Interesting Overheat Solution

Hi Randy,
Early last summer I wrote to you regarding my 86 Yamaha 150 overheating. Thank you for the reply.  (See
Motor Overheating)

The solution ended up as follows...
I noticed something weird with the problem... The engine would not overheat from hole shot to cruising at any speed as long as I went in a straight line.

As soon as I turned and straightened out again I lost water pressure. This alone is probably normal but pressure would not come back.

The sacrificial anode had fallen off (it was not tightened properly). This caused blisters in the paint on the lower unit.  Once air flowed past the blistered paint a "boundary layer" of air would stick to the gear casing. No matter what I did (go faster, slower, turn again) water would not break the air barrier and could not get into the grill.

After sanding off the blisters the problem was immediately fixed.  The boat ran great all summer.  It took my friend with a Masters in Mechanical Engineering to identify the problem.

Thanks again,



Glad to hear your problem is solved. I've had this happen a couple of times myself. To take it further we surmised that the air was only on the low pressure side of the case. Hence it would switch sides in different turns (direction). The intakes on Yamaha are not the best and the left intake vents to the right and vice-versa. During the turn the intake running in air would be the easiest one to pull from (air is lighter than water) and the motor waterpump would pull air during the turn. When the water flow resumed a nice steam pocket would form momentarily in the block and then, instant overheat. In those instances we made water intakes that rammed water into the gearcase at a pretty high pressure - enough pressure that it would fill the side of the gearcase that was in the air stream. If you have problems in the future that the contour won't cure, then try that in addition.

Anyway thanks for sharing your fix with us and hope your motor stays cool.




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.


[ Home l About I.H.R. l History l Models l Registry l Members' Pictures l Feature Article l Tech Talk ]
[ Literature l Events l Top Guns l Guest Boat l Classifieds l Video Gallery l Racing Corner ]
[ Prop Slippage Program l Animated Videos l Screen Savers l HIN Decoder l Links l Contact Mark ]