I have a 1999 500 Sportsman. I have always had some minor issues where my coolant overflow bottle will overflow a little fluid when I plow snow. Now I am having an issue where it seems to have a ton of preasure in the bottle (it is expanding like it is ready to blow). My HOT light has been coming on here and there too, which has never happened in the past. I have never had a problem in the summer with overheating. My coolant level is good (between the min/max in the overflow bottle). My fan kicks on just fine. I am wondering if this could be caused by a stuck thermostat perhaps? How could all this preasure build up in that bottle? Any help would be much appreciated!

 

It sounds like your machine is indeed overheating, judging by your coolant reservoir “balloon.” There’s a vent in the cap on that bottle, yours may be plugged, but that isn’t causing your overheating problem. Your cooling system can build pressure in a couple ways. Most likely it’s due to excessive coolant heat. Far less likely is a blown head gasket may be pressurizing the cooling system by leaking exhaust gasses into it.

I had a thermostat go bad on a ’99 SP once. Like you, I was surprised to be having the problem in winter when it had been fine all summer. That’s the first place I’d look. If it isn’t the thermostat I’d suspect a dirty / plugged radiator, except you’d think that problem would have shown up in summer unless you did some deep mudding as the seasons changed.

 

Any concerns about running this with no thermostat? Mostly as a test to see if the problem is cured. I realize that it will run cooler, but should that bother me or harm anything?

 

i know in the shop we have a guage that u put on the radiator and start the car up and if exhaust is leaking from the head gasket to the cooling system the psi will bounce. this is a auto test so i dont know if it will work w/ wheelers good luck

 

I guess I wouldn’t worry too much about running it without the thermostat for diagnostic purposes. It’s not like you’re going out for a day of trail riding. Just keep an eye on it and see what happens. It may not tell you anything definite but it’ll add a few clues to your mystery.

You can test the thermostat in pan of water on the stove and a thermometer to see at what temp it opens, or just to see if it will open at all. Those stats aren’t cheap, if I remember right they’re around $50 or more.