How To Fix Dirt Bike Coolant Spewing Out Overflow Hose

Has coolant recently started coming 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 most common reasons for a dirt bike leaking coolant from the overflow tube are:

  • There is too much pressure in the cooling system to hold the fluid
  • Bad Head Gasket
  • Warped Cylinder head
  • Bad water pump/impeller
  • Bike is overheating from riding too slow
  • Jetting is off

Can you overfill dirt bike coolant?

The short answer? Yes, anything above the main body of the radiator is overfill. However, this won’t hurt anything in the cooling system. As the coolant heats up, the excess will come out the overflow and then it will stabilize below the maximum fill line.

What if my dirt bike has an overflow reservoir?

A reservoir comes on some trail bikes (especially 4 strokes) because they are more likely to overheat and have coolant expand out of the radiator. Instead of going out the overflow tube and onto the ground, the tube goes to the reservoir so that it can be recycled back to the radiator when it cools down.

If you fill the reservoir and put too much in it, then the excess will just stay in the reservoir. If there’s another overflow hose then it will go out that.

Bottom line – it’s very unlikely to cause any damage by overfilling.

Should I add coolant to the radiator or reservoir?

If the engine is still hot from running, don’t open the radiator cap. Add coolant to the reservoir if it’s hot. Otherwise, when the engine and radiator are cool, then you should add coolant directly in the radiator.

Put A Cap On 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.

Coolant Leaking Out Overflow Tube How To Fix Dirt Bike Coolant Spewing Out Overflow Hose
Radiator Low On Coolant

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.

Let The Cap Tell You

If you need to replace the radiator cap and want peace of mind in the future, install one with a temp sensor built in to it (Amazon). That way you can quickly check while out riding.


You don’t want to be way out in the mountains or woods and have your bike overheating on you without even knowing it. Click the image below to purchase a new radiator cap from Amazon.

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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 overflow because the cap cannot hold that much pressure.

Don’t Just Install A New Gasket!

Before you install a new head gasket, though, you need 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!

Straighten A Warped Head DIY Version

If you want to save money, find a perfectly flat piece of glass or metal and put some 320 or 640 grit sandpaper on it. Spray some oil on the paper and then rub your warped head back on forth. It might not take much. All you’re doing is cleaning up the surface until it’s completely been sanded and straight again.

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 technical trail-riders have is riding too slow for long periods of time.

Coolant Recovery Tank For Security

If you have a liquid-cooled dirt bike and do a lot of trail riding at low speeds then you’re bound to run into this issue of the radiator spewing out coolant onto the ground. Even if your bike is running fine otherwise then you’re still losing that coolant every time it gets hot. Instead of losing that coolant you could install a coolant recovery tank for your dirt bike (Amazon). It’s super cheap, easy to install, and it simply works.

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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 Water Pump 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.

Going to an aftermarket water pump impeller kit may help, but then you’d want to go with bigger radiators to compensate for the extra water circulation. The cooling system is kind of like an engine system. Everything has to work together in order to be efficient. Change one thing and the rest of the system doesn’t work the same.

If there’s damage to the water pump impeller, I suggest simply buying a new impeller and going on with your riding.

How to prevent coolant coming out overflow for Free

Maybe you don’t have any of these mechanical problems that are causing coolant to spew out the overflow. It could be something more simple. Click here for 3 free ways to prevent coolant coming out the overflow.


Kelley Fager

I help newer riders learn to safely ride with confidence by teaching basic techniques in a way that's easy to understand. Learn To Ride With Control Here

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