Overheating

Mariner 9.9 (1981) seems to be getting hot and begins to lose power after 30 seconds or so going full speed. Telltale stream is strong, but nothing but a little mist coming out the exhaust relief hole on the leg. Is the telltale just indicating that pump is working, but not necessarily that the engine is being cooled? Shouldn't the water be coming out that exhaust relief hole fairly strong at full speed? If it is not, could that be an indication of blockage in the cooling system? Or something else?

Re: Overheating

OK, I'm assuming this is a Yamaha-built design and looks something like this:

http://www.mercruiserparts.com/Show...=492&bnbr=7&bdesc=CRANKCASE+AND+CYLINDER+HEAD

If so, you may have thermostat issues. Pull the thermostat cover (piece #16) and check the thermostat. Replace if it looks real crusty.

You can test the 'stat with a pot of hot water on the stove and a candy thermometer or equivalent. Suspend the 'stat with a wire so it doesn't touch the sides or bottom of the pot, it should start to open somewhere in the 120 deg F range and if it's not fully open well before 140 deg F that's way too hot.

You could also fire up the motor and see if you get a strong flow out the open thermostat hole. That should tell you loads about the quality of water circulation in your powerhead!

As a final diagnostic tool, you could try running the motor with the thermostat removed (with the cover reinstalled, of course!). It should remain Stone Cold and probably won't idle real smoothly, but you'll be getting Max Flow thru the powerhead.

BTW if you haven't replaced the impeller in quite a few years it's probably time to do so.

HTH & keep us posted..........ed

p.s. don't bust the 'stat cover bolts, they're easily snapped. If they won't come out with 'normal' force, heat them up with a propane or MAPP torch.

Re: Overheating

Thanks. Sounds like good advice. I will check it this weekend and let you know....