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Mercruiser 3.0 uneven compression

L

Leland Morris

New member
I just picked up a 1997 Monterey 180 with a Mercruiser 3.0 in it. The motor only has 154 hours on it and seems to run well, maybe a little rough at idle. I decided to do a compression test and got #1-120psi #2-100psi #3-150psi #4-150psi. Could this be a blown head gasket? Cylinder 2 did seem to have a little rust in it I’m going to try soaking the cylinders in penetrating fluid. Just hoping to use the boat soon and not have to spend too much money on it.
 
I would think a blown head gasket would be much lower than these numbers. #2 is basically close to a dead cylinder . You can pressure test the cooling system to confirm.

bottom line if it runs use and see if the compression improves. Was it sitting a long time? 3.0 is never the smoothest running engine.
 
I soaked cylinders one and two for about 24 hours with marvel mystery oil and the ran the engine. The compression readings are now #1-150 #2-120 #150 #4-150. I’m guessing the rings were stuck. Still hoping to get more out of cylinder 2.
 
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