Pulled my themostat today -

I ordered the high pressure cap and themostat from Summers racing yesterday. So I pulled the stocker out today and sure enough it was locked open. There must be a 100% fail rate with that part. Is it still the same part in the 01 and 02 bikes?


It has to be a manufacturing defect. The pin should not have the opportunity to get sideways outisde of the crossbar. I saw this on mine as well after taking it out and heating it in boiling water several times.

The fix is to compress the crossbar down a little. This was not difficult to do. I tested it several times in boiling water to make sure the minor adjustment was working, keeping the pin inside the crossbar and allowing the thermostat to open completely.

What is the problem with leaving the thermostat "stuck open"? Mine is a 2000, and it has done this also.

thanks, LL.

Two points to consider:

1) A closed thermostat allows the engine to warm up quicker.

2) A stuck thermostat may not be fully open, reducing the ability of the system to circulate water/coolant.

If you are not having frequent overheating problems then the stuck thermostat is probably fully open.

One advantage of a water cooled motor is the stability of the operating temperatures. Imagine a temp probe in the cylinder head and the range of temps it would see depending on the conditions of the trail. This affects the way that motor runs. A funtioning themostat stabilizes the operating temps. A water cooled bike can be jetted much closer to optimum than a air cooled bike can. If you are just blasting across a desert wide open, a stuck open thermostat will work just fine.

Cool guys, thanks for the info, I actually had not considered those things.


If you run with the thermostat open, and/or not there at all, the coolant flows too fast thru the radiators, and defeats the cooling process by not staying in the radiator long enough to cool the fluid!

Why did you put a high-pressure cap on your bike? Was it overheating? I have read lots about the boil-over on the XR650; bottom line is that if jetted properly and uncorked it will not boil-over with the stock 1.1bar cap. A high-pressure cap puts lots of extra strain on the cooling system. Unless you race and can afford the repair bills I would not put a high-pressure cap on the bike.

Did anyone notice the Tecate 250? Xr650R's first through fifth. I'll bet every one of those bikes had a high pressure cap.

The high pressure cap actually does what it says, by allowing the temp to be a little higher before releasing the pressure into the overflow bottle. In normal riding conditions, it isn't recommended, but in hard racing conditions, it is needed. A coolant hose shouldn't blow with the higher one on, but if it does, then it was weak to begin with. Make sure to keep any oil away from the rubber hoses, and replace the factory clip-clamps with real hose clamps for a secure fit!


I have been running with the high pressure cap for about 1 1/2 years. I think it was one of the first things I bought. The first time I took the bike out new, I don't think I made it an hour before it puked coolant all over. Of course it was in the high 90's when I rode it. I put the cap on shortly there after and have only 1 or 2 instances of boil over which was from a long period of idle. I have about 3,000 miles on the bike now and changed the coolant 2 weeks ago. There are no signs of leaks anywhere. My thermostat seems to be fine and working. I run the bike in the So Cal desert and am often out there when the temps are high - die hard!

I remember in high school physics, heating water then adding pressure then applying a vacuum. That determined at what temp the water boiled. The higher the pressure the higher temp the water would boil.

By the way - if you are running the stock thermostat it is probably stuck open.

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