Mercruise 898 overheat at full throttle

Hello
I am new to this forum and new to boating. I bought a 1983 Larson 21 cuddy with a 898 Mercruise. used only in fresh water.
The problem I have is the engine will overheat to 200 when at full throttle and about 160 at anything below full throttle.
The thing is that it does not always over heat at full throttle. Just the odd time.
I am looking for some help if anyone can give me a hand.
I have the repair manual and will repair mostly anything (execpt rebuild entire engine)
I gave the engine a new Manifold and riser on the starboard side and new bellow on the same side. could it be the port side needs a cleaning out?
I never took out the stat could this be the problem as well.

Again thanks

Re: Mercruise 898 overheat at full throttle

Howdy,

An overheat at full throttle is frequently related to clogged manifolds, risers and possibly worn raw pump impellers.

If you have had a raw pump failure recently, or other failures in the past, you may also have partial blockage of the raw water line from the transom to the thermostat housing. If you have a power steering and/or an oil cooler, those raw water pump impeller parts will frequently get lodged in the oil/PS cooler and will restrict raw water flow. You won't over heat at low power settings but will overheat at higher settings.

Also, if you have an older engine, and the engine and manifolds/risers are raw water cooled and you discovered blockage from corrosion you may also have corrosion and subsequent blockage in the block and heads.

The T-stat could be defective. You can rule that out by putting it in hot water and testing to see if it opens at the right temp. You also my have raw water pump fragments in the t-stat housing.


Regards,


Rick