Why Is Coolant Coming Out The Overflow Tube?

Has coolant recently started peeing out the overflow tube on your dirt bike? Do you have to refill the radiator with fluid after every ride? If you’re lucky, it’s a cheap and simple fix. But if you choose to ignore the problem, it will only get worse. The two most common explanations for coolant coming out the overflow are: the bike is getting too hot, or there is too much pressure in the cooling system to hold the fluid.

Put A Cap In It!

The easiest problem to fix could be a faulty radiator cap. It may not be able to hold the pressure because it is worn or broken, allowing coolant to leak past it and out the overflow. Without spending any money, take a cap from another bike and put it on to see if that’s the problem. Ride around until the bike is warm, and if it doesn’t puke out any fluid, problem solved! If it does continue to spew coolant, then you know the radiator cap is not the problem.

Radiator Low On Fluid

Can’t Handle The Pressure?

Another common problem that will not only cause the bike to overheat, but will over-pressurize the system is a blown head gasket. It will bring engine temps up greatly, and possibly cause the coolant to boil over. Then the engine leaking compressed air into the system just blows it out the radiator because the cap cannot hold that much pressure. Before you install a new head gasket, though, you will want to check and make sure the cylinder and head are perfectly flat and not warped (usually caused by improperly torquing the bolts). If it is warped, you’ll want to take that to a machine shop and get it fixed. Putting a new gasket in with a warped cylinder/head will do you no good, so don’t ignore it!

Liquid AND Air Cooling

Radiators don’t cool the engine by themselves. They NEED air to keep cool as well, so if they’re not getting any air-flow, it’s probably going to overheat the engine. A common problem that beginner and trail-riders have is riding too slow (More on this Here). Also, if a radiator is all bent up or many of the fins are twisted out of shape, that will reduce air-flow to the radiators. You can usually tweak most of the fins back if you’re careful, but it’s best to get a radiator fixed by a professional if it is badly bent or smashed. To increase air-flow even more, you can get a Vented Front Fender, or make one yourself if you have the patience.

Check Your Propeller

A cause of overheating may be, although not as often, a broken impeller. If it’s cracked or part of it is broken off (yes, even one small piece) it won’t provide sufficient cooling to the bike. If none of the above are the problem, just pop off the water pump cover to see if the impeller is bad or if there’s corrosion. If there is damage, it’s probably going to be an expensive fix if you buy OEM, so you might as well buy a Boyesen Water Pump Kit for a little more and be done with it, for good.

If none of these are the solution to your dirt bike’s problem, then you have bigger issues. If that’s the case, check out my article Why Does My Dirt Bike Get So Hot?

-Tom Stark


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  • shawn` says:

    hey man just asking i left me yz250f to warm up so left it for a bit when i came back all water was on the ground what could of cause this and what damage would of this done.

  • Tom Stark says:

    How long did you leave it running for?

  • shawn` says:

    for about 4 mintues

  • Tom Stark says:

    Yeah, I usually let my 250f’s idle for 30-60 seconds before riding. Four-stroke motocross bikes get hot fast if they don’t have air-flow. Make sure the radiators are topped off with coolant, and check out my post “Why Do 4-Stroke Motocross Bikes Get Hot?” for more tips.

  • shawn` says:

    thanks man for all ur help i will have look

  • Tom Stark says:

    No problem, good luck!

  • Greg says:

    I’ve recently bought an 05 Kawasaki 450f and was told it was normal for some coolant to leak out of the overflow and to just keep topping it up. But after reading this im thinking there may be an issue?

  • Tom Stark says:

    First of all, 2006 was the first year of the KX450F. Sounds like it has the typical overheating problem that most early model Kawasaki four-strokes had. How long does it take to get this hot? I’d check the water pump. Most people buy an aftermarket water pump for these bikes for better cooling, and that usually fixes it.

  • Greg says:

    Probably after about 10 mins at idle it starts to drip, but sometimes can leave quite a pool of coolant on the floor. I’ll try the water pump then and see if it improves, thanks for the reply mate!

  • Tom Stark says:

    Yeah, and if you’re letting it idle for 10 minutes at a time, any four-stroke motocross bike is going to get hot.

  • Shane says:

    I recentally just got a 2002 yamaha wr250f and i just cleaned out the carb after its been sitting for a year and just started it up today and then i started to notice coolant coming out of the over flow and the pipe where is comes off the block right at the bend was starting to glow. but idk if its a blow head gasket or the water pump or whatever someone help me

  • Tom Stark says:

    How long does it take for it to go out the overflow? Does the head pipe get red hot?

  • Thomas says:

    Wow what a relief! I got a 2012 kx250f and had the same problem. I just changed the oil today and when I was done I turned my bike on and let it idle and revved it up a little and after about 5 min. of doing that coolant came out of the overflow. I thought I some how messed it up, and I’m not a very knowledgeable mechanic on dirt bikes so I was worried. After it did it the first time I let it cool down I checked the coolant level and it was fine so I did it again and same thing happened, I was going to take it to ridenow but after reading the comments I think it is just normal overheating from not riding it and getting airflow while it is on. But I’m gonna keep an eye on it next few times I ride and if it happens again I suppose I will have to take it to get looked at. Anyways thanks!!

  • Ben says:

    I have a rm250 and I’ve changed just about every gasket you can replace including the head, base, and now the water pump cover gasket. It’s still leaking coolant and I’ve also checked the radiator hoses no clogs or anything like that. So I’d like to know if there’s anything else I can do or if it’s a lost cause

  • Tom Stark says:

    Where is it leaking from? If it’s coming out of the overflow, you may need a new radiator cap. Try swapping one from another bike to see if it works. Otherwise your cylinder head may be warped, causing coolant to leak past the gasket.

  • Stephen says:

    I do a lot of trail riding on my 2006 CRF 450R. I am thinking its just my bike getting hot from not always moving fast but am curious if thats the only reason. My header pipe gets red hot where it comes out of the engine block so I am curious if that is a tell tale sign of something more??? Please help!

  • Tom Stark says:

    If you’re riding really slow in the woods then that is pretty common. If it gets worse, some things that can help are an aftermarket water pump, aftermarket larger radiators, or a different coolant mixture. Also, check your spark plug and make sure your bike is jetted correctly. A lean condition can burn the engine down. Otherwise just ride faster so your bike breathes more!

  • rich says:

    My bike is a 94 cr125 and when I start it up it runs for a minute then dies out..what could this be ? I’ve made adjustments to the carb although I’m not to sure what I’m doing, but seems to get the same reaction

  • […] Coolant – Pop the radiator cap off and check the coolant level. It should be at or near the top. If it’s low then the engine may have overheated and spit some coolant out the overflow tube. […]

  • Lucas says:

    My klx-650r is leaking out the overflow tube in the resoviour tank also coolant is fouling the plug but none in the crankcase

  • Matt says:

    My yz 426 has been leaking a little coolant less than an inch every ride. I just noticed her heating up faster then normal i jiggled the main radiator hose and found a pin sized leak. Small enough to hold fluid from leaking but big enough to cause the coolant to bubble. Everyone is so quick to cry head gasket.

  • Tom Stark says:

    I see your point, but this post is referring to coolant coming out the “Overflow Tube”, not just leaking coolant in general…

  • Tom Stark says:

    Sorry for the late response, but that sounds like a bad head gasket to me. Let me know what you find out…

  • Ron says:

    i have an 08kx250f and it used to spit coolant through the over flow tube I changed the cap and is spits less could it be boiling over cause I have tap water n not coolant in my radiator.? What else should I check

  • Marcos says:

    I have a 05′ YFZ 450. I just got a carb clean and put a bigger jet in it because it’s a 480 now and it keeps overheating regardless of how I ride. It boiled out a lot of my coolant so I had to throw in my water just too keep fluids in it so I could get back to camp. I know my bike runs lean from the factory but now it’s even more lean with the fact that it has a bigger piston and a smaller jet.

  • Tom Stark says:

    Are you putting around on it at low speeds for longer periods of time? What does the spark plug look like? The pilot jet may be too lean/small.

  • Marcos says:

    What is considered slow 2nd gear? becasuse the only way it wouldnt over heat is if i was in 4th or 5th hauling ass. The jet could be too small but its already had a bigger one put in it. Its a 480 now but im still confused. It didnt do this after it was rebuilt. I also haven’t flushed the coolant system so it could have had an airbubble.

  • Tom Stark says:

    An air bubble wouldn’t cause it to overheat like that. Did you already try topping it off with coolant and it still overheated? The 480 jet is the main jet, which only affects 3/4 to full throttle jetting. My guess is the pilot jet is too lean, and possibly the needle position. Have you looked at the spark plug yet? What color is it?

  • Kyle Mccauley says:

    I have a 2005 yzf250. Recently my bikes spewing radiator fluid out the over flow. it happens within seconds of starting and its gushing out. (not a drip.) I have changed the radiator cap but its still leaking. What could the problem be be? It still starts but only runs for a short while and then I cant start it again. Any help will be greatly appreciated!

  • Tom Stark says:

    That’s odd that it would occur within seconds of starting the bike. You tried a new cap, or at least one off of a used bike that works? When did it start doing this? Has anything been done to the engine?

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