1985 Mercury 50 H.P. overheat alarm

[Bob Burgan]

When I get my boat up to plane the overheat alarm goes off and stays on til I slow down to about 5 knots. There is water comming out the tell-tale hole and out the prop. We tested it in the tank but it just ran nicely no matter how fast we ran it. we recently took the oiler tank off the system and started mixing the gas w/oil because the oiler wasn't working correctly. The only other thing that has happened is that my son ran it aground in the shallowsw amd chopped up a log before we could turn it off. I t came right back on and ran except for the alarm. Water pump has been rebuilt and engine was tuned...any ideas....Bob

 

[Texasmark]

Re: 1985 Mercury 50 H.P. overheat alarm

Not familiar with that engine. If you have a 3 cyl and an overtemp alarm, you must also have a thermostat and a pressure relief valve. Going to assume you do......makes the answer easy.

The thermostat is working, the pressure relief is not!!!

The opening in the press rel is many times larger than the stat. AT low speeds/rpm's the press rel is closed and the engine runs off the stat.

Any time the engine rpm's get up (2500) or the RAM water pressure entering your lower unit gets above a certain pressure, it unseats the pressure relief and lets considerably more water through the powerhead. This isn't happening for you. 8)

Mark

 

[lisfisher]

Re: 1985 Mercury 50 H.P. overheat alarm

Also , check your regulator. I had issues last year where as soon as I got on plane the alarm would go off. Slow down and it would stop. Higher rpm's sent my voltage through the roof , so i put in a new regulator and it solved the problem. I also asked Mercury if a bad reg can trigger the alarm and they said it can