Thread: Mercury Marine 25 HP 2-stroke overheat

Back flush when lower unit is remove to change inpeller

Didn't thank about seizure, normally Galled and stop.

Were u mixing fuel ?

I took the lower unit off and removed the water pump housing. Impeller was half destroyed. There’s ur problem!

Bad news is the lower cylinder has quite bit less compression than the upper. I had checked them at the beginning of the season and they were pretty close.

Part of me says I had better expect to have to rebuild the engine. I’ve done quite a bit of my own car and boat motor work, but I’m not sure what all I might be getting into


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Couple of points to make.
The 2 stroke mercury 25 does not have any form of overheat alarm or derate function.
The 94-95 year models were in transition between nikasil and mercosil design blocks. Short of having the block sleeved, neither should be considered rebuildable if deep cylinder scoring is present.
The lack of a thermostat is common place, especially if it's a transition mercosil block. Those would dribble water out of the tell-tail until the thermostat opened. A restrictor washer should have been in place of the thermostat to ensure proper cooling operation but not all mechanics are aware of that.
The compression on that motor would have been 120-130psi when new. You didn't give the compression numbers but anything below about 95psi is below acceptable.
If the block has been toasted, a replacement block is your only real option.

Originally Posted by sliderman

Couple of points to make.
The 2 stroke mercury 25 does not have any form of overheat alarm or derate function.
The 94-95 year models were in transition between nikasil and mercosil design blocks. Short of having the block sleeved, neither should be considered rebuildable if deep cylinder scoring is present.
The lack of a thermostat is common place, especially if it's a transition mercosil block. Those would dribble water out of the tell-tail until the thermostat opened. A restrictor washer should have been in place of the thermostat to ensure proper cooling operation but not all mechanics are aware of that.
The compression on that motor would have been 120-130psi when new. You didn't give the compression numbers but anything below about 95psi is below acceptable.
If the block has been toasted, a replacement block is your only real option.

I have never done an outboard, but on chainsaws and other 2 cycle OPE, unless they toss a clip or break a ring, the aluminum transfer "scoring" in the cylinders can be cleaned up 90% of the time and a new piston will fix it.

For a chrome bore block, common in small engines and older 25 mercs, your description is accurate with what is routine. Nikasil and mercosil is a completely different animal.

Any good sources for a replacement block? I can’t remember if I listed it, but the s/n is 0G002xxx


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