1GM10 - thermostat

[Drinky Crow]

Here is an image of my thermostat heated up to fully extend "flange" (closing water flow from bypass).
Note piston at rear of thermostat is still extended.
In this config is thermostat allowing water to pass from water jacket into wet exhaust?
Asking because at room temp I have to fully compress piston flush with bottom of cylinder for the "flange" to extend as in image.
Run at 1800rpm engine overheat alarm will sound after 20-30mins and no water discharge can be observed from wet exhaust.
Removing thermostat, clamping bypass hose, water will circulate freely through engine and wet exhaust.
All intake hoses wet exhaust passages from seacock to transom exit are clear of any obstructions.
Impeller is new functions properly.

1st pic is thermo at ~ operating temp. Went full open at 63C. Manual say 71C to start 85C running temp. 2nd pic is Frankin exhaust created by prior owner which will shortly be replaced by standard elbow. FWIW the Frankin has worked fine for 6 yrs.

 

[Alan Gomes]

If you look at a flow diagram for the cooling on the 1GM/1GM10, you'll see that water should always be flowing to and out the exhaust elbow. When the engine is cold, the water flows straight from the raw water pump and dumps it directly into the exhaust. As the engine heats, the thermostat then routes the water through the engine block passages and from there into the exhaust. Either way, water should be coming out of the exhaust.

 

[Benny17441]

Here is an image of my thermostat heated up to fully extend "flange" (closing water flow from bypass).
Note piston at rear of thermostat is still extended.
In this config is thermostat allowing water to pass from water jacket into wet exhaust?
Asking because at room temp I have to fully compress piston flush with bottom of cylinder for the "flange" to extend as in image.
Run at 1800rpm engine overheat alarm will sound after 20-30mins and no water discharge can be observed from wet exhaust.
Removing thermostat, clamping bypass hose, water will circulate freely through engine and wet exhaust.
All intake hoses wet exhaust passages from seacock to transom exit are clear of any obstructions.
Impeller is new functions properly.

1st pic is thermo at ~ operating temp. Went full open at 63C. Manual say 71C to start 85C running temp. 2nd pic is Frankin exhaust created by prior owner which will shortly be replaced by standard elbow. FWIW the Frankin has worked fine for 6 yrs.

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Sounds like the problem could be some obstruction blocking an adequate flow of water, Broken impeller vanes are likely the most probable obstructions found.