2005 300x overheat (intermittent)

[Fryguy]

Hey folks thanks for approving my account. Love all the information available here! I'm pretty new to outboards but have been a jet boat for years.

So having an issue with my 300x serial 0T998888. Usually the temp fluctuates between 118 and 122 which tells me the t-stats are working. But sometimes when running hard I will get the guardian alarm and power is reduced. When this happens the smartcraft displays the "temp" icon and displays about 150. Other times I can stand on it for miles with no issues.

I have a guy with a Rinda tool but could not locate any FF data for the failure, and of course could not duplicate the issue with it connected. In most cases intermittents are electrical issues but I feel like this could be mechanical. I say this because this engine has 3 temp sensors. 2 for the ECM and smartcraft(I guess) and a third for the aftermarket gauge in the dash. Both the smartcraft and the dash gauge showed over normal temp and that's while I feel like it's a real overheat problem and not just a sensor failure. Water pressure is solid 12-15 lbs.

Any advice would be greatly appreciated.

[EuropeanAM]

Welcome to BBC!

Really need to keep that engine at 15+ at WOT (prefer to see 18+ for safe measure in turns).

When was that water pump impeller (and gaskets) last replaced with OEM Mercury/Quicksilver components?

Anytime you experience a cooling system/overheating problem, it is best to start AT THE SOURCE: The Water Pump.

[Fryguy]

Yes sir Don, understood. I put a new quicksilver water pump in it several months ago. Once I began having this problem I dropped the LU to check it and it looks brand new. Getting a good solid pee stream as well. I removed the tstats and tested them in hot water. No debris, and they open/close normally.

[EuropeanAM]

Did you pull the metal plate below the water pump impeller plate, and inspect the low water inlet passages from the pickups all the way to the impeller plate (wood floating around in here will make for a fun diagnosis).

[Fryguy]

Yes I did. I looked all down in there and also blew air through each hole in the LWP. You mentioned water pressure being on the low side, what could that be? Poppet? Other than then t-stat sticking randomly, what else is mechanical in the cooling system?

[Rip Bass]

Having the engine mounted too high can also cause low water pressure.

[EuropeanAM]

Correct.

Any air leaks between the surface of the water and the water pump can also cause low water pressure (or aerated water supply) (example would be a torn or deteriorated gasket).