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Thread: l29 misfires vortec black box

  1. #1
    Fuel Injected! ralmo94's Avatar
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    l29 misfires vortec black box

    I have a 1998 k2500 7.4 vortec 454 4l80e 3.73.

    A little background,

    I bought the truck with the typical hot start issue.
    Got worse after I got the truck.
    Replaced injectors and FPR.

    That was 2019.

    Thought for a while I had a bad head gasket,
    but turned out to be a bad intake manifold gasket.
    Finally got around to changing it, in 2020.

    First startup and it knocked. Bad.
    Turns out, something had been screwed
    to the firewall at some point in time. Well there was a
    self drilling screw that landed in number 3 cylinder.
    Replaced num 3 piston and rings. What a job. Lol

    New Distributor, ceramic boot plug wires, Blu streak cap and rotor.
    New NGK plugs. Deleted EGR, and cats. Exhaust is true dual 2.5, with
    glass packs in cat locations, flowmasters after that with rear exit.

    I have a missfire at idle.

    No codes. But random missfire code is set to no error reported.
    Thought maybe my truck had been tuned before I got it,
    But reviewing other stock files from HPT respository with the same OS
    Also had no error reported.

    I seen once a YouTube video of a guy that had
    replaced his injectors and had misfires,
    And his injectors were not seated all the way in the hole.

    Has anyone else ever ran across this before?

    Missfire, missfire code, or anything else helpful?

    I know these trucks seem to be like a unicorn, not a lot of info on them with OBD2 454.

    Thanks

  2. #2
    Super Moderator dave w's Avatar
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    Often an infrared thermometer ( https://www.homedepot.com/p/Milwauke...s&locale=en-US ) is helpful to measure the exhaust manifold temperatures. Measure the temperate of each exhaust port close the cylinder head. A cylinder that is misfiring will have a slightly lower temperature. Perhaps identifying the cylinder that is misfiring is the next step.

    Perhaps an injector connector or injector wire is causing the misfire.

    dave w

  3. #3
    Fuel Injected! ralmo94's Avatar
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    Quote Originally Posted by dave w View Post
    Often an infrared thermometer ( https://www.homedepot.com/p/Milwauke...s&locale=en-US ) is helpful to measure the exhaust manifold temperatures. Measure the temperate of each exhaust port close the cylinder head. A cylinder that is misfiring will have a slightly lower temperature. Perhaps identifying the cylinder that is misfiring is the next step.

    Perhaps an injector connector or injector wire is causing the misfire.

    dave w
    Seems like I remember trying that before without anything conclusive.
    How much of a difference are we talking, a hundred degrees, or less?
    I'll check again as soon as I get a chance, and record the temps on a paper
    And post them here.

    I don't think I mentioned before, it's not like a constant missfire
    And doesn't seem to be noticable above 1k rpm, but when noticed,
    I can see wideband go rich, then stft seems to lean it out for a second.

    Idle AFR seems to be 14.6, 14.7 when smooth,when missfiring, goes down to 13.9
    then up as high as 16 before settling back out to 14.6, 14.7

    Also I have new denso narrow band sensors and a 180 thermostat with an ⅛" hole drilled in it.

    Thanks

  4. #4
    Fuel Injected! ralmo94's Avatar
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    Temps are
    390. 7
    485. 5
    475. 3
    400. 1


    388. 8
    417. 6
    450. 4
    450. 2

  5. #5
    Super Moderator dave w's Avatar
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    Quote Originally Posted by ralmo94 View Post
    Temps are
    390. 7
    485. 5
    475. 3
    400. 1


    388. 8
    417. 6
    450. 4
    450. 2
    I agree, inconclusive data. What are the exhaust manifold temperatures at a steady 1500 RPM's?

    Fuel / Air / Spark are the basics for an engine to run.
    MAF Grams per second flow at idle vs. 1500 RPM's?
    Drive a steady speed for a few miles, then shut off the engine and coast to a stop and look at the spark plugs?
    Flow Rate of each injector before and after being cleaned?

  6. #6
    Fuel Injected! ralmo94's Avatar
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    to keep idle at 1500rpm and
    check temps and record maf flow, and compare to idle, all
    Tables seem to be lb per hour this generation.
    I haven't cleaned injectors, or flow tested them.
    After I got the truck in 2019, I replaced the multecs with brand new
    Bosh injectors. Supposed to be stock replacements from a Ford application
    For the engine cut plug review, I don't know how feasible that is at this time
    I am in a city right now with a 18 ft box trailer in tow.

    I am wondering if I should pull the plenum and inspect the injectors. I'd rather not as
    It can be hard to find a place on the road to do it. Does it sound like it could be a
    fuel caused missfire? Only thing not new in ignition system is module and coil.

    Incase nobody has worked on one of these, the l29 plenum covers the injectors and harness.
    When I had it apart last was when I was replacing the number 3 piston.
    The plastic fuel rail is what holds the injectors in the hole of the lower intake.

  7. #7
    Super Moderator dave w's Avatar
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    Typically injector failures are constant, meaning once the spray pattern is degraded the spray pattern will remain degraded. Possibly a "Cure-in-a-Can" fix will help improve an injector with a degraded spray pattern? Typically an injector with a degraded spray pattern can be spotted with the exhaust manifold infrared temperature test.

    Maybe try a cylinder balancing test, to monitor how each cylinder effects vacuum and MAF readings. Turning off a cylinder with good ignition and good fuel flow will affect the baseline vacuum and MAF readings. If the cylinder balancing test reveals that all 8 cylinders have consistent vacuum and MAF readings then perhaps the random misfire is ignition related? Perhaps a stretched / worn timing chain is affecting the Cam Position Sensor?

    dave w

  8. #8
    Fuel Injected! ralmo94's Avatar
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    Did a Cylinder balance

    Open loop locked IAC at 30 counts and disabled injectors one at a time.

    running on all 8 about 980 rpm 30kpa

    920 32kpa 930 32kpa

    920 33kpa 925 32kpa

    925 32kpa 925 32kpa

    920 32kpa 920 32kpa

    MAF was the same the whole time at about 1.41 lb per min.

    Seems to me to be inconclusive.

    Again, the misfire is only noticeable on idle.

  9. #9
    Fuel Injected! ralmo94's Avatar
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    Quote Originally Posted by dave w View Post
    Typically injector failures are constant, meaning once the spray pattern is degraded the spray pattern will remain degraded. Possibly a "Cure-in-a-Can" fix will help improve an injector with a degraded spray pattern? Typically an injector with a degraded spray pattern can be spotted with the exhaust manifold infrared temperature test.

    Maybe try a cylinder balancing test, to monitor how each cylinder effects vacuum and MAF readings. Turning off a cylinder with good ignition and good fuel flow will affect the baseline vacuum and MAF readings. If the cylinder balancing test reveals that all 8 cylinders have consistent vacuum and MAF readings then perhaps the random misfire is ignition related? Perhaps a stretched / worn timing chain is affecting the Cam Position Sensor?

    dave w
    Been thinking about what you said.

    If an injector is not seated in the hole all the way, would that also be a steady missfire?

    When I installed the new distributor I set the offset to 0.25 deg.
    Could a stretched timing chain still be a culprit?
    If so, would a crank relearn cure it?

  10. #10
    Super Moderator dave w's Avatar
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    I agree, the cylinder balancing test is not conclusive . . . but maybe inconclusive means fuel / air / spark are good. Looking at mechanical possibilities, the timing chain connects the crank to the cam. Typically a loose or worn timing chain is likely on a high mileage engine. Seemingly a loose chain would be problematic at idle and less problematic at higher RPM's. Crank learn requires higher than idle RPM's.

    dave w

  11. #11
    Fuel Injected! ralmo94's Avatar
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    Just so I understand, if it is missing from a stretched timing chain, it
    Isbecause of actual valve timing, and not injector timing? If so,
    I guess I will just have to live with it until I get a chance to tear it down.
    Also, am I correct in assuming from the cylinder balance test,
    That it probably is not missing from leaky valves?
    Thanks

  12. #12
    Super Moderator dave w's Avatar
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    Cylinder balancing test indicates good cylinder combustion, all eight cylinders.


    Engine basics:
    Crankshaft connected to the camshaft with timing chain.
    Distributor connected to the camshaft.
    Camshaft position sensor is connected to the distributor.

    Timing chain worn or loose?
    Distributor gear worn?
    Distributor shat bussing worn?

    Maybe the misfire is a tolerance stack of worn parts?

    dave w

  13. #13
    Fuel Injected! ralmo94's Avatar
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    I know when I had the old distributor out, the gear was plenty worn.
    When I set the distributor I got the offset within 1/4 of a degree, the tolerance
    is supposed to be 2 deg. Am I correct in thinking this would deem a crank relearn
    Unneeded? And that the cam position sensor should be accurate?
    Are you thinking it might be a symptom of cam sensor position being faulty,
    Thus causing the injectors to fire at the wrong time? If so, would it be worth messing
    with the EOIT for injection timing?

  14. #14
    Super Moderator dave w's Avatar
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    A crank learn is something worth trying. I don't know if a crank learn will or will not fix the misfiring, but seems like an affordable troubleshooting option. Sometimes trying stuff will lead to a solution.

    I'm left wondering, what are worn out parts and what are good parts?

    dave w

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    You could do some testing. Does this happen in open loop, closed loop, or both? If you force the mixture rich by decreasing the O2 signal voltage slightly does the problem go away? If so, is it possible the O2 sensor signal is biased? Have you tried manually turning the crank back and forth while watching the distributor rotor for motion to estimate the number of degrees of travel allowed by the chain? If you've got less than 7-8 degrees of travel it's not likely you have a misfire due to a chain problem. Are the valves in cylinder 3 ok? Exhaust being forced into the intake might cause some misfire issues at idle. Is the engine equipped with the OE temperature thermostat? Front cylinders exhaust temperature readings look low. It's possible that cool cylinder temperatures are affecting combustion.

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