How To Service
the Cooling System on A Dirt Bike

When working properly, the cooling system on a dirt bike does an excellent job of efficiently keeping the engine cool and this is true whether the motorcycle is air cooled or liquid cooled although for a liquid cooled dirt bike, there several different components which all work together to provide efficient and relatively maintenance free cooling performance, yet these components do need routine maintenance and upkeep which we're going to go over below.

Whether the bike is air cooled or liquid cooled, a dirt bike demands periodic attention be brought towards the cooling system in order to keep the engine cooling properly. Routine maintenance can frequently prevent overheating or other engine problems, and this is true whether you're riding casually, or you're racing motocross. Pounding down an off-road trail or course, or if you're hucking gaps and working on new FMX tricks, the cooling system can't be neglected.

Remember This When Working On A Liquid Cooled Engine:

  • Use Extreme Care When Opening a Cooling System. In normal operation, the engine's coolant becomes very hot and is under pressure so hot coolant can easily burn someone if the cooling system is opened while hot and under pressure.
  • Most coolants are deadly poisonous to pets and wildlife, as well as humans so don't leave this stuff laying around in a pan, and never pour it into a container for storage that could be mistaken as a drinking container. (IE: Empty Gatorade or other drink bottles)
  • Only dispose of used coolant properly by taking it to a recycling center or contact your local auto repair shop or parts store as they will likely dispose of your used coolant free of charge.

On liquid cooled dirt bikes, there's a lot that's going on while keeping the bike cool in the heat of battle whereas an air cooled machine relies on cooling fins which are integral to the cylinder and cylinder head to dissipate heat away from the engine, a liquid cooled bike uses multiple parts all working together to provide a much more efficient and quiet running engine.

The Basics of keeping a liquid cooled dirt bike cool:
(In addition to the anti-freeze regulating freezing and boiling temperatures)

  • The coolant provides constant lubrication for the water pump seals.
  • Radiators are dispersing heat from the engine.
  • An Impeller is moving the engine's coolant through coolant passages within the engine and to the radiators where the engine's heat can be released.

Bottom line is that by maintaining the health of the cooling system components such as the radiator, cooling passages / jackets, hoses and impeller should leave you with one more way to get another advantage and have maximum cooling performance so here we go...

How to Inspect the Cooling System

Tips on Cleaning The Radiator(s)

As recommended in our article on washing a dirt bike, it's best when cleaning the dirt bike to use a lower pressure of water than that of a power washer around the radiators otherwise the radiator's core or tank seals could be damaged and have a resultant leak.

Beginning with the Exterior: It's really no secret that a liquid cooled dirt bike has a radiator or radiators placed in a rather vulnerable position being just behind the front wheel where flying rocks, roost and incoming sticks and branches are the norm and can puncture a radiator or cause a leak, so be sure after each ride to visually inspect the radiator's condition while also ensuring that the front of the radiator(s) are not covered or blocked with mud, leaves or any other debris that could obstruct air flow.

Additionally, you'll want to inspect the remainder of the cooling system looking for leaks which sometimes become evident during a ride by a smell of anti-freeze, as well as checking the condition of hoses for any irregularities such as bulging or other damage.

Once the engine has cooled it's also wise to check the coolant level as you want this to always remain full and clean with fresh coolant. If the coolant is constantly low, there may be an internal or external leak that will need to be addressed although if there is not an evident leak, you may find success in simply replacing the radiator cap.

Water is Running out of a Hole onto the Ground or My Oil Looks Milky

If you have noticed coolant pooling on the ground under the motorcycle, or if the oil is looking milky, it's highly likely that the water pump seals are worn out and the cooling system is in need of service such as replacement of the water pump shaft, seals and bearing although if the oil is milky, be sure to change the oil at this time as well.

Looking Under the Radiator Cap

With an engine that has cooled to a point where the radiator and engine can be comfortably touched and the hoses feel as if they are not under pressure as can be felt by gently squeezing the hose with your fingers, the radiator cap can be removed for inspection of the engine's coolant.

With the radiator cap removed, you'll want to look into the filler neck at the condition and level of the coolant. If the coolant looks like it did when it was poured in then it's probably alright, although if the coolant has not been changed in quite some time, you may want to see the article below on changing the coolant as the coolant does break down over time and it's anti-corrosive and cooling properties do become diminished over time.

If the coolant doesn't look clean, and you notice any dark substances such as oil or black residue floating in the water, this may indicate a leaking head gasket or water pump seal which will need to be addressed. If the coolant looks a reddish brown, this may indicate that the cooling system has not been cared for and is in need of service.

With the cap off, it's also a good time to check the flow of the coolant and this can be done with the coolant level filled to just above the cores of the radiator and then starting the motorcycle and revving the engine slightly while looking into the filler neck at which time you should notice the coolant circulating.

Inspecting the Cooling System From the Inside Out

If you've had the bike for a while, recently purchased a used dirt bike, or are having overheating problems, be sure to drain the coolant and remove the water pump cover to inspect the water pump impeller and housing for any signs of corrosion or other deterioration, while also checking to ensure the water pump bearing is not worn by holding the impeller with your fingers and feeling for any up or down movement. If any movement is felt when attempting to move the impeller up or down, the waterpump bearing is likely worn and the bearing, shaft and seals should be replaced as a set.

Lastly, be sure to hold the impeller and attempt to turn the shaft by hand. If the impeller turns at all without the engine turning, this would indicate that either the impeller or shaft splines are stripped or that the water pump is driven by a plastic gear which may be melted or damaged, all of which need attention.

How do I Replace the Coolant in My Dirt Bike?

Picture of A Technician Removing the Coolant Drain Bolt

It's important to remember that engine coolant has properties at work which help regulate the boiling and freezing points of the engine's coolant, as well as the coolant possessing other properties to help prevent oxidation and corrosion of high dollar aluminum parts from within.

Additionally, considering that coolant breaks down, the coolant should be replaced periodically per the manufacturers recommendations which can be found in a factory service manual specific to the motorcycle being serviced, or on the label which accompanied the coolant that is currently being used.

Once you're committed to replacing the coolant here's how:

A Tip
Trapped Air

When refilling the engine coolant, be sure to inspect the motorcycle for any type of bleeder on the cylinder head or radiator. If there is a bleeder, it's critical that the trapped air is allowed to escape during filling otherwise the engine will likely develop an overheating condition caused by trapped air.

  • With the engine cool and the radiator cap still tight, hold a drain pan at an angle underneath the motorcycle and below the water pump, then remove the coolant drain bolt.
  • With the drain bolt removed and the coolant coming out in a trickle, slowly start to loosen the radiator cap while positioning the drain pan so as to catch the coolant as it will begin to rush out and you'll find that it is necessary to angle and move the drain pan so as to catch the coolant and not make a mess.
  • Once the coolant has finished draining, reinstall the drain bolt and if using the same coolant as before, refill the coolant to just above the core, then start the bike and monitor the coolant flow and level.

When Changing Coolants such as when changing from OEM to an aftermarket coolant such as Engine Ice, Spectro or Water Wetter, be sure to flush the cooling system with clean, distilled water before refilling.

Once the coolant level is full, and any trapped air has been released, replace the radiator cap and you're ready to go.

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