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  1. #1

    Question Oil in one cylinder

    I have a 1989 Sundancer 268 with a 7.4l GM big block V8 and a Bravo One drive. I put new plugs in last year and on the last trip of last year, my engine was running rough. It started out with me wondering if the timing was off, but it got progressively worse to the point where it was missing, shuddering and generally unhappy. I pulled the plugs when I got home and they all looked normal... except for cylinder #3 - completely fouled with oil. I changed the #3 plug and put it away for the winter.


    We got a late start this year, but I had it out for the first time yesterday and immediately noticed the same rough running. It was hesitating, surging and I was getting more vibration than normal. I decided not to stress things and stayed off plane and putted around. Got home and pulled the plugs out of cylinder #3 and #5. #5 looked fine and #3 was oily. Here is a picture of the two plugs, #3 is a new plug with only 5 hours of running:

    Click image for larger version. 

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    My googling says it could be the valve stem seal or maybe the rings. What do you guys thing I'm dealing with here? How does one diagnose this?


    Thanks for any help!

  2. #2
    Join Date
    Apr 2016
    Location
    barnegat NJ
    Posts
    3,143

    Default Re: Oil in one cylinder

    A compression test is in order

  3. #3
    Join Date
    Sep 2003
    Location
    New Tripoli, PA, USA!
    Posts
    15,692

    Default Re: Oil in one cylinder

    Right, that's the first thing to do.

    Don't automatically assume that's oil on the plug. If your ignition wire to that cylinder is defective, then the plug will simply foul.

    Jeff

  4. #4

    Default Re: Oil in one cylinder

    Quote Originally Posted by fastjeff View Post
    Right, that's the first thing to do.

    Don't automatically assume that's oil on the plug. If your ignition wire to that cylinder is defective, then the plug will simply foul.
    Well, these are new-ish wires (installed in 2016)... how long do wires last?

    So the compression test - what will this tell me?

  5. #5
    Join Date
    Nov 2005
    Location
    Mt. Zion, Illinois,
    Posts
    770

    Default Re: Oil in one cylinder

    Quote Originally Posted by Kilo7Mike View Post
    Well, these are new-ish wires (installed in 2016)... how long do wires last?

    So the compression test - what will this tell me?
    Did you use marine grade spark plug wires or generic automotive wires? There is a difference, see here: http://www.marineengine.com/boat-for...marine-vs-auto

    Good wires should last a while, at least 2 seasons. But, if you used generic wires it's possible you have corrosion at the terminals or internally that are creating resistance and killing spark to that cylinder.

    Compression test will help confirm if your bad plug is oil fouling or fuel fouling. If compression is low on that cylinder it could mean you have a failed oil ring on the piston. If compression is even across all cylinders then most likely your 'oily' plug is fuel fouling due to bad ignition source to that cylinder.

    KJ

  6. #6

    Default Re: Oil in one cylinder

    Quote Originally Posted by kevinj View Post
    Did you use marine grade spark plug wires or generic automotive wires? There is a difference, see here: http://www.marineengine.com/boat-for...marine-vs-auto

    Good wires should last a while, at least 2 seasons. But, if you used generic wires it's possible you have corrosion at the terminals or internally that are creating resistance and killing spark to that cylinder.

    Compression test will help confirm if your bad plug is oil fouling or fuel fouling. If compression is low on that cylinder it could mean you have a failed oil ring on the piston. If compression is even across all cylinders then most likely your 'oily' plug is fuel fouling due to bad ignition source to that cylinder.
    Oh, definitely marine wires - Sierra spark plug wires that are for the V8 7.4l with Thunderbolt ignition.

    Alright... compression test time. Thanks for the reply.

  7. #7
    Join Date
    Apr 2016
    Location
    barnegat NJ
    Posts
    3,143

    Default Re: Oil in one cylinder

    Keep us posted Kilo7Mike

  8. #8
    Join Date
    Nov 2005
    Location
    Mt. Zion, Illinois,
    Posts
    770

    Default Re: Oil in one cylinder

    Quote Originally Posted by Kilo7Mike View Post
    Oh, definitely marine wires - Sierra spark plug wires that are for the V8 7.4l with Thunderbolt ignition.

    Alright... compression test time. Thanks for the reply.
    just to cover bases you can swap the ignition wire between 2 cylinders, run the engine, and see if the fouled plug stays I the same cylinder or travels to the new cylinder. Oh, be certain to swap at both the plug and the distributor. Also yes I’ll want a new plug to replace the fouled one.

    If #3 is bad after the swap you may have a deep issue. If the problem follows the plug wire then you have a simple fix.

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