1999 Mercury 40 hp 2 stroke Overheat

[Scoggs4]

‘99 mercury 40 hp 2 stroke overheat alarm sounds after 5 mins of running. Has yet to sound when in gear WOT. Typically sounds only in idle after warmed up. Telltale has water coming out. I have replaced impeller (including housing/gaskets), tested and confirmed thermostat opens. Removed thermostat and ran, alarm sounded same as usual. Water comes out from top cylinder brass plug and bottom plug when tested. Will shoot block with infrared temp gun soon to confirm temp. What am I missing?

 

[QBhoy]

Sure it’s not a low oil alarm ?

 

[Scoggs4]

Deleted the oil tank/alarm

 

[Chris1956]

Compression leak can cause overheat, however, that usually occurs at high RPM. It still makes sense to take a compression test.

 

[Scoggs4]

Compression leak can cause overheat, however, that usually occurs at high RPM. It still makes sense to take a compression test.

I did a compression test, however, it was a harbor freight kit. They all showed 90 psi, then 80 once warmed up. I returned that kit and need to buy a brand name and retest.

 

[Chris1956]

As long as all cylinders are within 10% of each other, the actual compression value is not important.

Put a torque wrench on the head bolts and see that they are tight.

A couple of things, in case you do not know. Mercs will overheat at idle if the impeller is chewed up on it's bottom. However, they will usually not overheat at fast idle or any cruising RPM. You say you replaced it, so that shouldn't be an issue.

The thermostat controls cooling at idle and above until about 2000RPM. At that point the poppit valve opens cooling the engine.

From your description, neither of these are happening to you.

Is your idle pickup timing set properly? Not sure if this would cause overheat, but it is worth checking.

You might stick a finger in the thermostat hole and see how much scale and other stuff has built up in the motor. I doubt that is the issue, but also worth a check.

 

[Scoggs4]

Someone mentioned a facebook group it could be the gasket between power head and midsection letting exhaust gases mix with water? Inside where tstat is looked clear of buildup.