OMC 3.0 compression tests

cobra 3.0

Lieutenant Commander
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Jul 31, 2003
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My engine has always run well, but I was curious about testing the compression on it as it is a 1986. To my astonishment, cynlinder 1 and 2 were at 115...and 3 at 80 and 4 at 75! I read in my Clymer that when two side by side cylinders have lower compression readings, that it may be the head gasket.<br /><br />I still need to do the wet test by putting oil in the cylinders...<br /><br />My question is, do I need to remachine the head flat before installing a new head gasket?
 

Boatist

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Apr 22, 2002
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Re: OMC 3.0 compression tests

You can check it your self with a good square. Unless it is really bad head gaskets today do a pretty good job of sealing. If you pull the head I would check the vlaves. All your cylinders are low. I would expect 125 or more in all cylinders. If this is a saltwater engine and manifold is old cylinder 4 and 3 first to get saltwater and rust valves to point they will not seal.
 

cobra 3.0

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Re: OMC 3.0 compression tests

...aaahhhh! Boatist, I don't know the boat's history whether it was ever in salt water. I don't think it was in salt, but the manifold is old and probably the original.<br /><br />I reset my valves yesterday late in the day and the pressures did go up some...125, 135, 120, 90. I'm still off with these readings. Guess I need to pull the heads and take a look.<br /><br />I really hope that my problem isn't more than the head gasket and the valves. I don't want to start<br />with changing rings and rehoning cylinders...<br /><br />The "funny" thing is I thought everything was running fine until my curiousity made me do a compression check.
 

cobra 3.0

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Re: OMC 3.0 compression tests

Re adjusted the valves again today and got 150, 150, 135, 115. I'm going on vacation for a week up north and I won't be doing anything with it until later. I don't suppose any harm will come from running it this way for a week.
 

Boatist

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Apr 22, 2002
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Re: OMC 3.0 compression tests

Cobra Sounds like you know engines. Running the valves so tight they do not seal will cause them to overheat and burn but running just a little loose will no no damage. If fact if they were too tight before it may help them.<br />Have a fun vacation
 
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