Overheating at Idle

[Holden441]

Hello all,
I own a 1983 Ski Supreme with ford 351PCM. I have had problems overheating at idle. For example, I will start the boat, could be the 1st or 5th start of the day- it does not matter if boat is cold or warm. The temp gauge slowly creaps up to above operating temp (usually 160 at idle). All it takes is an increase of RPM's maybe 12-1400 to get the temp back down. Then the temp is fine at idle or high rpm until i shut the boat off and start again. I have not a clue of what this could be. This is what I have done: replaced impellar, replaced all cold water hoses, checked for clogs in transmission cooler. Belt tension is fine on raw water pump. Tested thermostat on stove, it opened (not exactly what temp). Any thoughts to this problem. Thanks for all your help!

[lazeyjack]

have you checked the jacket water pump? once I had an engine that had sheared a key, , somehow it picked up speed at higher rpm

[whipper]

The only thing I would say is when you tested your T-Stats you might want to just replace them for what there worth. Because you dont know what temps they opened they still could be your culprit? I tested mine that way before and tried to check the water temps also thought every thing was OK. I ran the motor and same over heat problem? I just replaced them and over heat problem solved. 10 bucks each. The only way to eliminate them as a problem is to replace them. They could have opened at way to high a temp for all you know right.

[Holden441]

The first thing i am planning on is buying a new thermostat and throwing it in there and hopefully its a quick fix, but would that matter if it's overheating at an idle and not a higher rpm (remember just after starts) then it is fine the rest of the time, even at idle. It's like it needs to create a vaccum again. I removed the screen and eliminated it all togother just to test and i still had the problem, is it possible to be sucking air? Everything appears tight with now leaks. Thanks for the replies.

[TollyWally]

On my inboard freshwater cooled boat you are moving the very least amount of water through the exhaust system at idle. Perhaps pull the thermostat out entirely and make a few runs. It should run colder and take longer to warm up. Some of the drag boat guys that have problems with overheating at idle plumb suplemental electric pumps that are not dependant on engine rpm for output into the system.

Sometimes it's just the fine line of being on the ragged edge. My boat exhaust runs a little hot on one side and I have fooled around and "fixed" it by taking everything apart, cleaning, and putting it back together again without ever really finding anything wrong.

Good luck

[CDK]

The circulation at idle rpm has stopped or almost stopped. Most probably the pump's rotor is corroded to the point where it can only swirl some water around in the housing. I think this pump has an alloy rotor, the only alloy part in an otherwise cast iron engine, so it is slowly sacrificed.

[tuantom]

Have you checked the impeller on the external water pump? They get hard and brittle over time and that could cause it to lose its seal and prime - so it needs the higher rpms to pull the water up and get working again.

Can you check the exhaust to see if it's pumping water after you first start it?

EDIT - just reread your first post and see that you replaced the impeller already.

[mongo75]

How about clogged / corroded exhaust riser not letting enough raw water escape? That could cause a slew of back-ups in your cooling system. If they are corroded, you can give them an acid bath in a 50/50 mix of pool acid and water or replace them