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Oil in one cylinder

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Kilo7Mike

New member
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:

IMG_6063.jpg


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!
 
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
 
fastjeff said:
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.
Click to expand...

Well, these are new-ish wires (installed in 2016)... how long do wires last?

So the compression test - what will this tell me?
 
Kilo7Mike said:
Well, these are new-ish wires (installed in 2016)... how long do wires last?

So the compression test - what will this tell me?
Click to expand...
Did you use marine grade spark plug wires or generic automotive wires? There is a difference, see here: http://www.marineengine.com/boat-forum/showthread.php?432102-Spark-plug-wires-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
 
kevinj said:
Did you use marine grade spark plug wires or generic automotive wires? There is a difference, see here: http://www.marineengine.com/boat-forum/showthread.php?432102-Spark-plug-wires-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.
Click to expand...

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.
 
Kilo7Mike said:
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.
Click to expand...
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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