Motor Overheating

Hi Randy,
Great job you are doing.

I have a Valero with a 1986 Yamaha ProV150.  I just bought it at the end of last season and over the winter I had the impeller changed just to be safe. Now I am overheating even when running slow (25 mph) with the trim all the way down.  It did not overheat last year.

I have removed both thermostats - they are clean and function perfectly when I dip them in hot water.

I removed the 2 inch oval cover plate at the back of the motor at the bottom where there is a hose attached.  Inside there is a spring and a rubber/plastic valve.  ( I do not know what this is for but it is clean and seems to function properly except that I found a loose chip of plastic about 1/8" by 1/4" this could be nothing).

There is a water pressure gauge and the pressure is good when I start out cold but then after about 5to10 minutes the buzzer goes off and I look down to see the pressure is at zero.  If I leave it to cool for 30mins and then start it up again the pressure is back to normal.  There is always water pissing out of the back of the engine even when the gauge is at zero.  It will also cool down in about 2 minutes if I leave it at idle while the buzzer is going.

 What should I check next?  I can't take it to the shop due to finances so my choice is fix it myself or don't drive it.  (The later may be best due to gas prices anyway.)

I look forward to hearing from you,

Thank you,



You've done most of the checks and all seems fine. At this point I'd change the entire waterpump assembly. The plate and cup can warp or have grooves that will cause the overheat problem. Make sure you run with the thermostats installed as the motor will overheat without them in. The water flow is too fast with them out and the block can sometimes not be able to get rid of its heat. For a check you can install a hose fitting on the top of the block (fitting is facing up, one on each bank) and watch the water flow from the engine. If you have air pockets that burp out all the time then we'll have to search deeper. Buy a complete waterpump kit and start there.

Keep cool,   (couldn't resist)




Please submit whatever questions you have to  All questions and answers selected will be displayed on this page as they come in.  Note: though all boaters are welcome, priority may be given to I.H.R. members.  Randy can not be held responsible for any advice given.  Though his information and expertise is second to none, he has no control over what you do with your boat.  It is up to you to boat safely and act responsibly, and his advice is only to be used as a guidance for your high performance boat/motor of which you are the one responsible for the risks involved.

Mark C.


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