Thermostat Troubles....every year

So every spring I have owned this (97 sport GT 40) boat the thermostat seems to give me problems. I always just replaced it since it was cheap and things worked fine the rest of the year. This spring I thought I would just beat the problem to the punch and put a new one in as a normal part of maintenance. I had a few other issues out the gate this year but running it with the bucket in the yard and at the lake last week the temperature wasn't one of them. Today I all of the sudden have temp problems. So I pull the thermostat and everything cools right down when I run it without the thermostat. Put it back in and things heat up. This was a brand new thermostat.   I am beginning to wonder if I have a different issue don't have a weak raw water pump or something. I did not replace the impeller this year but I did pull it out of the pump last fall and put it in a bag.   Is there any reason why water would have trouble going through the thermostat? I'm going to put a new impeller and yet another thermostat in just for good measure and see what happens but hoping someone has some ideas.

I have tried both pcm brand from nautique parts and one from the local parts store and both seem to give me trouble. I just ordered another one from NP.

At speed

You might want to test the 'stat in a pan of water on the stove top. Use a thermometer to see what temp it opens up at.

My buddy has an I/O on which has same problem. He took it to a mech who replaced outdrive impeller (which didn't help) & then told my buddy he didn't want to work on it any further (is why I got involved). I searched online & saw that blocked exhaust manifolds/ risers is a common cause for high speed overheat.

I took off 1 riser & it was like new, so no cigar there.

We back flushed thru the hose leading to riser & that did reduce the problem considerably, so that points to sand buildup in system somewhere.

Good luck.

I had a weird overheat issue with my 79 tique 351. It would be cool as a cucumber up to 35 mph then heat up at barefoot speed. I thought it was a pickup problem because it was cool the rest of the time. Turned out to be a bad mechanical seal in the raw water pump. It would leak air which would collect at the top of the intake manifold causing high temp reading. I used an infrared thermometer to keep an eye on different parts of the motor. A Major RWP rebuild kit fixed it, now it runs cool up to 50 mph.

It doesn't heat up at speed without the thermostat in.

Hollywood wrote: I would be testing the RWP performance

x2 What he said

Not thread jacking hopefully--can I replace the T-stat without removing the hoses from the housing on my GT-40? Is there enough slack to lift the housing still attached to the hoses? Mine are stuck on and i just want to get the stat in since mine is stuck open.

Tryathlete: I was able to get mine out without removing the hoses.

I don't think the gauge or the sender on mine is wrong since the water was hot when I pulled out the thermostat plus I replaced the sending unit on mine last year before I figured out I had a bad thermostat at that time.

I am going to go ahead and order a new water pump. Pricey but I am a little suspect of it anyway and it has been making some noise lately. Will get the new stat and pump on with new impeller and see what happens. Might be overkill but I have some church camps I am committed to in early July and don't have a lot of time to be messing with things,

Hollywood--one side removal. This was key! Starboard side came off easily.

Not in brackish water. (Montana). I did see the rebuild on eBay. I Winterize by draining the engine. then sucking marine coolant through and then draining it again. Not sure if the coolant could be causing problems but again put a new stat in as part of the dewinterizing process.

Me too!
I wonder if maybe it's getting hung up on the housing some how. Going to look at that. i know that they have changed the design of the Ford Thermostats in the last few years and you can't hardly find one like the original. Even the oem ones from PCM have a different design.

tryathlete wrote: I would call and inquire what the "rebuild" includes. Ebay water pump

Cut and paste from ad,

PRODUCT DESCRIPTION: OEM Sherwood G20 / G21 Raw Water Sea Pump. Disassembled / Media Blasted / Inspected / New Parts Installed / Painted / Tested. New bearings, SS bolts, impeller, o-ring, gasket, mechanical seal and retaining rings are installed. Grooved wear plates are resurfaced or replaced. The pumps are painted Black and the body is left brass. They are tested and lubricated with glycerin impeller lube.

So I took the boat out today with the new raw water pump PCM Thermostat. Seemed to run fine for a while then it heated up suddenly again.   I pulled it and it cooled right down. Put it back in and it ran fine for a while right at temperature and then suddenly heated up again. I don't understand why it runs fine without it.   I took it home and tested it in water. It opens very wide at 190 and about 1/4" at 170 and not at all at 160.
Maybe I have bigger issues like a cracked head but I am at a total loss right now.   

I did pull one of the plugs and started the engine and revved it and water sprayed out.   According to Skidim that is supposed to be how to test if the circulation pump is working or not.