ATV Coolant In Cylinder (This is why)

The coolant inside the cylinder is a pretty serious condition, and if ignored, could grenade the motor. But, the problem may not be as severe as it first appears.

The top 3 reasons for coolant in ATV cylinder include:

  1. Blown head-gasket
  2. Loose cylinder head bolts
  3. Cylinder head fault

In this post, you’ll learn about the most common causes for coolant inside your ATV cylinder, how to diagnose them and how to fix them.

1 Blown Head-gasket

A head gasket is a graphite material that’s sandwiched between your cylinder (Jug) and the cylinder head. Its function is to seal – seal the compression chamber, seal the coolant passages and seal the oil passages.

Head gaskets work super hard and are under extreme stress. It’s not surprising they’re a common failing component. Gaskets can fail in a few different ways.

Symptoms of a blown head gasket vary, depending on engine type and how the gasket fails. The coolant inside the cylinder suggests a failure of the gasket between the cylinder and the coolant passage.

This type of failure allows compression gasses to escape into the coolant system, which may present itself as coolant blowing from the rad cap or a hose pipe blowing off. Compression gases escaping into the coolant system can cause rad damage too.

This type of failure also allows the piston to suck coolant into the cylinder, where it tries to burn off the coolant with the fuel mix. This presents itself as white smoke and which smells sweet.

How to diagnose:

There are a few ways to diagnose a blown head gasket, a chemical test of the coolant system, a compression test, a coolant system test.

I like to use a leak-down test kit. This is a simple test and involves removing the spark plug, fitting an adaptor, and compressing the cylinder with air at TDC (Top Dead Center).

A pair of gauges help add and gauge pressure loss, but you’ll usually hear where the problem is. In our case, we’ll expect to see air coming from the radiator, so top up your rad and remove the rad cap.

The compressed air will cause the coolant to bubble up.

What causes head-gasket failure: Wear and tear, old age, old coolant, and overheating.

Overheating is one of the more usual and immediate causes of blow head-gaskets. Your bike depends on air rushing past the radiator to cool the coolant. Slower-paced trail riding leads to overheating, and that leads to lower coolant levels, which exacerbates the situation.

Not all bikes come with a rad fan as standard, so if you do a lot of trail riding and you notice your bike dripping coolant, consider fitting a rad fan.

Old coolant or using straight water in your coolant system causes corrosion, and that can lead to early gasket failure and some other problems, like:

  • Warped head
  • Frozen engine
  • Electrolysis
  • Corroded frost plugs
  • Damaged seal

How to fix:

The only fix here is to remove the cylinder head inspect all components, and replace the gasket. It is a job you can take care of yourself, but you’ll need to be mindful of timing the engine correctly when reassembling.

2 Loose Cylinder Head Bolts

As you already know, as gaskets age, they degrade, and cylinder bolts can become loose. Loose bolts will reduce friction between the head and cylinder.

And by loose, I don’t mean hand loose. I point out of specification.

Because the combustion chamber is under such pressure (about 200psi), it will find any weakness. Torque specifications are very particular to each manufacturer, and the sequence of tightening is particular too.

A new gasket must be replaced whenever the cylinder head is removed, and a torque wrench used to tighten the head bolts to spec.

Borrow or buy a torque wrench, get your engine torque specs and sequence and check your head bolts. You’ll likely need to remove some components to gain access to the bolts.

Re-torquing the head may not be successful. The correct fix is to replace the gasket.

3 Cylinder Head Fault

Your engine may look pretty solid, but in fact, it’s hollow and not as strong as it seems. Coolant moves around an engine in a circuit. The cylinder head and cylinder contain hollowed-out passages that allow the coolant flow.

If an engine overheats, the walls of the water jacket or head passages can develop hairline cracks which will allow coolant to enter the cylinder, either directly or through the intake valve.

This is the worst possible scenario as this type of damage can’t be repaired.
Another common failure is a warped cylinder head. The warping may be caused by overheating, broken head bolts, or incorrect torquing specification or sequence.

A warped head can be repaired, but it will need to go to a machine shop for analysis and skimming.

You may find these posts helpful:

ATV coolant in the oil

Riding bike without coolant

Two stroke overheating

John Cunningham

John Cunningham is a certified mechanic and writer on I’ve been a mechanic for over twenty-five years, I use my knowledge and experience to write articles that help fellow gear-heads with all aspects of ATV ownership, from maintenance & repair to troubleshooting.

Recent Posts