1990 Yamaha 150 overheating

[papawoulfe]

I am having a lot of trouble with my 1990 yamaha overheating. It has less than 700 hrs. It started with getting the boat up to 25 mph and I got the overheat alarm and the motor slowed down to a fast idle. Water was coming out of the p hole. Changed out the water pump and still the same result. I changed the temperature sensors and still the same result. Changed the thermostats and now after 5 to 10 minutes it overheats regardless of the engine speed and when it gets hot the water stops coming out the p hole. I did a compression test as a precautionary measure and all 6 cylinders are between 101 and 105 lb. per sq". Anybody have any suggestions? Am I missing something here?

 

[birdbrain]

Re: 1990 Yamaha 150 overheating

I'm not the expert here so let the others check in. If it were my engine I'd establish if there was sufficient water flow. I may consider starting the engine briefly with the thermostat covers off, to see if water was making it that far up the engine. I'd decide my next move after this test.

 

[scottcmb]

Re: 1990 Yamaha 150 overheating

check your poppet valve at the bottom of the block between the heads.....if the spring inside is broken or the valve inside is stuck open...water wont reach the top of the block while running......it is designed to control the water pressure in your block, it is basically a water pressure relief valve......if open all the time.....the water will just dump out instead of rising to the top of the block and through the thermostats.

when inspecting, check the spring, check the rubber seal, check the plastic valve itself and clean out the housing and cap.....when reasembling make sure that nothing will bind on the valve.
DONT run your engine for any length of time until you can establish that you DO have water rising to the top of the block!!!
hope this helps you

 

[scottfromoz]

Re: 1990 Yamaha 150 overheating

check above as suggested, but you may have to take off head covers and clean out water galleries - I threaded recently about it for my 93 115

 

[scottcmb]

Re: 1990 Yamaha 150 overheating

another thought after re reading your thread......seeing that your motor is at the 700hr mark, has it had the 500hr rebuild service?( replacement of all power head gaskets and water gallery de-scaling and cleanout, carbs etc etc)

If not, i would seriously suggest you get this done!

 

[scottfromoz]

Re: 1990 Yamaha 150 overheating

Im not suggesting that a strip down and clean out of all galleries is a bad idea (I have just done the head galleries) but does Yammy actually recommend it for salt water motors (even properly flushed with fresh) at 500 odd hours? I am interested. I have the owners manual which does not mention it but is there a tech bull' or workshop manual that actually says it should be done? Lot of money in gaskets and time cleaning.

Cheers