i have 01 wr250f and i keep losing coolant through the overflow (overflow tank removed). the bike is not over heating, but as soon as i start it coolant will just come out of the overflow drop by drop. it off the water pump cover and the impeller seems to be in good shape and it rotates with the kick starter. i have rode the bike without any problems, but i don't want to be on a long ride and lose all the coolandt. could it be i have a bad radiator cap?


could be the cap, good place to start. take your cap to auto shop and ask them to pressure test it, they should do for free. let me know. I was also thinking that there could be a block in the sys. if the coolant cant flow then it will try to escape somewhere else.

I had a similar problem. Turned out to be a blown head gasket, leaking cylinder pressure into the water jacket and pushing the coolant out. I was also getting a buildup of black stuff (carbon??) on the bottom cap seal, this would prevent the cap from sealing and coolant would continue to leak after the engine was turned off.

Get your cooling system pressure tested to see if thats whats causing it.

Hakka is more than likely correct. I had similar problems but ingored it and had to replace a warped cylinder and have the head milled

the reason you are losing coolant is because you need to keep the coolant overflow tank installed, it works just like an automotive system.when the motor is hot or even warm the coolant sits in that reservoir and when your motor cools down it then returns to the radiators.so if your reservoir isn't there your coolant will just drip onto the ground via the overflow tube and your motor will eventually seize because it got too hot.keep the reservoir installed.

not true. the yzf's dont have them and they dont seize!!!!

Levi, you are wrong, coolant does not return to the radiators when the engine cools down.

When the system overheats pressure forces the bottom seal of the radiator cap open, allowing coolant to exit out the overflow hole, which is located above the bottom cap seal.

When the engine cools this seal closes up, so no coolant can flow in or out.

The cap seal opens when the pressure in the raiators reaches 180psi, the overflow bottle is not pressurised - so its all one way traffic.

What is the point of having an overflow then?

It works like a car. When the coolant gets hot it expands, thus overflowing into the overflow. when the coolant cools down it shrinks and gets pulled back into the radiators.

Notice the marks on the overflow bottle. why would there be a low mark if the bottle just gets filled and never has any pulled out of it to refill the radiator.

Other bikes work diff. when they get hot they dump the coolant out never to be returned to the radiators, so you have to check the coolant level moore often.

Hakka writes:

coolant does not return to the radiators when the engine cools down

haaka, check your math on that one; it does indeed return.

when the pressure in the raiators reaches 180psi


jim aka wrooster

uh lets see the bike that guy was talking about was a WR250F and yes they do have overflow tanks under the rear fenders and yes the coolant does return to the radiators after the motor cools down and yes the motor can get too hot and sieze if there is no coolant in the cooling system and you continue to twist the throttle as hard as i do.


So how does the coolant get back in once the cap seal closes?

Read my first post about how I was losing coolant out of the overflow pipe, after each ride I had to add between 300-500ml of coolant to get the radiators back up to the correct level, the whole time my overflow bottle was full, on top of that my cap wasn't sealing properly because of the carbon buildup and it still wasn't drawing coolant back into the rads. (I sat there one day after a ride watching the coolant flow out waitng to see if it went back in. I also checked to make sure there wasnt any obstuction in the pipe from the bottle to the bottom of the bike that could cause probs.)

On other occasions when I have boiled the bike in tight trails and had to add colant to the radiators, despite the fact that there is plenty in the overflow bottle.

Wrooster, in reply to your comment check your math re: 180psi, from memory the cap is rated at 180psi - please explain what you mean here.

We check the math and find the standard radiator test pressure to be 180 kPa (kilo Pascals) which tranlates to 25.6 PSI used only when checking for leaks in the system. The radiator cap (provided it's in good working order) opens at 110 kPa (15.6 PSI). When the radiator system pressure exceeds 15.6 PSI coolant is expelled in to the overflow bottle. When the bike is cooling off, the pressure inside the cooling system drops creating a vacuum drawing coolant from the overflow bottle back into the radiator, thereby keeping it topped off. The valve in the radiator cap is a two way valve, same as in an automobile allowing for coolant to flow in both directions.

180 PSI would be like riding a two stage air compressor. :)

I could be wrong, but on a semi sealed system like the wr. The overflow tube should come from the overflow tank. The only need for the tank is added coolant. A Wr should cycle the fluid in and out of the tank during normal operation. If it ever got hot the tank should purg the fluid. On a yz it might spit during normal brake in, or after a fluid change but under normal conditions should hold the same level.

Just my understanding of a cooling system

bye the way I have never inspected where the overflow tube is located on a wr but I cant understand how it would be from the radiator.

compare it to an old car and a new on. My first car never had a tank and the one I have now does. either way if I am spitting fluid there might be a problem

I had my PSIs and KPAs mixed up, sorry.

So, the cap opens at 15.6 psi, allowing coolant to flow into the overflow bottle, i'm assuming the pressure cant go higher than 15.6 psi?? So as soon as the temp comes down slightly the cap seal closes, lets say at 15.5psi. The overflow outlet is located between the battom cap seal and the top cap seal. So as soon as the bottom seal closes no coolant can flow back into the radiator.



reread the tail end of the post by dmp437...

> The valve in the radiator cap is a two way valve, same

> as in an automobile allowing for coolant to flow in

> both directions.

jim aka the wrooster

Just had a look at the cap. The diameter of the bottom cap seal is about 10mm larger in diameter than the hole at the top of the radiator, so it can only open in one direction, upwards. A vacuum in the system cannot push the cap upwards.

give it up Hakka you're wrong. the coolant will flow back into your radiator from the overflow bottle. If it's not something is wrong with your cooling system. I had a hard time believing it myself. but while I do not exactly understand how it gets past the radiator cap back into the radiator, I have watched my overflow container fill up after heating my bike up and then then empty back to original level as the bike cools down. I would love to read an explanation of how this happens but trust me it does happen.

I might well be wrong but I want someone to explain how it works.

I've watched my bottle fill up and stay full.

I may be wrong now that i think abount it. but the mentality still may be right. sucks when cold spuses when hot. dumps on the ground on a yz and never ciezes. unles you neglacte it

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now