Overheating Mercury 50 hp Four Stroke

A simple question to you all - Where is the thermostat on the Merc 50 four stroke (1999)? Is it an "on water" or an " in workshop" sort of job?

The engine runs fine for a few minutes but quickly gives its warning of engine overheat, the powerhead itself is too hot to touch and the engine cuts back to half speed.
After 1 minute at idle it will run fine again for another couple of minutes.

The engine has a new water pump impellor and there is plenty of warm - not hot - water coming out of the telltale.

Any thoughts

Hugh Mattos
Devon England

 

A good friend of mine has the same problem. The thermostat was an easy thing to change. In his case the thermostat was not the problem. and also in his case the water pump impellor is new. Also we noticed at high speed, there is no water coming out the telltale. At idle the water comes out the telltale, but not forcefully like it did a year ago. I think the Mercury outboard has a poppet valve that bypasses the thermostat when it fails anyway. Let me know what you find, I'll keep digging as well.

 

Solved at last

A solution at last to the overheating Merc 50 Four stroke.

I am told that this was a field campaign item some years ago, but Mercury Mariner dealers will no longer pick up the bill.

The problem is corrosion of the engine lower tray which produces so much oxide that it compresses the rubber seal where the cold water feed pipe from the pump comes up to the powerhead. This seal in turn squashes the thin walled metal pipe so that insufficient water can flow - hence the OK cooling at idle but overheats at full throttle

Not a major job, but it does require removal of the powerhead and then all is obvious.

The next step depends upon how bad it all is.

In our case we were able to just clean up the corrosion and used a punch to swell out the feed pipe again and then re-assembled it all with some extra sealant around the rubber bush.

All solved in a few hours and now it goes like a dream.

Good Luck
Hugh Mattos
www.explorermarine.co.uk