High Temp Alarm

[LCheap]

I have a 1984 200 hp Mercury and when I get up on plane with the boat , above 2500 rpms, I get a high temp alarmbut run under those rpms and no alarm I have changed the water pump and the popett valve. I can run the motor hooked to garden hose and run up to high rpms and can not duplicate the alarm. Open to suggestions.

 

[PolishPrince]

Re: High Temp Alarm

It may not be a high temp alarm. My '07 90HP 4 stroke alarm kept going off. The Owners Manual said the alarm was for high water temp or low oil pressure, but my Merc dealer said it also a low fuel pressure alarm. There's a big age difference between the engines, but don't limit yourself to high water temp.
Now if it is a high water temp alarm, the reason it doesn't do it with your garden hose hooked up is that under a load the engine is using more fuel to maintain a given RPM. Under no load (out of the water) less fuel is used to achieve the same RPM. Energy can not be created or destroyed so more fuel used = higher combustion temperatures = higher water temp.

 

[j_martin]

Re: High Temp Alarm

That engine has only the temp alarm, and maybe an oil alert which is a different beep. No other foolishness.

Either you are overheating, or the temp sender is failing.

If it's hot enough to set off the temp sender, the heads would be painfully hot to the touch. You could duct tape yer wife's remote reading oven thermometer to a head and see what the temp is at speed.

Bet is, you are actually overheating. It could be caused by lean mixture, overpropping, leaking gaskets, plugged water passages, or a combination of the above. Any one of them could be fatal to the engine, so you must figure it out.

hope it helps
John

 

[CharlieB]

Re: High Temp Alarm

I have a 1984 200 hp Mercury and when I get up on plane with the boat , above 2500 rpms, I get a high temp alarmbut run under those rpms and no alarm I have changed the water pump and the popett valve. I can run the motor hooked to garden hose and run up to high rpms and can not duplicate the alarm. Open to suggestions.

Did you change the whole pump or just the impeller?

A warped plate or lower pump housing will allow the pump to suck air on the boat gets on a plane and the gearcase rises enough that the pump is then above the water.

Another possibility is a split gasket under the powerhead that will allow cooling water to divert and leak into the exhaust, reducing high speed cooling.