overheat warning buzzer issue

[daveg734]

2001 Mercury
50 horse
4 stroke
bigfoot
Serial number 0T23141

I am having an overheating issue with this engine. I started it and idled for about 5 minutes and the overheat buzzer went off. I changed the impeller and lake tested it. Again, overheat buzzer if left idling. If I brought the revs up to just under 2k it would not overheat. I ran it this way for quite a few outings hoping it would improve. I tried to let it idle again and within 5 minutes I heard the buzzer. I swapped out the thermostat. When I removed the old one it was full of sand and zebra muscle shell parts. I wiped the crud off he housing and installed the thermostat. Again, tried to idle and heard the buzzer. Tried the higher idle tactic and it worked. A couple outings later I was running full throttle and the engine stalled and gave me overheat buzzer. I swapped the overheat sensor and again, same issues. Compression is fine, plugs looked good.

Could there be debris in the cooling system? Is there a way to flush it?

I am out of ideas.

 

[daveg734]

This boat is not equipped with a poppet valve. I pulled the stat and flushed both positive and negative side. Still overheating.

I would assume its the power head to coolant tube seal. Would anyone agree? Is there anything else to check?

 

[Texasmark]

Test 1, Do you have a non contact Infrared Thermometer? If so, with things all closed up like when you are running on the water , run it and when the horn sounds, shoot the area around your spark plugs and over to your exhaust manifold and get some temp readings. Post back.

Test 2, Put the engine on muffs and a water hose and pull the stat and leave the hole open where it mounts. Crank the engine and run at idle long enough to get a data point. How much water is coming out the stat hole and is there any crud coming out with it?

 

[daveg734]

When I tested it without thermostat installed and I was getting no pressure at telltale. Water appeared to be running down the inside of prop shaft cowling.

My theory is that the thermostat became plugged and the water had nowhere to go which caused the upper coolant tube seal to come unseated.

I do not have the tool you described, but I may end up purchasing one later today. If I do i will post the results.

 

[daveg734]

Absolutely disgusted with Mercury. That upper tube seal should be something serviceable.
I removed every bolt.
I removed the power head.
I removed the lower unit.

The intermediate housing just will not come apart.

I checked the impeller, its fine and in the correct orientation.
I checked the thermostat, it looks fine.
I removed the cover for the cooling passages and everything looks fine.

This is the only thing left and I cannot for the life of me gain access to it.