Overheating problem

I am still waiting on my manual, so this is just something else I am stumped with.

I did some trail riding this morning with a few friends. One trail ended and instead of turning around, we chose to go thru some thick brush/thorns,etc.(Not smart) When we made it back to the trail, I stopped to wait on them and my bike started shooting antifreeze out from the bottom. There is a good size hole in the black line which goes down the left side of the frame(Front)and goes into the bottom guard. It looks like the hose comes out of the top of the motor(Near the carb, or on top of the carb) Is this a drain hose? I let it cool and it was fine for a while. My buddy fouled a plug and I tried to tow him out of the woods. My bike overheated again. I had to let a Honda tow him. NOT GOOD. Does the 426 overheat a lot? I just got the bike and never checked the radiator fluid. Any suggestions or advice will be greatly appreciated. You guys are a big help and I thank all of you......................................


God Bless

the yz will run hot if you let it iddle more than a couple of minutes. As for the coolant, are you sure it was coming from the hose one the left side of the bike? That sounds like the breather hose for the cylinder head and should not have coolant coming from there. If you are sure it is then I don't know what to tell ya..

If its not coming from there than I have something for you to check out...

There is a small weep hole on the bottom of the water pump cover that will spit out coolant if the bike gets too hot. when the coolant gets to hot and builds pressure it will vent out that weep hole rather than blow through one of the seals or gasket. I noticed the same problem and thought I had a bad water pump or gasket. I found that having too much coolant in the radiator will cause the same thing too.. leave the level about 1/2 inch lower than the radiator cap opening and don't let it sit at iddle too long and it should be ok.. I would check it carefully though...


Tom N.


This is normal, don’t worry about it. Like your car the radiator cap is set to release fluid when the system reaches a certain pressure, so if you fill your bike completely or spend a lot of time at low speeds you will end up venting some coolant. I think you could say that the 426 is a little prone to overheating at low speeds but will run all day long in very hot temp.s as long as sufficient air is reaching the radiators.

I’ve heard of some guys switching to a Kawasaki radiator cap with a higher release point but I don’t think it is a good idea to increase the pressure the cooling system has to deal with just to avoid a little mess.

I check my coolant before every ride and fill to about an inch below the top. It usually ends up loosing a cup or so during a long ride. If this bugs you the WR has a coolant retrieval system, I’m sure you could retrofit the appropriate parts or fashion one yourself.

Hope this helps!

Thanks guys. No, I'm not sure it's coming from the black hose. It was coming out from beneath the plate under the engine. I saw a hole in the black hose and thought it may have caused it. Is it ok if the breather tube has a hole in it? I'm guessing it is open at the end. Thanks again guys. I feel much better.


God Bless

Sounds normal, my 00 426 will push out fluid from the overflow hose after about 5 min of idling if I overfill the radiator. Keep the water level in the radiator 3/4" below the cap. If I keep the radiator as full as yamaha says in the manual it will spit the excess out just warming it up for an oil change. Hopefully this is your only problem.

Hi, I have some info that may help all 426 owners. I have extensive experience with this bike and can offer the following suggestions. First of all, The 426 does vent coolant on a regular basis. It should be coming out of the pink overflow hose ONLY! not out of the black vent hose or the waterpump weep hole. Coolant coming out of the weep hole is a sign of a bad waterpump seal and should be replaced immiediately. If you vent excessive coolant try switching to a radiator cap rated at 1.5 instead of the stock 1.1 cap. Always top off your radiator when you check it before your rides, it will find its own level quickly after starting your ride. Be sure that your radiator vanes are clear of all obstructions and aren't all bent up. Keep an eye on your water pipes, they are prone to cracking under extreme use and can leak coolant. Owners of bikes 99 and 98 should switch to WR radiators, they have an extra cooling vane for more volume. You could also switch to 00 and 01 radiators but you will also have to switch to 00-01 shrouds. This should cure most of your cooling problems. Good luck


Yamaha race team.

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