What compression for my 1991, 3cylinder, 90hp Mariner/Mercury??

I have a 1991, 3 cylinder, 90hp Mariner engine, with the clam shell cover. I have recently noticed some fuel/oil mix leaking from the top of the crank case, it seems to be coming from under the fly wheel somewhere, I havnt taken the fly wheel off yet. It is only a small amount of fuel/oil and the engine runs fine. I asked my friend who does all my outboard work for me and he said we should do a compression test. Can anybody tell me what the readings should be? Is it measured in psi? or something else?
Why is the engine leaking from the top? Could it be a gasket on its way out?

Thank you.

Re: What compression for my 1991, 3cylinder, 90hp Mariner/Mercury??

All three cylinders should compress within 10-15%. It doesn't matter what the psi is as long as it is above 90spi.

Re: What compression for my 1991, 3cylinder, 90hp Mariner/Mercury??

psoyes said:

Why is the engine leaking from the top? Could it be a gasket on its way out?

Click to expand...

There is a driveshaft seal at the top which sounds blown on your motor.

Re: What compression for my 1991, 3cylinder, 90hp Mariner/Mercury??

Ok guys thank you very much, I will get the compression tester on it tomorrow. I will get me friend to have a look at the drive shaft seal aswell.
Thank you.

Re: What compression for my 1991, 3cylinder, 90hp Mariner/Mercury??

My manual on later versions of that engine says "expect problems below 120 PSIG" JB's remarks are the rule however as there are a lot of variables and if all cylinders read the same, you can pretty much figure your engine is good to go. But obviously, as the manual states, there is a lower limit to be considered.

The crankshaft comes up out of the block for attachment of the flywheel. Obviously there is a seal there. I don't personally correlate the seal leaking to compression but when you think about it, compression blow by could fill the crankcase area and put pressure on the seal in excess of it's design. Course if you are suffering from low compression, given wear, not bad maintenance, then surely other things are going to be worn also.

Mark

Re: What compression for my 1991, 3cylinder, 90hp Mariner/Mercury??

Thanks for that Mark. Im not too good with engines but Im pretty the engine isnt too worn, not long before I bought the boat the engine had the head taken off and lots of work done on it. When the boat is in the water she runs like a dream, so I guess things are good there. I noticed the fuel oil leak 2 months ago and the engine seems to run fine. I like my engine to be good and if it is leaking then I want to find out why and get it fixed. Im going down to the boat in the morning with my brother in law who does all my engine work and Im sure he will be able to shed some light on it. It was him who asked me to find out about compression numbers for my engine.

Thanks for your advice.