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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 include:

  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, you’ll learn how to diagnose them and what you need to do to fix them.

1 Blown Head-gasket

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

ATV head gasket

Its function is to create a seal between the water passageways, the oil passageways 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 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 the coolant in the oil, at this stage, it would appear your head gasket has failed between the coolant passageway and the oil passageway.

This isn’t the most common way for the head-gasket to fail, but it does happen. The more usual way is coolant inside the cylinder.

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 pressurized system (1 bar/15psi). There are several ways to test for a failed head gasket. I like to use a leak-down tester but it is possible to use a coolant system test kit too.

Leakdown tester

The leak-down tester pressurizes the cylinder and checks for pressure loss over a given period of time.

However, a failed gasket usually makes itself known by the telltale sound of air from, and where the air leaks from is where the gasket has failed.

ATV coolant tester

To use a coolant system test kit remove the cap and pressure the system using the hand pump tester, and check for leaks.

It is similar to the leak-down tester in that the sound of air leaking is the indicator of where the gasket has failed.

As our coolant and oil are mixing we’ll expect to hear air leaking from the oil filter/ dipstick port. Go ahead and remove the dipstick and listen for air escaping from the crankcase area.

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

Twin cam ATV engine timing marks

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

2 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. The 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.

ATV coolant pump

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 to 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. Suppose your 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 allows play and that 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.

3 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 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 it. 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

ATV thermostat

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 the thermostat), and antifreeze strength checked well before winter arrives.

As coolant ages, it loses its strength and turns acidic, and can eat your engine from the inside. Old coolant not only risks your engine from the effects of 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 realize. 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

You may find the following posts helpful:

ATV coolant in the cylinder

Ride bike without coolant