For what it's worth, I just had this exact same thing happen on my '99 diesel sportsman. It would boil the coolant over, fan wasn't turning on, so I figured maybe the thermostat, but what it ended up being was, on mine anyway, there's a flange nut that holds your water pump impeller tight on the shaft that drives it. Mine backed off, and the impeller no longer was spinning when the engine was running, causing the hot coolant to stay in the motor, and the cold coolant staying in the radiator, tricking the fan sensor into thinking all's well... well all was not well, and it damaged the pump shaft in the process, which is also your oil pump shaft and counter balancer all in one, at least on the diesel. That's where I'd look first, as long as you don't have any obstructions in your lines or radiator. Also, try taking you're thermostat out, and either try running it without and see if that fixes it, or just try putting it in a pot of water and start bringing it to a boil and see what temp it opens up at. Good luck!

 

Should have talked to you before buying the new sensor?
I will have a look at it as soon as I get some time and warms up a bit,hopefully I didn't damage anything, how can I tell if my pump shaft is damaged?

 

You'll know if your shaft is damaged, Just take the impeller cover off and you'll know right away. What happened with mine is the flange of the nut was rubbing on the threaded part of the shaft right in the middle of the threaded area ( the nut fell down in between two fins on the impeller). Instead of doing what any professional would do, since the threads were good on either side of the damage, I got a long joiner nut and drilled and tapped it to the right thread, and loctited the snot out of it and let it cure for a while.(in my case over a month, but that's cuz it was at the cabin) This way it bridged the damage, and still had plenty of solid thread engagement. Only reason I did this instead of replacing the shaft is 1) I'd have to split my crank case, and I wasn't that ambitious at that point, and 2) I can't get the shaft from polaris anymore, so I found a used one at a salvage yard to have on hand in case it happens again. As far as having a diesel wheeler, like any of the other guys on here with one would say, the speed and acceleration is enough to make a turtle beg for mercy, however, it'll tow your house up to 35 miles an hour. Last summer A good buddy of mine who's severely handicapped was at the cabin, so I put on a two up seat and we went out riding. I did the math, and there was right around 1400#'s sitting on those tires, and it had no problem out climbing any of my other buddys' machines. Best part is in roughly 60-70 miles of pretty much WOT riding, I filled up afterwards with 1.5 gallons of fuel:top: ...plus it's kinda fun to roll coal climbing hills or taking off from a dead stop!

 

So today I checked my thermostat,put it in water and heated it up and it opened so that's good.
Opened up water pump, impeller looks ok.......is the fins supposed to touch the inside of housing at all like in an outboard motor. Mine doesn't.
The nut was tight, if I pulled on my pull start the impeller turns. So everything looks like it's working.
Put everything back together,filled it up and same thing.....it boiled over without fan coming on. I unplugged the sensor and plugged old one in and put sensor in rad fill, 5 seconds in the hot water in rad and fan came on. So for some reason fan,sensor both work but not when plugged into rad.
Is there anyone on here that can chime in and give me some pointers.

 

Couple things to try , have you bled the radiator for trapped air ?
Try leaving the sensor bypassed and ride it - does it overheat with the fan constantly running ?

 

I have not bled radiator, how?
I did drive a short distance with sensor bipassed and didn't overheat.

 

Excerpt from another forum -

Jack up the front of the machine with tires off the ground,add coolant and let the fan cycle on and off about 3 times with the cap off.This should remove any trapped air pockets. Top off the radiator and overflow bottle.

 

Read the last couple of posts in this thread -

02 polaris sportsman 500 - ATVConnection.com ATV Enthusiast Community

 

Thanks for all your help,it was a damn air lock,who would have known this would cause me all these headaches. I borrowed a rad pressure tester tried it and it held good. Then I got an idea, fill the rad as much as I can then hook up pressure tester and pump it up. I did this a few times adding more water each time and opening thermostat housing to let air out until one time water came out. Then I started it with the fan on bipass running steady until I seen water starting to circulate and steaming. Plugged fan back in ran it for quite awhile letting fan kick on 3-4 times and it's perfect. Wish I knew this before buying a new sensor and taking the front end apart to get rad out. I noticed the rad cap doesn't really lock on like a car,it kind of just sits there but doesn't leak.
Is it ok to remove thermostat and run it without one for better circulation.

 

Thanks for all your help,it was a damn air lock,who would have known this would cause me all these headaches.

One person knew ... :smile