, then I took it for a short trip, and BAM, misfires and dead cylinder again...

  3. #3
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    What do the plugs look like? What's oil pressure? Any engine noise? What did the injectors look like when you were swapping things around? Did you try knocking them on a hard surface to see if they had any trash in the top of them?

    It almost looks like an injector or lifter issue from the log with cylinder 1 being the constant issue...
    2010 Vette Stock Bottom LS3 - LS2 APS Twin Turbo Kit, Trick Flow Heads and Custom Cam - 12psi - 714rwhp and 820rwtq / 100hp Nitrous Shot starting at 3000 rpms - 948rwhp and 1044rwtq still on 93
    2011 Vette Cam Only Internal Mod in stock LS3 -- YSI @ 18psi - 811rwhp on 93 / 926rwhp on E60 & 1008rwhp with a 50 shot of nitrous all through a 6L80

    ~Greg Huggins~
    Remote Tuning Available at gh[email protected]
    Mobile Tuning Available for North Georgia and WNC

  4. #4
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    Plugs looked good when I pulled them, didnt pull them today after this log though. I can grab another scan log and look at Cyl 1 maybe tomorrow evening though.

    Oil pressure is ~40psi at cold start, slightly under 30psi warm. Admittedly needs an oil change badly, as I havent seen the truck since April. It is on the to-do list this week.

    Stock injectors looked fine, orings were clean. I have 60# injectors just sitting in a box waiting, but I didn't want to compound issues, and i don't have the exact tables to get them dialed in.

    Engine is as smooth sounding as off the factory line, zero engine/lifter noise. That was my first though, given it has 150k on the clock.

  5. #5
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    Got freed up for a min to grab another scan log. You'll notice in the middle of the scan it gets stuck at 10 on Cly 1, and nothing was reported for a minute for misfires. I could notably tell it was still missing pretty bad, but just wasnt reporting for some reason. This scan has me even more confused. Until the pump went out, the truck was driving perfectly fine according to my mechanic up in IN. Pump was apparently on its last leg and gave up once I started backing it off the trailer when it arrived to TX... I am lost

    03 Sierra 01082017 after drive(2).hpl

  6. #6
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    If you know how - do a test when it's misfiring on the ignition system with a plug load tester of some sort - same for fuel injector - that's if you have a noid light or something to install... If spark is present and good and the noid light is flashing - pull the valve cover off of bank 1 (you can pull one coil in the front on the driver's side to get the bolts loose to remove the cover) and fire it up and let it idle while it's still showing misfires... You might find the lifter coming and going... If valves are honestly opening and closing like they're supposed to when misfires are showing present then it's either seriously carboned up or you have injector issues... That's if all of your electrical checks out... If it didn't start investigating grounds since that's the only thing I can think of effecting multiple cylinders like this...

    You've already checked compression and I assume it's the same or at least close cylinder to cylinder... Oil pressure is a little low... You might but I really doubt it will cure your problem - even though it sounds like it needs to be done anyway - but you might be able to give the crankcase a good flush and see if things improve any at all since you have an oil change coming up...
    Last edited by GHuggins; 01-08-2017 at 06:06 PM.
    2010 Vette Stock Bottom LS3 - LS2 APS Twin Turbo Kit, Trick Flow Heads and Custom Cam - 12psi - 714rwhp and 820rwtq / 100hp Nitrous Shot starting at 3000 rpms - 948rwhp and 1044rwtq still on 93
    2011 Vette Cam Only Internal Mod in stock LS3 -- YSI @ 18psi - 811rwhp on 93 / 926rwhp on E60 & 1008rwhp with a 50 shot of nitrous all through a 6L80

    ~Greg Huggins~
    Remote Tuning Available at gh[email protected]
    Mobile Tuning Available for North Georgia and WNC

  7. #7
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    My mechanic back in IN checked out all of the ignition, down to the signals coming from the PCM. I have an extra set of coil packs as well, that I have swapped in and out to diagnose. I will have to look more into pulling a valve cover, as my gameplan after the maiden voyage was supposed to be the Triple 12 cam install I have waiting. Seems like its always something.

    I did install a new batter when the truck arrived, as something is drawing current while its off. I pulled the radio fuse under hood, and havent had an issue with it starting since. Likely need to remove the radio and see whats up. I did notice that with the new e-fans, my voltages drops down to around 13.5 when they come on. Thinking I need a bigger alt, but that shouldnt be related to this. I can start checking grounds, didnt really think to look at them...

  8. #8
    Tuner in Training boostbuick's Avatar
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    Another problem u all have over looked is the maf sensor is it dirty and did u cleen it cause i know they can and will cause misfires also but only i stress this!!!!!! Only clean the fass air flow sensor with fass air flow sensor cleener only!!!!!!! do not use anything else but this because it can damage the resistance and cause a differnt out put signal to the computer causeing a lean or rich condition and if you rollin like i do over 100 mph yah dont want the dam thing lean on yah and go boom lol ok hope that helps ttyl boostbuick

  9. #9
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    Well, fuel pressure dialed back to 58psi at key on, instead of idle. Oil change done as well for good measure. No change, still runs like a slug.

  10. #10
    Tuner in Training boostbuick's Avatar
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    maby yah got to torque management too high and it will linit the power output can u post your tune plz

  11. #11
    Tuner in Training boostbuick's Avatar
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    ok I see your tps sensor dosnt have 5 volt ref signal and ure misfire is number one cylinder big time

  12. #12
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    Tune is posted in the first post.

    Check the second scan, its all cylinders in that one.

  13. #13
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    Have you tried a running compression test when the misfire is most noticeable?

  14. #14
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    I ran compression tests and everything checked out within 20psi. If you check the 2nd scan, you will notice that nearly ALL cylinders started misfiring. My assumption is that some sensor or something is having a fit or went bad... Just no idea which. 02, maf, crank, etc.

  15. #15
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    Couple quick notes:

    Cylinder 7 is the only cylinder that hasnt misfired through all of this. Cly 2 is a close second with only a couple. All other cylinders are basically misfiring at will.

    Noticed my ignition voltage was lower on my scan today, like dipping into the 12's. An hour after shutting the engine down, the Alt was still hot as hell. An hour after that, still hot. So the alt is coming out for an HO. Now that I have e-fans, probably better to upgrade it anyways.

    All that being said, the alt going out will not fix the misfiring issues. Pulled a couple plugs, just had a fuel smell and slightly wet, what I expected. No other signs of anything. Still at square 1. Going to pull the new fuel filters tomorrow if I have time to check them out.

  16. #16
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    How is that square 1? That's somewhat of a direction to go if you had raw fuel on the plugs when missing... When its missing - its not sparking the fuel - so either no spark or no air movement... Have you physically installed a spark checker and checked it when it was misfiring yet? Have you checked valve train when it was misfiring yet? Does it try to clear out if you rev it up while it's misfiring?
    2010 Vette Stock Bottom LS3 - LS2 APS Twin Turbo Kit, Trick Flow Heads and Custom Cam - 12psi - 714rwhp and 820rwtq / 100hp Nitrous Shot starting at 3000 rpms - 948rwhp and 1044rwtq still on 93
    2011 Vette Cam Only Internal Mod in stock LS3 -- YSI @ 18psi - 811rwhp on 93 / 926rwhp on E60 & 1008rwhp with a 50 shot of nitrous all through a 6L80

    ~Greg Huggins~
    Remote Tuning Available at gh[email protected]
    Mobile Tuning Available for North Georgia and WNC

  17. #17
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    When it is missing, it is commanding spark, but I have not physically hooked up a spark checker yet. As I do not have one at the moment, will have to purchase. The wetness was on the threads, not on the node of the plug (likely residual fuel and remnants of anti seize). I have not pulled a valve cover to check while misfiring, I want to finish evaluating the fuel system before I go into the engine. While I rev the engine, yes it tries to clear up and sounds better. When I hold a steady RPM, it breaks up. I have an entire extra set of coils I can start swapping out, I am just perplexed as to why 7 cylinders all decided to start misfiring at the SAME time.

    I likely will not run it much until I replace the Alt now, as I do not want to burn up a new FP from low voltage. I can instantly hear the pump bogg down when the voltage goes below 13.5.

  18. #18
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    Have you checked back pressure yet - could be common restriction between banks? Alternator can cause weird voltage back feed issues especially if there's much ac voltage on the line which can cause problems. O2's can cause bank issues, but wouldn't think both...
    2010 Vette Stock Bottom LS3 - LS2 APS Twin Turbo Kit, Trick Flow Heads and Custom Cam - 12psi - 714rwhp and 820rwtq / 100hp Nitrous Shot starting at 3000 rpms - 948rwhp and 1044rwtq still on 93
    2011 Vette Cam Only Internal Mod in stock LS3 -- YSI @ 18psi - 811rwhp on 93 / 926rwhp on E60 & 1008rwhp with a 50 shot of nitrous all through a 6L80

    ~Greg Huggins~
    Remote Tuning Available at gh[email protected]
    Mobile Tuning Available for North Georgia and WNC

  19. #19
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    I have not checked backpressure, as it is basically a hacked off stock exhaust with a turn down. Not even cats if I recall correctly, I will have to jack it up and verify, since it is like 3" from the ground I never really checked.

    Alt is on the way, I do not have high hopes that it will fix my misfire issue at all, but it will at least fix the voltage drop, amp drop with e-fans, and the on fire alt after running 5min. Debating buying a 5.3 from LKQ with 6mo warranty and just swapping the engine out... This is 1.5yr of fighting at this point.

  20. #20
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    Well, said F it and ordered an LKQ 5.3 for the truck. Figured I can install everything on the stand, mount it to 80e, and drop it all in at once.

    I did install the new Alt, currently waiting for the battery to charge and going to fire it up to see if anything changed.