My inlaws' Sprtsman 400 is overheating. I've eliminated the fan and fan switch as culprits, and the radiator is clean. It appears that the coolant isn't circulating when I run it with the cap off.

Is it possible that it's the thermostat, or is it pretty much a given that the water pump is bad?

If the thermostat is a possibility can I remove it and drop it in a pan of boiling water to see if it opens like you can a car thermostat?

Last question, if it's the water pump is this a job I should tackle myself in the backyard? I've done pretty much all the repairs on my own vehicles for 25 years but have never messed with an ATV. If it were mine I'd tear into just for the experience, but I don't want to start on this one unless I know I'm going to be able to finish it up in a day.

Couple of other things, we haven't noticed any leaking from the weep hole, and I've always though it ran a little warm so this could have been a progressive problem.

Any advice is greatly appreciated!

 

You can test the stat in boiling water. Mine also looked like the water was not flowing when looking down the rad filler hole.

Turned out the cooling system had a coolant air lock. I had to burp the system.

All I did was fill the rad slowly while the motor was running and shake the ATV at the same time,the coolant did a burp and then it was flowing normally again.

You know you have a air lock if the top rad hose is hard and hot while the bottom rad hose is soft and cooler. Both hoses should be hard and hot if the coolant is flowing OK.

Is this the older 2 stroke 400 sportsman or the 4 stroke 400 Sportsman?

 

It's a 4-stroke.

Thanks for the reply!

 

What year model?? If a 2005 model and hasn't had the ecm recall done or even if it has ,could be an ecm problem or the thermal sensor if there is no coolant loss?? OPT

 

I don't know the year, sorry. I'm confident that it's a circulation issue rather than an electrical issue, though, because when the engine is hot enough to boil over the radiator is still cool to the touch. The coolant sensor circuit will close when dropped in a pan of boiling water, but the coolant in the radiator just doesn't get anywhere near hot enough to close it. I've confirmed this with a continuity tester.

 

Then I would leave the radiator cap off as suggested to eliminate any air pockets,also rarely happens,but impellers have spun on the shaft and I've even had the impeller fins melted limiting flow(usually if coolant got real low).I've only replaced a couple thermostats over the years,but you can test it out as you mentioned,But I would still check if its under the recall.The ecm can do some crazy things.Plus have you checked that the radiator itself is clean especially around the back? OPT