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Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 11071
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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I have a 1993 Merc 350 Magnum 5.7 L with Alpha 1 outdrive.

Resolved Question:

I have a 1993 Merc 350 Magnum 5.7 L with Alpha 1 outdrive. Engine overheats under heavy throttle (usually runs about 170 according to gauge), but not while keeping below 3000 rpm. Checked flow from outdrive impeller, which was strong when throttling up in neutral. Replaced water pump and thermostat (thermostat and thermostat housing was replaced last year; thermostat is 160 degree). Pulled thermostat and ran engine without at idle (no throttle)...cooled off as expected. Put thermo back in, and once again heated up past 180 degrees and alarm went off. Any ideas?
Submitted: 5 years ago.
Category: Boat
Expert:  Tan replied 5 years ago.

Hi my name isXXXXX will assist you.


When the engine starts over heating under a load at higher rpms this is a sign of blockage in the manifolds and risers. The other place to look for blockage is in the oil cooler. Remove the risers and check the water passages for excessive corrosion. Post back with findings.

Customer: replied 5 years ago.
If the problem is with the risers or manifolds,is the only solution to replace them?
Expert:  Tan replied 5 years ago.
Yes they cannot be cleaned as the integrity of the metal is deteriorated so bad they will develop holes and start leaking internally filling the engine with water.
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