Skip to Content

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? Whether it just started happening or it’s a regular problem, you’re in the right place!

In this article, I’ll cover the most common problems that cause coolant to come out the overflow on a dirt bike, why it happens, as well as how to fix it and prevent it from happening again.
If you’re lucky, it’s a cheap and simple fix. But if you choose to ignore the problem, it will only get worse.

Where is the coolant overflow hose on a dirt bike?

It’s generally located down the frame rail pointed down at the ground. You can follow the tube that comes out of the top of the radiator and follow it down your dirt bike frame for the exact location where coolant leaks out.

How to know if your dirt bike is leaking coolant out the overflow

Liquid-cooled dirt bikes have a hose that’s connected to one of the radiators (usually the right right) that is routed down the frame and drips down on the ground. When the coolant gets too hot or the radiator cap can’t contain the pressure, it blows past the seal on the cap and shoots out the overflow hose.

If you start seeing a liquid on the ground when your dirt bike is idling, there’s a good chance that it’s leaking coolant. Look for it coming out the hose – it will also have a “sweet smell” to it, which is quite a bit different than gas leaking.

YZ250F Dirt Bike Leaking Coolant Out The Overflow Hose How To Fix Dirt Bike Coolant Spewing Out Overflow Hose
Coolant leaking out the overflow hose comes out on the floor

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 it expands, and the excess will come out of the overflow and then it will stabilize below the maximum fill line.

A common symptom of coolant leaking out of your dirt bike overflow hose is that there's no visible coolant when you take the cap off.
Radiator low on coolant = potential problem

Can you fill the radiator with tap water?

Yes, water is actually one of the best “coolants” you can use to keep your engine running cooler. There are two slight problems that you must keep in mind.

Tap water is going to be more corrosive, especially if it’s not distilled water. The higher concentration or mixture you run of water vs antifreeze, the more likely it will be to corrode if you let your dirt bike sit for long periods of time.

The main problem with using straight water as coolant

The other problem is that water has a lower boiling point than antifreeze. This means that it will steam and push out of your radiator at a lower temperature.

That’s why most “coolants” are a 50/50 mix of antifreeze and water – a combination to keep it running cooler and prevent corrosion and boiling over, which leads to coolant coming out the radiator overflow.

Radiator keeps pushing water out?

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.

The engine leaking compressed air into the system just blows it out the radiator overflow because the cap cannot hold that much pressure. It doesn’t take much of a ‘flaw’ to have a poor seal on the cylinder head.

Cyl. Head 1 How To Fix Dirt Bike Coolant Spewing Out Overflow Hose
Blown head gasket from CR125

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.

Milky oil?

Maybe you noticed that your coolant level is down and noticed that your dirt bike’s engine oil is milky gray when changing it. That’s not a good sign, but it doesn’t mean there’s a major problem yet.

Milky White Oil How To Fix Dirt Bike Coolant Spewing Out Overflow Hose
Oil looks “milky” when mixed with water/coolant

Milky oil is usually caused by:

  • Coolant or water mixing with the oil
  • A bad head gasket
  • Cracked cylinder or engine case
  • Bad water pump seal

If you’ve changed the oil twice and it’s still milky, then you know there’s a serious problem that needs to be fixed ASAP or you might get stranded from an engine failure. If you’ve done all the simple things, then you will probably have to start taking the top-end apart or check the water pump system.

Coolant coming out overflow but not overheating

Maybe your dirt bike isn’t overheating, but it’s still pushing coolant out of the radiator overflow hose. This is usually caused by one of two things.

A bad radiator cap and a blown head gasket will push water/coolant out because there’s too much pressure. Once it leaks out too much coolant, it will eventually start to overheat.

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.

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.

Check your water pump impeller

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.

Should you upgrade your water pump impeller?

Going to an aftermarket water pump impeller kit (Motosport) may help, but then you’d want to go with bigger radiators to compensate for the extra water circulation.

Boyesen Supercooler Water Pump Kit Blue How To Fix Dirt Bike Coolant Spewing Out Overflow Hose

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.

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 hard enduro trail riders have is riding too slow for long periods of time. This builds up heat in your engine, especially if you have poor clutch control.

Colorado Trip 2018 Edit 22 How To Fix Dirt Bike Coolant Spewing Out Overflow Hose
Trail riding a CRF250X on a technical trail requires using the clutch more

Idling too long?

Maybe you’re new to modern 4 stroke dirt bikes and you started your bike up and let it idle for about 5 minutes to warm up. That’s great that you’re letting it properly warm up before riding, but this is too long for most liquid-cooled 4 stroke dirt bikes.

Not only can coolant leak out the overflow hose, but you might see the head pipe get red hot. The answer? Don’t let it idle for too long before riding.

Jetting too lean?

Another common, yet hidden cause is poor carburetor jetting. You see, when the jets are too small, they won’t allow enough fuel to get to the engine.

This causes a lean jetting condition, which makes your engine run hotter. That will result in coolant leaking out the overflow tube much quicker than if the jetting circuits were properly tuned.

Keihin PWK carburetor on a Honda CRF230F trail dirt bike
Dirt bike carbs are pretty simple, but they can cause a lot of headaches if you don’t pay attention.

This starts with getting the air screw properly set if you have a 2 stroke dirt bike, or the fuel mixture screw if you have a 4 stroke.

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 of 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.

An overflow tank recovers the coolant leaking out of your dirt bike radiator.
An aftermarket or DIY “catch can” can save a lot of coolant if your bike overheats

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.

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.

Is your radiator blocking air flow?

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.

Dirt Bike Radiator Fins Bent How To Fix Dirt Bike Coolant Spewing Out Overflow Hose
Radiator fins bent, blocking some air flow

To increase air-flow even more, you can get a vented front fender (Amazon), or make one yourself if you have the patience.

Coolant leaking out the weep hole

If you have coolant coming out of the weep hole by your water pump cover, this is a good sign that something needs to be fixed. It doesn’t mean there’s a major issue yet, but it’s a common sign that the water pump seal is failing.

When you use coolant that’s not aluminum-safe, it will corrode the aluminum more easily. This will degrade the seals, and then cause coolant to leak out the weep hole.

In this case, you’ll most likely have to remove the right side case cover (water-pump side of the engine) and replace the rubber water pump seal.

Bottom line: Why is coolant leaking out of your dirt bike?

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 poor clutch control at low speed
  • Jetting is off

How to prevent coolant coming out overflow for FREE

Coolant comes out the overflow when the fluid gets too hot, so you need to figure out why it’s getting too hot if there’s not a mechanical problem. Either there’s not enough water/coolant in the radiator, there’s mud covering the radiators blocking the airflow, or you have poor clutch control at low speed.

When you slip the clutch too much, the clutch plates spin and cause heat from the friction. Excessive heat is an engine’s worse enemy, so being more efficient with your clutch control is the easiest way to reduce heat and make your dirt bike last longer without having coolant leak out. Want to have better clutch control? Click here to get started with my free guide.


Thursday 10th of November 2022

Hey was just wondering if any one can help me I just recently had my Yz450f 2009 model fully rebuilt not even 20 hours ago I decided to change the radiator hoses and put new coolant in it well I have done that then I have gone to start my bike and took ages to start finally got it to go left radiator cap off for a bit to get rid of the air well after a while I put the cap back on stoped and tried to start it again and wouldn’t had to take the radiator cap off again to even start it then after all that I decided to check my oil and was milky what could be the problem cause I can’t see how that can happen by just changing hoses on a radiator any help would be appreciated heaps aye

Kelley Fager

Thursday 10th of November 2022

Hey Josh, sorry to hear about your issue. It doesn't sound like changing the hoses caused it. More than likely there's another problem, such as a bad head gasket, water pump seal or a cracked engine part, unfortunately. Let me know what you find out!


Friday 13th of May 2022

Hi everybody, I don't if you still answer. Anyway I have a crf 450r 2008 and the overflow leaks when I'm riding and when I get to a stop and I turn it off it leaks a lot more. Leaving a pool in the floor. I already change the water pump and I put a cap of 0.8 lb. Can I put a higher pressure cap like 1.6lb?

Any other solution?

Kelley Fager

Friday 13th of May 2022

Hey Luigi, sorry to hear about your bike problem. Is that 0.8 cap the original radiator cap? I believe that 1.1 is stock, and 1.6 would be fine. Just know that having a higher pressure cap allows the coolant to get a few degrees warmer before it leaks past the cap.

Al Anderson

Saturday 23rd of April 2022

Hi I have a 1992 Honda Shadow 1200. After riding for 10 mins steam shoots out of my antifreeze overflow line.Filled with antifreeze again same thing happens. Sounds like a tea kettle !!!

Kelley Fager

Saturday 23rd of April 2022

Sorry to hear about your issue, Al! Did you do anything to the bike before that? Have you tried a different radiator cap? Otherwise it could be a blown head gasket. I don't have as much experience with that bike.


Monday 2nd of December 2019

If got a kmt 450 2005 sx and just serviced it but when I leave it running got about 10 min it starts letting off loads of steam out the over flow can u help

Kelley Fager

Tuesday 31st of December 2019

Are you letting it sit and idle for 10 minutes? It needs air circulating through the radiators to cool it down.

Bobby Douglas

Tuesday 20th of August 2019

What would make the coolenf poot óut of the over flow when idialing