Bad Compression

[longisldr]

I have a 1985 mercruiser 3.0L 140 hp engine. Serial #GM 2774531. At the end of last season, I heard a weird noise from the engine compartment. When I removed the engine cover, my oil dipstick shot up about 50 feet in the air and into the bay. My engine got real hot and smelled like it was burning oil. I covered the dipstick opening with a rag and chugged back to the dock at 3mph. I just recently began troubleshooting last years problem, first on my list was a compression test of all 4 cylinders. #1 was 150psi, #2 was 130psi, #3 was 0, yes 0, and #4 was 150psi. I added the teaspoon of oil to #3, gave it a few cranks, and did another test, and got another 0 psi on #3. I am praying it's not a hole in my piston or the rings are shot. The only other thing I can think of is a burnt valve, or worn guides. Any other suggestions or tests I can do to get a definitive answer before I get prices on a rebuild. Thanks for your help.

 

[WillyBWright]

Re: Bad Compression

I'd pull the head and see what's up. Could just be a head gasket. No compression probably means a valve is sticking. Or a piston disentegrated. You'd have a destroyed sparkplug in that hole.

 

[longisldr]

Re: Bad Compression

Thanks Willy, I dont believe a piston "disintegrated" because that would have made a heck of a sound. I definitely hear an abnormality when the boat is running. It starts up no problem. Idles a little rough, and when I put it into drive and get above 1500 rpm's, it bogs down. Would I have gotten that far with a blown piston? Or is it a good hunch it is a valve? I would give anything for it to be a valve right about now. The flounder run is upon us, and I got a guy that can do the head for me in a couple of days.

 

[Haut Medoc]

Re: Bad Compression

Only one way to find out....It is a bad valve/ valve spring, collapsed lifter maybe.....<br />Pulling the valve cover & running/observing will tell you something....<br />You are getting exhaust into the crankcase for sure.....<br />If you have a bad piston from poor timing or lugging, the rest are not in great shape most likely........JK

 

[longisldr]

Re: Bad Compression

Thanks Haut. I did remove the valve cover and to my surprise everything looked normal. I cranked the engine and the springs, push rods and rocker arms looked great, almost new.

 

[longisldr]

Re: Bad Compression

 

[Haut Medoc]

Re: Bad Compression

If they look ok you need to pull the head ..<br />Sounds like a bad piston.....JK

 

[longisldr]

Re: Bad Compression

Thanks for the help, I'll give you an update within a few days. My fingers are crossed.

 

[Bondo]

Re: Bad Compression

Ayuh,........<br />I can see a Hole in the piston from Here......<br /><br />How long has it been since it's last Tune-Up,..??..<br />Was the Timing set Properly,..???....<br />Sounds like you may have Detonated it to Death.....<br /><br />Good Luck......

 

[longisldr]

Re: Bad Compression

The last tune up was about a month before I started having problems. I am not sure about the timing.Before the tune up, the boat ran pretty darn good! It was just routine maintenance.

 

[Don S]

Re: Bad Compression

You have an 85 model, and unless it was changed over, it has points. Points MUST be set with a dwell meter on a boat, and since dwell effects timing, so the dwell must be set then the timing. It's also necessary to check the advance timing when doing a tuneup. All MUST work together to prevent holes in pistons.

 

[rodbolt]

Re: Bad Compression

overheating and poor /degraded fuel will blow holes in pistons as well. if your crankcase had that much pressure check the valve cover vent as well.<br /> about the only thing that will pressurize a crankcase that bad is a blown piston.<br /> valve train defects wont create crankcase pressure.

 

[longisldr]

Re: Bad Compression

well guys, i pulled the cylinder head off today and found a "dime" size hole in the piston. now i am contemplating a remanufactured engine or rebuilding this one on my own. a new replacement engine made by Barr/EDM Marine goes for about $2350 at a local marine store. should i just go for that? sounds like a good price. comes with a 1 year warranty.

 

[FreeBeeTony]

Re: Bad Compression

Where on Long Island is this marine store?<br /><br />Just curious............

 

[rodbolt]

Re: Bad Compression

remember, a hole in a piston is a symptom of a problem, it was a creation not a cause. if you dont find and fix the cause it will create another hole. most hole in piston failures WILL NOT be covered by warrenty.

 

[Bondo]

Re: Bad Compression

Originally posted by longisldr:<br /> The last tune up was about a month before I started having problems.<br /><br /> I am not sure about the timing. <br /><br />Before the tune up, the boat ran pretty darn good! It was just routine maintenance.

I'm Guessing,......<br />That was Cause,........... The Piston was the Results.............

 

[longisldr]

Re: Bad Compression

babylonboater, the marine store is lighthouse marine in riverhead. i have dealt with them in the past and they seem to know what they are doing. i am planning on having everthing done professionally so i dont run into the same problem. i was told i could have the motor in a few days. do you know of anyone else?

 

[FreeBeeTony]

Re: Bad Compression

No........I do most of my own work, was just curious.