Hello everyone, this is a follow-up to a post a few weeks ago called "water-wetter didn't work for me" Quick background, I have a '99 400 that I race enduros with.

My bike is still overheating and losing coolant intermittently. Some days it runs perfectly, with no coolant loss. But yesterday, at an enduro, it used over a quart of fluid that I had to keep filling along the trail. No external leaks, no trace of coolant in the oil and no smell of coolant out the exhaust. Here is what I have tried, not necessarily in this order and with different combos:

Replaced o-rings

50/50 with waterwetter

distilled with WW

Engine Ice with WW

Boyesen pump impeller

New rad. cap

Replaced head gasket and bolts

I have a WR catch bottle installed and I notice when it was running hot in the tight stuff that steam was billowing out of the upper hose of the bottle and I could hear it boiling away. Is it possible for anti-freeze to evaporate just like that? If so, should my bike be running hot enough to do it? I'm really at a loss, along with many of my knowledgable friends, about this situation. One more thing I want to try is the higher press. cap. But I'm concerned about blowing seals or anything like that. Any feedback(pros or cons) on the high press. cap would be great! If anyone could help me out with my nightmare, I would REALLY appreciate it! I'm losing sleep over it! Later!



You say you have been racing enduros. Are they on tight trails? Ones that do not let you get enough air flow past the radiators? If it is marginal, how is your riding style? On my street bike, moderatly modified with high compression pistons, etc. I had to change my riding style on hot days, or when there is not enough air passing past the oil cooler..... if you don't already, get one of those oil dip stick temp gauges. I got mine for my 426 from a place out of the magazines called XR's Only. It will not be something you will want to ride around with in all the time, but it helps you get a feel for exactly how hot your bike is running. Anyway on my street bike (with a digital temp gauge) you would be surprised to see how little things make a big difference in engine temperature. For example when coming up to a stop light, if I just pull in the clutch and use the brakes instead of down-shift to stop, the temp can go down as much as 10 degrees (F). In town on the side streets if I give it a little gas and then coast, the temp goes down. I know it does not sound like much, but a little bit here and a little bit there makes a difference. I am not sure of your exact riding style, but if you use alot of clutch coming out of corners or up hills, this could be building up termenous heat. The more time you can spend coasting (I know in a race that is probably the last thing on your mind) the cooler your engine will remain. Also riding in a higher gear might help also, less rpm, less heat build up. Other than that, if you have made sure you do not have a partially clogged cooling passage, I am out of ideas.

hope this helps


Do you have a lighting coil on your bike? If so then you may want to try one of those small radiator fans that they put on some on/off road bikes to see if that helps.

I have read that the 400 ran hot from the factory. When Yamaha made the 426, they added an extra core to the radiator. It doesn't take much radiator squishing to start the overheating problem. Have you checked your spark plug? An engine that is running too lean can create a lot of heat. A higher pressure cap will raise the boil-over temperature a little, so will using more anti-freeze. Read the labels, some anti-freezes have higher boil over points, especially when you mix them with 20% water instead of 50% water. I also have noticed that the oil lines are accessable enough that you could install an oil cooler. This is probably cheaper than buying 426 radiators!


Roswell, 2000 YZ426. Do you need to ride out of control to really know the limits?

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