I have a 1987 88hp SPL Evinrude 2 stroke outboard (premix). I love this engine it seems pretty much bullet proof and has never let me down. I sometimes get an overheat alarm only at low idle. A faster idle, or trolling never have an issue. So far I have changed the entire waterpump including impeller, Thermostats, took water jackets off and cleaned them. Flushed entire water flow system with Rydlyme. Still get alarm at low idle, almost like low idle does not push enough water through it. I have not removed heads to clean them out for fear of breaking head bolts due to galvanic corrosion. I only run in fresh water, but prior owner may have run in salt many years ago. I purchased the boat and engine in 1994. This problem has puzzled me for a while now.

It is not a huge problem, I simply do not let it low idle for more than a couple seconds. I flush it on muffs or a water trough every couple months. If any of you 2 stroke mechanics or techs have an idea let me know. Thank You in advance.

I had a 225 Merc Optimax that would over heat at idle in just a few minutes.  I spent a lot of money on it and never got it fixed.  So I just made sure to not idle very long.  It was still going strong after 14 years when I traded it off.

Mine does that ! It’s the intake tube. I had a Buddy adjust it one time and it stopped for several years. It still does it on extremely rare occasions, where I idle at the lowest speed in gear for several minutes straight.

Learn something new every day...I did not know there was such a thing as an "overheat alarm."

 

My old 1970 Mercury 2 stroke's overheat alarm was the day I noticed water not peeing from the engine.

 

Better shut her down!

 

A cracked wire in the wiring harness can throw an alarm at idle because the excessive vibration at idle causes flex in the cracked wire. It doesn't happen at higher speeds because the engine runs smoother with less vibration. It happened to me on a 250 Pro XS. Hard one to diagnose.    

7 hours ago, DaubsNU1 said:

Learn something new every day...I did not know there was such a thing as an "overheat alarm."

 

My old 1970 Mercury 2 stroke's overheat alarm was the day I noticed water not peeing from the engine.

 

Better shut her down!

 

Mine will alarm while it’s peeing 

As mentioned, a number of things can be causing it.  If the motor has ever been run in saltwater, there could be a corrosion build up with the passages that's preventing enough water circulating at lower rpm.  I have pulled the heads off old crossflows that it was hard to recognize the water jackets in the block.  

Pulling just the top part of the head off really does nothing for diagnosing the water flow problems.  You pretty much have to pull the heads off, and doing that on an older motor, ESPECIALLY if it has ever been in saltwater can open up a whole can of worms you really don't want to get into.  Without experience with them, you could very easily make a good running motor junk. Broken head bolts, tops of cylinders corroded out to where they have to be built up with Devcon to fix the destroyed sealing surfaces, etc.

It is easy enough to see if it's a wire shorting out just by unplugging the sensor.  If you get the alarm with the sensor unplugged, then it's something in the wiring.

The water by pass is what you all refer to as peeing. The bye pass indicates the water pump is working and over heating indicates the thermostat hasn’t opened allowing water to cool the power head.

Tom