Broken spark plug

Mknupp

Mknupp

Member
Sep 25, 2019
73
finger lakes
Boat Info
1995 330 Sundancer
Engines
7.4 454 v drives
5.7 mercruiser alpha drive. Spark plug on one cylinder blows off porcelain after running above 2500 rpm.. compression good , heads have been redone, new carb and fuel pump, timing is spot on....no clue as to why this happens, also no water in the suspect cylinder, temp reads normal... only thing that hasn’t been done is manifolds and risers, however temps are normal and no water ingestion evident...completely stumped on the cause?...
 
Is this a one time occurrence or has it happened several times on the same cylinder?
 
Happens on same cylinder only when running above 2500 rpms, heads were checked and redone by a reliable machine shop, compression did not appear to be higher than specs
 
So the steel threads of the plug remain intact but the porcelain blows out through the middle? Correct?

Double check the firing order and routing of the spark plug wires.

Maybe a sticking exhaust valve?

Can you remove the valve cover to inspect the valve train?
Maybe the camshaft lobe for the exhaust valve on that cylinder has flattened?

Is there any popping through the intake before the plug blows out?

Have you done a compression test recently?
Although I’m not sure that would prove anything.
 
yes steel threads remain in head, just porcelain blows out, firing order has been rechecked, never heard any popping before it happened however possibly it did just couldn't hear it. cam shaft ? I see were your going with this. Guess pulling valve cover and putting on some oil deflectors on rockers then starting it up to see what looks strange will be next step.. thanks for the help... yes compression numbers are all within spec
 
Does it have the flat top style distributor cap? A cross fire in them is not uncommon. Have you tried replacing it?
 
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