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mhook
09-16-2009, 03:33 PM
My boat is a '95 Maristar, 5.7 indmar, 240 hours. Twice in the past five years the spark plug in the same cylinder came apart. The insulator came completely out of the metal body. There was no damage to the electrode or the tip of the plug. The boat runs great other than this. Could the cylinder be running hotter than the others and cause this to happen? I have no idea where to start.

EricB
09-16-2009, 03:52 PM
Pictures please. That will help with diagnosis. What cylinder? Is the plug wire properly retained or have excess cable hanging and creating a weighted vibration?

Pictures will tell.

Hoosier Bob
09-16-2009, 04:11 PM
Is it hard to reach? They are easier to crack than you think. Bad angle and you may be cracking the insulator. :confused:

mhook
09-16-2009, 05:47 PM
Sorry no pictures. I tossed the two pieces out. It's occurring on the #4 cylinder. The plug wire is ok and access isn't an issue. The two sections of the plug were in perfect condition. The base of the plug was still screwed tight into the plug hole. A friend suggested that the cylinder may be running hotter than the others and causing the plug to heat up and separate. Maybe I'll just keep a spare plug in the glove box!

TMCNo1
09-16-2009, 08:24 PM
Sorry no pictures. I tossed the two pieces out. It's occurring on the #4 cylinder. The plug wire is ok and access isn't an issue. The two sections of the plug were in perfect condition. The base of the plug was still screwed tight into the plug hole. A friend suggested that the cylinder may be running hotter than the others and causing the plug to heat up and separate. Maybe I'll just keep a spare plug in the glove box!


If your changing your own plugs, be sure you are using a deep well spark plug socket of the right size for the plug that has the rubber sleeve inside to keep the plug lined up in the socket or you can easily crack the porcelain while installing if a plug gets crooked in a regular deep well socket.

1redTA
09-16-2009, 09:07 PM
I caught myself starting some plugs with the socket versus a few threads by hand and messing up the insulation, I can only guess the angle of the socket broke the porcelin.

JimN
09-16-2009, 09:13 PM
My boat is a '95 Maristar, 5.7 indmar, 240 hours. Twice in the past five years the spark plug in the same cylinder came apart. The insulator came completely out of the metal body. There was no damage to the electrode or the tip of the plug. The boat runs great other than this. Could the cylinder be running hotter than the others and cause this to happen? I have no idea where to start.

When you say it's the #4 cylinder, is that #4 on one bank or #4 in the sense that you started at the farthest forward and alternated sides when you counted? The reason I ask is that A) people who are familiar with Ford motors and not GM will often count along one side in sequence and B) the #4 cylinder isn't usually a problem but the #8 can be if the PCV valve hose is attached to the runner for that cylinder instead of the base of the intake manifold. If the plug is in the cylinder that has the PCV hose, it tends to run lean, which also means it runs hotter in that one than the rest.

mhook
09-16-2009, 09:58 PM
Facing the front of the engine, it's the 2nd plug on the passenger side. I thought that was firing order #4, but maybe not.

Hoosier Bob
09-16-2009, 10:49 PM
I have a Ford but I thought Chevy was 1 on passenger side, 2 Driver, 3 pass and 4 driver? Are boats bass ackwords?:confused:

JimN
09-16-2009, 11:59 PM
Facing the front of the engine, it's the 2nd plug on the passenger side. I thought that was firing order #4, but maybe not.

You got the cylinder number right for a GM. I just didn't know if you're a Ford guy and I have seen people designate the rear on one side as #4, the same way Ford does. It's cylinder #4, but not #4 in the firing order.

JLeuck64
09-18-2009, 01:32 AM
Curious....
Did your spark plug look like this one?

mhook
09-18-2009, 09:52 AM
Well, kind of like the last picture minus the rust! The plug looked to be in perfect condtion. Just in two pieces.