Intermittant miss
Thread Starter
Junior Member
Joined: Sep 2009
Posts: 12
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From: Houston, Texas
Intermittant miss
Can't figure this out. Supercharged 5.4 that loses cylinder #7 when decelerating. Cylinder will "clean up" and run again shortly after event. Changed injector, plugs, coils and connectors for coil and all makes the truck run better for a while but cylinder loss continues. I have boroscoped cylinder and all looks fine. 54K on engine and well maintained. Don't get it. This all came about after extremely cold night when I stepped on motor and had high boost and broke a plug. Ever since then I can't find problem to fix it permanently. Code thrown from computer is 354 which is for coil "D" misfire. Tried swapping coils.....no change. Help please
Moderator (Ret.)
Joined: Sep 2008
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From: Lake Mary Florida
Do a compression test on that cylinder, and from a few others. I suspect either oil is fouling the plug from the oil ring not seating, or from the valve guide. Boroscoping the cylinder will not show the conditions of the rings seating against the cylinder wall, or the condition of the valve guide seal.
A dry then wet (squirting some oil into the plug hole then testing again) compression test will rule this possibility out, or prove it is the culprit.
If a dry test shows low compression, do the wet test. If compression increases, either the rings are bad, or the valve guide seal is bad, allowing oil to enter the cylinder and oil fouling the plug.
A dry then wet (squirting some oil into the plug hole then testing again) compression test will rule this possibility out, or prove it is the culprit.
If a dry test shows low compression, do the wet test. If compression increases, either the rings are bad, or the valve guide seal is bad, allowing oil to enter the cylinder and oil fouling the plug.
Thread Starter
Junior Member
Joined: Sep 2009
Posts: 12
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From: Houston, Texas
Did a compression test and cylinder leak down, all is well. Did not suspect that to be the problem since the truck runs excellent until cylinder drops. Boroscope still shows cross hatching in cylinder bores, truck really is in good shape. Cannot understand why it is so intermittent. Will try recording some data on the drive home. Just wondering if anyone has ever run into a similar problem. Thanks
Thread Starter
Junior Member
Joined: Sep 2009
Posts: 12
Likes: 0
From: Houston, Texas
Found the problem. Coil "D" is for cylinder #4 not #7 plug had incorrect gap probably due to heavy knock under high boost. Kept checking everything on cylinder #7 because original code thrown was for that cylinder. Replaced plug and truck runs great.
