Hello,
I have a 1988 Evinrude 150 I bought about a year ago. It keeps overheating and setting off the warning alarm. I had the thrermostats replaced took it once and it did it again. The mechanic replaced one of the thermostats again and said it was covered with a white powder almost as if it had been in salt water. I've never had it in salt water and as far as I know one has. I took it out yesterday and ran it for a short time and the alarm went off again, the temp gaugae read 175. The gauge does read about 25 degrees high all the time. I let it cool for a while took the cover off and ran it back to the dock, did okay until I idled in and it went off again.
Any ideals of what could be causing this? This is really taking the fun out of fishiing. Can't afford a new motor.
I'd appreciate any suggestions.

Forgot to say it does have good water pressue the impeller was changed last year just after I bought it.

sounds like the impeller...

could be the thermostat...

The impeller has been changed and the water pressure is good. The thermostats have both replaced, one has been replaced twice. Both times after the replacing the boat was run one time and it alarmed out again.

Could have a problem with the water deflectors around the cylinders. They are rubber tubes that direct the water flow in the right direction around thr cylinder but the only way to check them is to pull the heads...OPTI

I'm going to back opti on this one. An 88 model should have already had the diverter tubes replaced. If not, for absolute sure that would be the problem. The white/cream colored powder is not from saltwater but it is instead calcium from the fresh water lakes. We commonly call it "hard Water" it's the same as what builds up around your kitchen faucet. The clearer the water the more Lime/calcium you have.

Opti,
If those tubes are bad, would that cause the thermostat to stick?