About a month ago I finished up doing a head gasket on a 2002 sportsman 400. I took the head off and had it checked for warpage and any signs of cracks and bought a engine gasket set and proceeded to put it back together. Flushed the cooling system on the block and radiator and put a new thermostat in for good measure. Bled the system, started it up, let it run for about a half hour and the fan cycled 3 or 4 times and no leaks. Everything was good....or so I thought.

I had only driven the machine for a little bit, usually putting on a couple miles at a time. It would get up to temp, fan would cycle and never ran hot. No leaks at all to be found. My brother in law wanted it for a cabin four wheeler in the summer and to plow snow in the winter. He made me a deal so he took it home and put it in the garage. This weekend he brought it out to my campground. Drove it a bit but never really a lot to warm it up. We finally went on a decent ride where we got up to decent speeds and put more than a few miles on. We get back tithe campground and he has coolant bubbling out the overflow a little. My heart sank and I thought, "did I get another bad gasket set?". I ran into a winderosa set on a 400 two stroke motor I rebuilt that gave me fits till I replaced it with an OEM set and the problem disappeared.

I don't know if this is a fluke. I know I kinda over filled the overflow jug when I bled the system. It's a habit of mine when I change coolers on a vehicle when I fix them for damage. It always seems to draw in the excess once the engine cools and any excess air that is trapped gets out of the system. Maybe I just had the system bled completely and there was too much in the overflow jug. I know it was over the "max" line a bit. The other issue is the radiator cap. Could that cause coolant to be forced out if it was weak? I put my tester on it and pumped it up to 13 psi. It dropped to almost 10 psi in a little over 5 minutes.



The other thing I thought was the head was bad again so I tested the system. I pumped it up to 10 psi and let it sit. It sat for 4 hours and didn't drop a bit. I then tried it at 13 psi and let sit for another 4 hours and it dropped slightly, probably bot evennthe width of the needle on the gauge, so not even a half psi.



If the head gasket was leaking again, wouldn't it leak when pressurized? If it was forcing pressure into the system from the combustion chamber, wouldn't the pressure test do the same in reverse? The machine never runs hot and the fan kicks on and cycles for a few minutes and goes off.

I ordered a new radiator cap regardless. Is this just a combination of me having the overflow filled to much and the cap getting week or do I have cause for concern of a bad head gasket that didn't seal when I replaced it? I don't remember the name brand of the gasket set but I am sure it was either Cometic or Winderosa. Either way, if this is a gasket failure, I thinkni will stick with OEM gaskets months future.

Thanks,
Mike

 

If the system didn't drop while pressurized,then sounds like the cap is hopefully the problem. Just make sure you always bleed any possible trapped air from the 10mm bleed screw on the head also.

 

Thanks OPT,

I didn't realize the four strokes had a bleeder on the head. I just found the one on the water pump which I bled. I got the new cap today and put it on my tester and it actually held 15psi for quite awhile so I know it is good. I ran it for a bit and it seemed to hold pressure and never pumped any coolant into the overflow. I will take it out tomorrow and run it quite a bit to make sure everything is in order.

Thanks,
Mike

 

My fault again. When you said "400" I was thinking about all the 2 strokes you have in your stable. Yes the 4 strokes have the bleed screw at the w/p housing. Holding 15 psi sounds like you've solved the problem with the new cap.