MerCruiser 5.7L Low-ish Compression

[jimmbo]

Just the potential of Downgrading both heads

 

[jamesm113]

Finally got around to tearing it down. We also learned it's not the original motor, was repowered sometime in the 80s or 90s.

Discovered milkshake underneath the intake. Suspect the intake gasket went bad and leaked coolant.

Poured gas on both cylinder heads to test for leakage on both heads. All cylinders would leak - some on the intake side, some on the exhaust side and some on both. Pulled valves on the known bad head. At least one intake valve is completely shot, the others are borderline.

Need to figure out what destroyed the intake valve(s) and be 100% sure what caused the coolant to get into the oil. Hopefully just the intake gasket.

Crank turns over nice and smooth, used a socket extension and hammer to test each piston for knock, everything was solid. Cylinders look maybe a touch questionable with slight surface rust, but they are smooth to the touch.

 

[Scott06]

What do spark plugs look like ? Wonder if it was running lean and that smoked the intake valves

might have overheated and ruined head gaskets ?

 

[jamesm113]

What do spark plugs look like ? Wonder if it was running lean and that smoked the intake valves

Hard to say, spark plugs were covered in gunk when we first got it running. I think the current ones got pretty dirty in the tear down process - they were black.

might have overheated and ruined head gaskets ?

We tried doing a float test a few weeks ago. Never left the dock, because it started overheating. (We later found out the the impeller was shot, never had issues with city water) Temp on the gauge never got beyond the green area but the risers were hot to the touch before we shut it off. Visually checking the HGs it didn't appear there was any coolant leaks. Nor does it appear there's a path for oil and coolant to mix through the HG.