Overheating suggestions?

[Formulaz]

This is our first season with this boat. It's an 02(newer hull) 21ir with the 350 mpi mercruiser. Before winterizing I water tested the boat and everything worked perfectly.

While running the boat in the driveway this spring using a fake a lake it was running hot (approx 200 before I shut it off).
I immediately suspected a sticky thermostat and replaced it. Still having the same problem, I eventually undid one of the blue winterizing plugs at the stat housing and there was no water, then after a few seconds I had water flowing out and the temp seemed to be holding at approx 170.

Since I was set to take the boat up to the lake (for the season) that day and I was reasonably convinced that I had experienced some sort of airlock I went ahead with the launch.

Well, It still seems to be running hot (200ish). So after several attempts at solving the issue I have;

-replaced thermostat twice
-replaced impeller (old one was worn but intact)
-cleaned out inlet to transmission cooler (there were a few bits of old impeller in there)
-replaced serpentine belt
-ran engine with no thermostat briefly and it ran totally cold

So I'm heading up this weekend to;

-back flush entire cooling system
-verify temp with infrared thermometer
-remove exhaust risers to inspect for blockage

Anyone have any other suggestions?

I should mention that the connector on my fake a lake came off so I did initially run it with no water for approx 1-2 min before I figured it out.

Should I just replace the circulation pump also? Do these things ever fail? I'd rather have some gas money left for the season if I can help it!

[chpthril]

If your Merc has the composite impeller housing, they are prone to getting hot and getting out of round. This effects the flow. Pull the impeller and inspect the inside for blueing and scoring.

I should mention that the connector on my fake a lake came off so I did initially run it with no water for approx 1-2 min before I figured it out

If this was after the new impeller, get another new one

[Formulaz]

Thx chpthril, impeller was replaced after that and the housing didn't show those signs

[Joeprunc]

Looks like you've covered all the areas I would look at.

How long did you run the motor without a thermostat, and how long did it stay cold?

Could it be your water pump is going bad? I always thought of water pumps going bad due to bearings going out, but is it possible that the water pump is not able to provide enough pressure to push water past the thermostat opening?

When running is there a lack of pressure in the hoses coming off the thermostat? Is it any different without a thermostat in the housing?

I'm just trying to think out loud.

[Formulaz]

Yeah thanks, I'll check those things tomorrow.

The circ pump is the only thing left I can think of without it being something serious. (hairline crack, head gasket)

I'm holding out hope that the flush is what it needs!

[Formulaz]

Whew!

Turns out its a faulty temp sender (or gauge)

The infrared thermometer revealed the temp to be spot on.

Thank everyone for your help.

For what it's worth I did find another bit of impeller in the intake side of the transmission cooler.