Yamaha f40bet overheating

chookstar

Cadet
Joined
Oct 12, 2024
Messages
7
Im hoping you can help me with my problem as i have run out of ideas. I have a Yamaha 40 HP 4 stroke outboard F40Bet I was cruzing along mid speed after a few minutes the temperature buzzer sounded, with cool water coming out very well from the tale tail and at a strong pressure. I checked thermostat but it has been removed by previous owner. Still after 4 minutes on idle the temperature buzzer would beep. I have removed and cleaned the exhaust plate, changed the impeller. Switched on the engine and still on idle after around 5 minutes the temperature buzzer went off. I am honestly out of ideas and desperate to find a solution. Engine is really getting hot especially around the thermostat area. I also tried changing the temperature sensor with a new one but still no change. When i switch on the motor without the thermostat cover water flows out. Any ideas what could be wrong? really appreciate your feedback.
 

99yam40

Fleet Admiral
Joined
Sep 7, 2008
Messages
9,086
you might try using a chemical descaler like Rydlime marine descaler.

or take the motor apart farther to find what is plugged up
 

cyclops222

Lieutenant
Joined
Mar 21, 2024
Messages
1,339
I once had 2 brand new thermostats AND 2 new thermostat gaskets.
Sadly The gaskets were new rejects. The gaskets would get pushed up at a angle and jam the thermostat at a partially open position.

Plugs blackened & started missing in a few hours. I removed the rubber gasket and it ran almost perfect for a month everyday.
 

chookstar

Cadet
Joined
Oct 12, 2024
Messages
7
I once had 2 brand new thermostats AND 2 new thermostat gaskets.
Sadly The gaskets were new rejects. The gaskets would get pushed up at a angle and jam the thermostat at a partially open position.

Plugs blackened & started missing in a few hours. I removed the rubber gasket and it ran almost perfect for a month everyday.
Ok cheers I will have a check
 
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