I have a 2006 Sportman 450 that flashed HOT after about 5 minutes of riding. The fan comes on early, followed by the check engine command 5 minutes later. I have replaced the ECM and the Thermo switch. The thermostat seems to be opening at 180-190 degrees. The water pump appears to be working sd well. Even when the HOT warning is displayed, the rediator is only luke warm (Hot on the intake side) and warm on the outflow side. The radiator cap is always cool.

 

Welcome to the forum! Jack the front end up with the tires off the ground, leave the radiator cap off and let the fan cycle on and off a couple times. Hopefully the problem is trapped air if you didn't purge the system after you replaced the thermo switch. Air in coolant system was a problem on the atvs along with faulty ecm modules,and it's not uncommon also to have a faulty radiator cap. Plus make sure the area behind the radiator around the fan is clean of any dirt/mud build up.Sometimes fans have to be remove to really clean the radiators up,plus sometimes the radiators have to be removed if it's mud caked to be able to clean properly. If none of this helps could be a faulty ecm right out of the box. Have had this happen before also. OPT

 

Thanks for the suggestion. I'm having the starter rebuilt right now because, as luck would have it, when I went to purge the system the machine wouldn't start. I had already removed the radiator and fan and cleaned the radiator out; that was step one. I guess it is possible I got air trapped when I tried to refill. I also thought about the radiator cap - how do you know if that is defective?

 

It's a 13 psi cap and available at almost all auto stores for about 6 bucks. It's already going on 7 years old so,if in doubt just replace it.Better than having it blow on the trail. One of the ways to tell is when you purge the system with the cap off and are positive all air is removed, fan is operating properly and has cycled on and off a couple times,radiator coolant is then topped off,replace the radiator cap, and fill the overflow bottle.Let the engine cool down. If the cap is faulty, coolant will bypass the cap seal,into the overflow bottle and spew out even before the engine has time to even get warm. Purge the air first,then see if the cap holds the seal. OPT

 

I think we got it - I took the termostat loose enough to bleed some air out when I first put some water in it. I then tightened everything down, got the front end elevated and then kept filling with the engine running. This time I could tell the thermostat opened as the water went down some as soon as the fan kicked in. I also had a guy tell me to drive it up and down a ditchbank a few times to shake any remaining air bubbles around enough to work them out. I was pretty sure it was working when the fan final shut off on its own. Thanks for your help!

 

When the fan cycles on and the coolant level in the radiator drops then most of the air is purged. Sometimes it takes a little more to get the rest.Be glad you don't have one of the older models with the return hose from the engine going back to the coolant filler neck!These are even tougher to bleed. Glad you solved the problem. OPT