Yamaha V8 5.7 YEMS 1990 Forum

The engine suffers from intermittent loss of cooling..
The drive belts are in good condition
The thermostat has been taken out and tested and appears to operate successfully
The boat is operated in a marine environment
Can any body help

ok

Did you run it in Salt water?

If so was it after this that it began to have the cooling problem?

The reason I mention it is because it sounds like you have a partial blockage in the Thermostat Housing. Remove all hoses and check and be sure all parts of the Housing are clear.

When ever this engine is run in salt it does have a tendency to crystallize in the housing and the block. Once done running in salt water (and this goes for any boat engine) put a freshwater hose to the water intake and run clean fresh tap water through it for a few minute to clean out the salts.

Also some lakes are more alkaline than others and as a precaution you can fill a 50 gal drum filled with 40 gals of water and 5 gallons of muriatic acid (or one gallon of full strength hydrochloric acid) hook a hose to your water pump and run through the system for five minutes.

If you boat has sat out of water for sometime without cleaning and clearing the housings and pump and cooling system it will also crystallize follow the muriatic acid program and it should clean out the system.

One other thing Brian

check your oil pump and see that it is working properly. A slow oil pump will also allow it to over heat.

Thank you for the information
The boat runs in salt water all the time
At present it is out of the water for a clean and general service
We will certianly follow you advice
many thanks

you can also just let the engine parts soak in the diluted muriatic acid. Never leave parts in the solution to long or unattended. take them out rinse then if needed soak them again.

good luck