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Gator 6x4 with yanmar 3cyl diesel. It overheats and I found no coolant in it with alot of rust. So I replaced water pump, tested thermostat to be fine. And replaced fan temp switch that was malfunctioning. Now this thing is still overheating. Coolant is filled and bled and circulating properly, fan turns on as it should but eventually it still over heats. And my coolant isn't leaking externally and the level is staying where it's supposed to.
 

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I flushed everything till it ran clean and it's not leaking. Still think that's a possible issue?
Just because it isn't leaking doesn't mean it's cooling enough. You don't need to reduce coolant flow much to start losing ground. Add corrosion that reduces efficiency and there you go. I have three Yanmars and note hot days and hills make for a system that keeps up but with little headroom.
 

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is your fan running at full speed? Ex: bearing friction, wire damage or corroded connections. Fans take considerable current and wiring needs to be in great shape to run at rated speed.

could outside or radiator be dirty restricting air flow?

did you measure temperature at which thermostat actually opens or just know that it opens? I verify with a pot of water and candy thermometer.
Air can be difficult to remove. Elevating radiator much higher can help reduce air pockets. Squeezing hoses rapidly can get fluid moving and burp the system.
Good luck


Internally collapsed hose?
 

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Gator 6x4 with yanmar 3cyl diesel. It overheats and I found no coolant in it with alot of rust. So I replaced water pump, tested thermostat to be fine. And replaced fan temp switch that was malfunctioning. Now this thing is still overheating. Coolant is filled and bled and circulating properly, fan turns on as it should but eventually it still over heats. And my coolant isn't leaking externally and the level is staying where it's supposed to.
I had the same problem and finally tried replacing the head gasket, which worked.
 

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I’ve done the following in the past to check head gasket as a crude check.

With cold engine remove radiator cap. Start engine and look for bubbles in the coolant. It helps if the engine can be under some load. Example brake firmly on while increasing throttle position

There is a chemical test that can be performed using the coolant. Kit can be rented or purchased at most auto stores and along with harbor freight.

Considering the labor involved, I’d verify before replacing HG.

 
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