I Just replaced the headgasket, mechanical water pump seal, and had my valves adjusted on my 500 sportsman. took it out today and it started over heating with coolant hissing out of the cap on the overflow resevoire. the fan does come on. Does anyone have any Idea's as to what I can do to fix this problem. I put alot of money into this atv over the winter and would love to enjoy it over the summer.

 

Did you jack up the machine at the front,leave the radiator cap off and let the fan cycle on and off several times to purge all the air from the system? If not,this could be the problem.

 

I had this happen. Did the head gasket and it was fine for a while. The next summer it started puking coolant from the overflow. It turned out that the plastic filler neck became warped and wasn't letting the cap seal properly letting pressure build and pushing coolant out. The filler neck is pretty cheap. I was at a lost so I brought it to my mechanic. He said it is fairly common especially if the head gasket was caused from overheating.