ATV Coolant In Oil (Mechanics advice)

Coolant in your ATV oil will grenade the motor, this is the kind of problem that needs immediate attention. But don’t panic, you’re in the right place, and very shortly you’ll have it figured out.

Top 3 reasons for coolant in ATV oil, includes:

  1. Blown head-gasket
  2. Failed water pump seal
  3. Leaking frost plug

In this post you’ll learn the top 3 reasons an ATV has coolant in the oil , how to diagnose them and how to fix it.

Blown Head-gasket

Head-gaskets fail all the time they work hard and are under tremendous pressure. What is a head gasket? It’s a graphite materiel sandwiched between your cylinder head and the piston sleeve.

It’s function is to create a seal between the water passage ways, the oil passage ways and to seal the combustion chamber so compression doesn’t leak.

What’s coolant passageways?

Your ATV engine creates a ton of heat and if it isn’t managed, the internal engine components would simply fuse together and seize.

Hollowed out passageways inside your cylinder head and sleeve carry cool coolant to and hot from the engine.

The hot coolant as you know is then transported to the radiator by the water-pump, where ambient air helps cool the fluid and the cycle begins again.

A head-gasket can fail in a few different ways and depending on the engine type and where the gasket blows will dictate the symptoms.

As your ATV suffers from coolant in the oil, and if the fault does indeed turn out to be the gasket. The gasket will have failed the between the coolant passage way and the oil passage way.

This isn’t the most common way for the head-gasket to fail, but it does happen. This type failure may be caused by old coolant or by using straight water.

The symptoms of coolant to oil passage gasket failure include:

  • Milky/tan or brown frothy oil
  • White scum on dipstick
  • Very high oil level
  • Low coolant
  • Unexplained coolant loss
  • White smoke

How to diagnose :

Your coolant system is a sealed pressurised system (1 bar/15psi). There are several ways to test for a failed head-gasket but for coolant in the oil, the best approach is to use a special coolant system pressure test kit.

Test kits aren’t expensive or have your local mechanic run this test.
Remove the cap and pressure the system using the tester, and check for leaks.

Remove the dipstick and listen for air escaping from the crankcase area, this is a strong indicator of gasket failure between the coolant and oil passages.

But as you’ll learn in the next section, it may not be the only reason you hear air escaping the the dipstick.

If you find your head-gasket has failed, it is a job you can take care of your self. You will need a torque wrench and a torque specifications, but it isn’t a hugely difficult procedure.

Over head cam engines will be more challenging, as incorrectly refitting a timing chain is easy to do and could potentially grenade the motor.

Failed Water Pump Seal

The water pump should really be called a coolant pump as it moves coolant. Anyhow, its job is to keep the coolant moving around the system. Cooled coolant is pumped to the engine and returns to the radiator to be cooled and so on.

The pump is usually mechanically driven by the engine. It’s positioned on the side of the motor. A shaft with an impeller on the water pump side passes through a seal and bearing to be driven from the crankcase side.

A failure of the water pump seal can cause coolant to enter the crankcase and mix with oil. However, if this is the problem, a weep of coolant should also be present at the bottom of the water pump housing, which should alert you to a problem.

The second possible failure of the water pump is model dependent. Some models use an o-ring or gasket to interface the pump housing with the engine. As the pump housing and side cover are integrated, a failure of this o-ring or gasket will allow coolant leak into the crankcase.

How to diagnose:

Coolant pumps wear out, just like brake pads, so if you own your atv long enough you’ll be replacing it.

If you water pump hasn’t been replaced in the last five years. Go ahead and replace the water pump seal, bearing and gaskets.

While replacing, inspect the impeller and shaft for wear and for evidence of failure. A worn bearing will cause the seal to fail.

Rebuilding the pump and replacing gaskets is worthwhile maintenance, even if it’s not the source of your leak.

Leaking Frost Plugs

Frost plugs are cup shaped metal components about the size of a dollar coin. They’re fitted to all water cooled motors and offer protection against freeze thaw action.

When coolant isn’t strong enough, it freezes and expands inside the engine pushing the soft metal frost plugs out of the engine.

Without frost plugs, the the forces are strong enough to break the engine apart.

Frost plugs don’t usually cause too much trouble. They’re press fitted into the cylinder head and block and are tight enough to create a seal. Common problems include corrosion and leaking.

Most frost plugs are fitted to the external of the motor, meaning if they leak you’ll know about. But some models fit frost plugs under the cam cover and a leaking frost plug here will cause coolant to migrate to the crankcase.

This is a rare condition, but I’ve met it a few times and worth checking before you pull the cylinder head, as you’ll have the cam cover off anyway.

ATV Coolant

Coolant contains anti-freezing agents and is often referred to as antifreeze, both terms are correct. Fresh coolant is important, it should be changed every three years (including thermostat) and antifreeze strength checked well before winter arrives.

As coolant ages it looses its strength, but also turns acidic and can actually eat your engine from the inside. Old coolant not only risks your engine from overheating and freezing but also risks corroding other coolant system components like:

  • Head-gasket
  • Frost plugs
  • Pumps
  • Thermostats
  • Rubber seals and gaskets
  • Internal engine water jackets

Coolant does a lot more than most ATV owners realise, your coolant is specially formulated and contains additives that:

  • Raise the boiling point
  • Lower freezing point
  • Protects internals from corrosion
  • Lubricates and protects rubber seals

John Cunningham

John Cunningham is an certified mechanic and writer on I’ve been a mechanic for over twenty 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.

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