Misfires on cylinder 6 - but only at idle

Discussion in '360/430' started by sanfran, Jan 13, 2018.

  1. sanfran

    sanfran Formula Junior

    Apr 10, 2004
    347
    LA
    Full Name:
    Spider
    Hi,

    Misfires are noticeable and loud. At idle you can hear popping in the intake plenum. It disappears above 2,000 RPM.

    I have changed coil packs and spark plugs around. 6 to 7. 7 to 6. Still misfire on 6.

    Both MAFs, all coil packs and plugs are new.

    I have sprayed brake cleaner around cylinder 6. No change.

    Will a faulty fuel injector lead to this behavior?

    I have cleaned all injectors with Electronic Parts cleaner and test sprayed them with a 9V battery. Spray pattern on all of them looked identical. No dripping.

    When driving there are no misfires above 2,000 RPM. Car drives great. Sometimes it dies at stop signs. I have cleaned 6 ground points so far. Have not cleaned grounds on exhaust/cat ECUs since I do not have a slow down light.

    What am I missing?
     
  2. tazandjan

    tazandjan Two Time F1 World Champ
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    Jul 19, 2008
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    Do a compression check and make sure you do not have a burned valve.
     
  3. WOFERRARI

    WOFERRARI Formula Junior
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    Sep 26, 2015
    800
    Sarasota Fl
    curious 360 or 430 or did i not see it stated
     
  4. sanfran

    sanfran Formula Junior

    Apr 10, 2004
    347
    LA
    Full Name:
    Spider
    It's a 360.

    Compression on all cylinders is even. Around 220.
     
  5. sanfran

    sanfran Formula Junior

    Apr 10, 2004
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    Don't you need a leak down test to see if valves are 100% sealing? I don't have the set up for that.
     
  6. mello

    mello Formula 3
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    Jul 12, 2013
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    Swapping the fuel injector is another test that you can do.
     
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  8. sanfran

    sanfran Formula Junior

    Apr 10, 2004
    347
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    I reused the old O-Rings. Maybe I should use new ones once I have the fuel rail out again. Maybe the injector is not sealing right.

    Keep it coming guys....
     
  9. slracing

    slracing Formula Junior
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    Jan 13, 2011
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    Europe
    Even temprature in both cats??
     
  10. 24000rpm

    24000rpm Formula 3

    Aug 31, 2012
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    Asia and the U.S.
    great insight, can you elaborate a little?

     
  11. RedNeck

    RedNeck Formula 3
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    Jul 8, 2016
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    I wouldn't discount intake manifold gaskets ether...common issue on our cars. I've been chasing down an intermittent CEL and will be replacing all of my plugs and coils soon, while I am working on my exhaust. Debating on pulling the intake and doing all the gaskets and heat exchanger while I'm at it.
     
  12. mello

    mello Formula 3
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    The leak-down test suggestion is good to eliminate any possible mechanical problems. The #6 spark plug would be my next guess.
     
  13. sanfran

    sanfran Formula Junior

    Apr 10, 2004
    347
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    I swapped fuel injectors from 5 to 6 and 6 to 5.

    Replaced cyl 6 spark plug with a new one.

    Same misfire code on cyl 6.
     
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  15. Michael B

    Michael B F1 Rookie
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  16. sanfran

    sanfran Formula Junior

    Apr 10, 2004
    347
    LA
    Full Name:
    Spider
    Did you have misfires and hear popping in the intake plenum at low RPM with these gaskets?

    I don't understand why I only have misfires at idle and very low RPM.
     
  17. vrsurgeon

    vrsurgeon F1 World Champ
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    Dec 13, 2009
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    This IMHO.

    You only notice them at idle.
     
  18. ar4me

    ar4me F1 Rookie
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    Apr 4, 2010
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    I would guess that the misfires, in fact, ONLY occur at idle: false air entering, un-metered, through the faulty gasket is significant at idle (causing the misfire) while at higher RPM it is an insignificant amount of extra air relative to the air through the open throttle plates - insufficient to cause a misfire.
     
  19. sanfran

    sanfran Formula Junior

    Apr 10, 2004
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    Only at idle.

    At 2,000 RPM everything is fine.

    The fact that the popping sound resonates in the red intake plenum would point to an intake valve not closing at top dead center.

    I took the plenum off and put a camera straight down to the three intake valves of cylinder 6. The valves looked good. Just like the intake valves on the other cylinders.

    Can it be that there is a tiny foreign object between one intake valve and the cylinder head preventing it from closing correctly?

    Would that not have been blown into the cylinder and out the exhaust valve at 8,000 RPM?

    I don't know much about valve seals, valve guides, and wear in this area.

    The fact that no misfires are audible above 2,000 RPM might still lead to an ignition problem which only occurs at low RPM. It is not the coil pack or the spark plug.

    What other parts of the ignition system can cause this RPM dependent misfire? It can't be wiring between the ECU and the coil pack, can it?
     
  20. ar4me

    ar4me F1 Rookie
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    I think that Michael and Curt may still be correct - as I said above, the false air entering through leaking intake gasket is insignificant at higher RPM, but not at idle = misfire at idle but not at higher RPM.
     
  21. sanfran

    sanfran Formula Junior

    Apr 10, 2004
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    I'll take a look at the MAF readings. If we are talking about a factor of 10x (I think the units are in LB/h) from idle to 3,000 RPM that analysis makes a lot of sense.

    So it would not be an ignition problem after all -- but most likely the gasket at the bottom of cylinder 6 leaking and pulling false air.
     
  22. ar4me

    ar4me F1 Rookie
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    Not sure exactly what you mean, but I think the suspect is the intake gasket for cyl 6.
     
  23. sanfran

    sanfran Formula Junior

    Apr 10, 2004
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    OK then. I'll order upper and lower intake gaskets.
    This is a ***** of a job. Is it easier on a Spider if you take the frame out of the engine compartment for better access? I can barely see the bolts for the intake stacks. Can this be done in one day?
     
  24. ar4me

    ar4me F1 Rookie
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    Well, keep in mind, that internet diagnosis is not very accurate :) Try do a search and see if anyone has listed how long it took them to pull and put back the intake. On the surface it seems doable in one day, but sometime the devil is in the details...
     
  25. vrsurgeon

    vrsurgeon F1 World Champ
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    Ar4me.. my experience is that the speed of the engine "masks" the misfire. It's still occurring.. but the higher speeds of the engine make it harder to notice. In the case of a broken gasket and unmetered air, it'll lean out the condition for that cylinder. Just my observations.

    OP, I would take carb cleaner and spray around the suspected intake gasket at idle and see if you notice a difference at idle before you order the gasket. When I had a misfire at idle it turned out to be a coil... IMHO by far the most common cause, but you said you've replaced the coil. I redid my gaskets needlessly apparently. :) But now I know they were done 3 years ago. :)
     
  26. 24000rpm

    24000rpm Formula 3

    Aug 31, 2012
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    so, your problem , I guess is the same problem we have encountered, only that we have multiple misfiring cylinders.

    I myself have done much more comprehensive tests than plug, injector, MAF. I even replaced all intake gaskets, still the same thing came on.

    A few days ago a member asked me in private if I resolved my problem, I said no and i now determined it a mechanical, rather than electrical issue

    as above, try spray carb cleaner onto the intake gasket area, then untilmately, leak down and compression are the way to go. I am just too busy to do it.

     
  27. sanfran

    sanfran Formula Junior

    Apr 10, 2004
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    I get no codes when keeping idle above 1,500 RPM.
     

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