Outboard Cooling

[Schnidly]

I have a 1988 Mercury 90 hp 3 cylinder outboard. Last year a new water pump was installed with the new houseing and this year I installed a new water pressure gage. When I run the boat at idle at startup the water pressure is between 10 and 15psi. When I throttle the boat the water pressure drops down to 5psi. Then when the motor is warmed up the fluxuation in the pressure depending on the throttle position varies only slightly from between 5psi and 10psi. Can anyone explain to me why this is happening. I have a good pee stream shooting out of the back of the motor.

 

[12Footer]

Re: Outboard Cooling

Sure can. The impeller in most outboards and raw water cooled I/O's are made of rubber. The vanes contact the walls of the puymp housing when idling, and it acts like a positive-displacement pump.<br />However, at high speeds, the vanes "tuck-in",and the pump acts like a centrifugal pump.<br />The pressure fluctuations you are seeing are normal,as the impeller generates higher pressure durring idle when it is acting as a PD pump. Also, take into account that the thermostat is opening and closing at it's rated temperature.<br />Because these impellers are made out of rubber, it should be considered a routine maintainence item,and be replaced every season.

 

[Schnidly]

Re: Outboard Cooling

Dear 12 Footer,<br /><br />Thank you so much for the info. I was told that on a motor above a 50 hp the impeller only needs changing about every 2-5 years. Also, do you know where I can get a good livewell timer for my boat? I bought one from a guy that makes them in Florida and I am not happy with it. My original one was cylindrical shaped and red. Do you know where I might find one like that?

 

[12Footer]

Re: Outboard Cooling

LOL,Shnidly. The timer on my livewell is an on/off switch

<br />I had a timer on my PennYan, but it broke after one season.<br /><br />But I digress. You can find all kindsa goodies in the "megamall" at the top banner of this page.<br />Here is another "link to links".<br /> http://www.wmi.org/bassfish/BASStore.HTMl