Overheat sensor or thermostat?

[Michael Beaty]

2004 mercury optimax 225 dfi
Serial:0t961388
Water pressure at idle: .5-1

got on the water today and in 29 degree air temp and 43 degree water motor pulled over heat within about a ‘minute. Killed motor waited and temp came back at 90degrees warmed up fine and idled out went to fish alarm went off a couple times. Each time on idle at neutral and each time I could put in gear and get rpms above 800 and temp would instantly go back down to 127 run fine without over heat alarm. New water pump less than 2 months old cleaned compressor strainer last week wasn’t bad or that dirty. Thinking maybe a clogged thermostat? Any ideas would be appreciated. Thanks

[Grant2]

I would definitely look at the thermostat more than likely has debris in it.

[apenland01]

If they are the original thermostats, I'd replace the darn things. I had to replace mine as soon as I bought the boat because they wouldn't let the engine come up to temps. I was scared I was going to cold seize the engine. As soon as I got the new thermostats, I haven't had a single problem. They aren't cheap though...

[apenland01]

Also, my idle pressure is between 2-3, so 0.5-1 seems low to me. Impeller installed correctly and peeing a healthy stream at idle?

[Michael Beaty]

Has a good stream I thought kinda low as well

[SilverFox579]

Minimum idle water pressure is 1psi. You cannot use telltale as an indicator with the Optimax, it is compressor cooling outlet only, not engine cooling system.

For cooling system issues always start at the source and service the water pump.

[EuropeanAM]

Minimum idle water pressure is 1psi. You cannot use telltale as an indicator with the Optimax, it is compressor cooling outlet only, not engine cooling system.

For cooling system issues always start at the source and service the water pump.

^^^^ Spot-on advice.

[Michael Beaty]

Thanks for the advice guys. New water pump and new thermostats going in this week

[apenland01]

Good call, and if you remember, loop back and tell us how it worked out for you!