Overheat Continued......saga

1998 Mercury Force 40HP 2 Stroke
Had it out yesterday...after checking the impeller which looks very good...
I believe the reason for alarm in my boat is because it is overheating. Here is why I think. With this motor the discharge is large hole on the back side of the motor. At idle speeds it is more of a spray. Normal for this motor.
At trolling speeds only...I overheat.
Folks I am unable to troll....or idle extended periods of time....how can this be resolved?

Check for a blown head gasket.------You checked the impeller and did not replace with factory new part ?----Why ?

How could it be a head gasket? It runs great!

As for the impeller...if it's not broke why fix it...you really think it could still be that? Maybe I should revisit it.

Another Question...could it be that the motor is trying to run to lean...meaning less oil also? I may need to visit the impellor again I guess also. It bounced back to it's normal shape though, no rot pieces missing...
The water here is 35 degrees...I don't understand why it is having a problem keeping cool...

Just because the impeller "looks" good doesn't mean it is.
How old is it? Pics of the impeller and the housing and wear plate after they've been cleaned up.

IF?? you troll a lot and run in shallow water?? the cooling passages could be clogged with sand/mud.
Pull the thermo and check the area behind it. Any corrosion/crud ??

The pic is of a motor that ran in shallow salt water and was never flushed after use
and over heated.
The passages were about 75% clogged.
Took me 2 hrs to pry the crud out.

You can test the temp sending unit.
You'll need a candy thermometer, water and a pan and analog meter.
Put it on the stove and turn on.
Hook up the meter and see when the sender closes the circuit.
If I remember your thermo is a 130d and the sender works at 145-150d

Or spend the $ and get a heat gun.
I spent about 16$ and have used it many. many times.

YOU ARE A WEALTH OF KNOWLEDGE!

That thermo is unrecognizable! I pulled mine and it opens fully at 107-108. Boiled water...poured in mug...it opened up. So fully opened at let's say 208-210 considering boiling point.
If water passage was partially blocked what is solution?

I will order new thermo and impeller or good measure....sounds like a good potential fix.

If the passages are partially blocked?
Then removing the head is the only way to access the passages.

If the thermo came out clean, then the passages are probably ok.

Pics when you take the impeller out.

Like the old adage, one picture can be worth a thousand words!!

I'm guessing my thermo has simply worn out but still opening under higher temp than it should.

They don't usually work like that.
I'd replace the impeller then maybe the thermo??

I'd still like to see pics?

This pics will have to wait till it's opened up again.

Tested therm old vs new side by side. New wins at 150 over the old opening 1/3 more than the old. I will be installing the new impeller also.

Good results.