Engine temp climbing at WOT

[Lift12]

Fresh water cooled Benny.

 

[GA_Boater]

Both thermostat housings removed, cleaned, painted, new gaskets, spacers and 140 degree thermostats. I will try swapping them between engines to see if it changes anything. One could easily be defective. Thanks for the thought.

BTW that thermostat is a $25 part here in Canada. Maybe more at my snooty marina nearby.

While the stats were out, did you test them in a pot of hot water? A side by side check to see if both stats open and close at the same rate.

This is a check, not a change, as Benny suggested.

 

[QBhoy]

Hi. Were the impellers etc replaced out of routine or did one or both of the engines overheat previously?
if they or it did...is there a chance you have bits of a melted and dislodged exhaust flapper stuck somewhere ?
failimg that, buy a heat sensor gun and verify the gauge isn’t telling fibs

 

[kenny nunez]

I am going to throw some gasoline on the fire.
When I had my shop I re-powered a customers 24 Stamas with a 260 Mercruiser package. He wanted the Mercruiser fresh water cooling kit. The kit came with a belt driven sea water pump and the instructions said to install the pump. Well I did not feel like cutting a hole in the hull. I took it to the lake and as soon as I started to push it over 3500 the temperature shot up.
Next day I installed the pump and the through hull fitting with a sea ****. PROBLEM SOLVED !

 

[Benny67]

Next day I installed the pump and the through hull fitting with a sea ****. PROBLEM SOLVED !

Stories like this are usually key in leading to solving problems.

Before I'm beaten up, I'm not saying that what Kenny is writing IS THE SOLUTION... But it does shed another light on the topic.

Maybe. OP doesn't realize that the system has these pumps or not... maybe the boat was upgraded incorrectly..maybe it has these pumps and one of them is shot?

Regardless, it's worth pointing out to put this on the table to ask if the boat has these pumps or not

 

[kenny nunez]

I am sure that if the O P would have disclosed that the engine was FW cooled it would have been of a great help with the diagnosis.
Since his boat has dual engines I know that trying to service belt driven sea water pumps is very difficult. Several forum posters have stated that having the drive pumps feed directly to the sea water pumps without a sea suction through the hull does not work.
Since the engine is FW cooled that rules out a head gasket leak. His heat exchanger on the sea water side is probably partially restricted.

 

[scoflaw]

Those heat exchanger gaskets swell and can block the tube bundle

 

[Benny67]

His heat exchanger on the sea water side is probably partially restricted.

I'd say it's "almost safe to say" it's a coolant volume issue.

Could be as simple as running a piece of buss wire through the intake passages on the drive.

I do that every year before it gets too cold to get in the LI sound. If I don't do it, by the time my boat is on stands in my driveway those passages are about 70% plugged.