Yamaha 50 HP 4 stroke outboard overheating. Cant solve problem

[KenKen700]

Im hoping you can help me with my problem as i have run out of ideas. I have a Yamaha 50 HP 4 stroke outboard F50DET. Motor has been at me for 15 years with no major problems. Used the boat twice this summer with no problems, until a few weeks ago i drove into a bay and reduced the 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. To cut a long story short, i removed the thermostat which was blocked in close position. Still after 4 minutes on idle the temperature buzzer would beep. My mechanic came over removed and cleaned the exhaust plate, changed the impeller and still after 4 minutes on idle the temperature buzzer would beep. We decided to have a look at the cylinder heads, and we found a lot of salt/calcium deposits blocking the bottom part of the bottom cylinder head. Obstructing the water passage way from the cylinder heads to the exhaust plate. Being confident that we solved this problem the mechanic reassembled all parts this morning. 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. We also tried changing the temperature sensor with a new one but still no change. When i switch in the motor without the thermostat cover water flows out. Any ideas what could be wrong? really appreciate your feedback.

 

[99yam40]

you mentioned the stat was not opening, or maybe it was blocked up with some debris.
But you did not say you put a known good (tested)stat into the housing?

Does this motor have a PRV that could stick open dumping water?
are you sure the pipe from pump to power head is sealed to make sure all the water to the power head gets into the powerhead?

have you replaced the water pump including the housing?

 

[KenKen700]

Hi thanks for helping out. Im still waiting for a new thermostat, but my mechanic said the engine shouldnt overheat without a thermostat.

My engine doesnt have a popper valve, checked this on service manual. Im really running out of ideas.

Regarding pipe from pump to power head i am assuming it should be ok as water is coming out from behind the thermostat when switched on without a thermostat cover.

We changed the impeller but not its housing. On inspection mechanic said housing looked ok. Would you suggest changing it anyway ? Water is coming out very well from tell tail even if connected with headphones.

What i forgot to mention is that my tell tail doesnt have the plastic nozzle which reduces the diameter of the tell tail pipe at the end. Its been without it since last summer and never had an engine overheat. Could it be that without it water is exiting the engine too quickly ?

 

[99yam40]

or maybe too much is flowing out instead of going thru the motor.

I have no idea on where that 4 stroke TT is tied in, but a lot of the old 2 strokes it just indicated that the pump was pumping, and the water coming out was not hot so it did not run thru the motor before exiting the TT

And I have always read that a stat forces the water to flow thru the whole motor the way it was designed to.
if you wanted to remove the stat, you needed to put in a restriction device, or remove the center part of the stat and put the outer part back in.
without doing that there will be hot spots and cold spots/areas

 

[KenKen700]

or maybe too much is flowing out instead of going thru the motor.

I have no idea on where that 4 stroke TT is tied in, but a lot of the old 2 strokes it just indicated that the pump was pumping, and the water coming out was not hot so it did not run thru the motor before exiting the TT

And I have always read that a stat forces the water to flow thru the whole motor the way it was designed to.
if you wanted to remove the stat, you needed to put in a restriction device, or remove the center part of the stat and put the outer part back in.
without doing that there will be hot spots and cold spots/areas

Yes it possible that water isnt circulating all the motor and is exiting faster via the TT. Will install a new thermostat when stock arrives but i doubt that will solve the issue. There must be a blockage somewhere which i still cant find

I will ask mechanic to change the pump housing and give that a try.

 

[KenKen700]

or maybe too much is flowing out instead of going thru the motor.

I have no idea on where that 4 stroke TT is tied in, but a lot of the old 2 strokes it just indicated that the pump was pumping, and the water coming out was not hot so it did not run thru the motor before exiting the TT

And I have always read that a stat forces the water to flow thru the whole motor the way it was designed to.
if you wanted to remove the stat, you needed to put in a restriction device, or remove the center part of the stat and put the outer part back in.
without doing that there will be hot spots and cold spots/areas

Update.

So installed new Thermostat and plastic bit found at the end of the tell tail which reduces the tell tail diameter.

Started engine in a water container (better then with headphones). Still the same overheating problem after 5 mins on idle.

My mechanic now wants to remove the engine block and inspect water channels underneath it. Does this make any sense? i am realising that the top 2 cylinder heads next to the thermostat are the ones that heat up the quickest. any suggestions?

 

[99yam40]

Update.

So installed new Thermostat and plastic bit found at the end of the tell tail which reduces the tell tail diameter.

Started engine in a water container (better then with headphones). Still the same overheating problem after 5 mins on idle.

My mechanic now wants to remove the engine block and inspect water channels underneath it. Does this make any sense? i am realising that the top 2 cylinder heads next to the thermostat are the ones that heat up the quickest. any suggestions?

I would have thought that was done when he had the motor apart last time.
chemical flush is the only thing you have not tried.
I read good reports on Ridlyme Marine descaler , but have not tried it myself

I have to wonder if the top cylinders are not getting enough water into block to cool them.
in other words not filling block all the way when idling

 

[chookstar]

Im hoping you can help me with my problem as i have run out of ideas. I have a Yamaha 50 HP 4 stroke outboard F50DET. Motor has been at me for 15 years with no major problems. Used the boat twice this summer with no problems, until a few weeks ago i drove into a bay and reduced the 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. To cut a long story short, i removed the thermostat which was blocked in close position. Still after 4 minutes on idle the temperature buzzer would beep. My mechanic came over removed and cleaned the exhaust plate, changed the impeller and still after 4 minutes on idle the temperature buzzer would beep. We decided to have a look at the cylinder heads, and we found a lot of salt/calcium deposits blocking the bottom part of the bottom cylinder head. Obstructing the water passage way from the cylinder heads to the exhaust plate. Being confident that we solved this problem the mechanic reassembled all parts this morning. 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. We also tried changing the temperature sensor with a new one but still no change. When i switch in the motor without the thermostat cover water flows out. Any ideas what could be wrong? really appreciate your feedback

 

[chookstar]

G'day mate
I see this is an old question but I am wondering if you ever fixed it please? I am in exact position as you have described. It is doing my head in.
Cheers chook