oil coming out of lawnmower exhaust

When you consider the complexity of a lawnmower engine, it is incredible that they can run consistently, with constant abuse for such long periods. The number of processes that work seamlessly at speeds of up to 96 times per second makes you realize how remarkable the engineering that goes into their manufacture is.

Three situations generally cause oil to come out of a lawnmower exhaust.

  1. The oil has been replaced or topped up, and the level is too high
  2. The operator tilted the lawnmower with the carburettor and air filter facing down
  3. The piston ring is worn or broken.

If oil comes out of the lawnmower exhaust, excessive motor oil has entered the top of the piston chamber and is being expelled through the exhaust valve.

There are reasons this could occur, and in most cases, it’s an easy situation to rectify.

Reasons Why Oil May Come Out Of A Lawnmowers Exhaust

There are several reasons why oil may come out of a lawnmower’s engine exhaust. The most common issues are simple to fix.

The leading causes are.

  1. The Oil Level Is Too High.
  2. The Operator Tilted The Lawn Mower The Wrong Way.
  3. The Piston Ring Is Worn.

The Oil Level Is Too High

If the lawnmower’s crankshaft has been filled past the dip stick’s maximum mark, you should rectify this immediately.

Overfilling a lawnmower oil tank can result in the oil being whipped into foam by the crankshaft, expanding it.

Four-stroke lawnmowers generally have a vent that draws air from the crankcase to the air filter box. If the oil is overfilled, the now expanded and aerated oil will travel to the carburetor via the crankcase vent.

It will result in a considerable amount of oil being drawn into the running engine through the inlet valve, and if the engine stays running, it will result in more smoke than you’d have thought possible

Excess motor oil will be forced out of the chamber when the exhaust valve opens because the oil enters the piston chamber from the top (through the inlet valve). It will cause oil to come out of the lawnmower exhaust.

In an extreme situation, too much oil can build up enough pressure in the crankcase to push out or deform valve seals and even the head gasket.

The Operator Tilted The Lawnmower The Wrong Way

The lawnmower being, so the carburetor faces down, the oil will flow through the crankcase breather into the carburetor.

Just as excess oil may enter the piston chamber through the inlet valve, the oil flowing to the carburetor will also do the same and be expelled through the exhaust valve.

It will cause oil to come out of the lawnmower exhaust.

The Piston Ring Is Worn

A worn piston ring may allow the oil splashed from the crankcase to the bottom of the piston chamber to move past the worn piston ring and into the top of the piston chamber.

Once the oil reaches this area, it will be expelled through the exhaust valve and come out of the lawnmower exhaust.

A piston ring will wear out for the following reasons.

The Engine Is Old And Tired

The engine is old and eventually requires a rebuild.

The Oil Is Not Changed Often Enough

The ring will wear if the oil is not regularly changed with the correct viscosity oil.

The Air Filter Is Dirty

If the air filter is dirty, dirty may enter the piston chamber and damage the ring.

The air intake is close to the dust, which is thrown up and often does not seal, resulting in a damaged ring.

The Lawn Mover Cooling Fins Are Blocked

A lawnmower engine will overheat if gunk builds up around the engine’s cooling fins, which may result in a damaged or broken ring.

It makes the cylinder and piston run at higher temperatures. The extra heat removes the temper of the rings, and they become soft.

The Valve Seals Are Damaged Or Worn

If the exhaust valve has become unseated, the engine may not entirely remove the exhaust gasses, and the motor could overheat, resulting in a damaged or broken piston ring.

The symptoms of a broken or damaged piston ring are.

  1. There is a loss of engine power.
  2. Large volumes of white smoke are blown out of the lawnmower exhaust.
  3. If a piston ring becomes worn or broken, the piston will start to knock against the cylinder wall.
  4. The lawnmower oil consumption increases.
  5. You may see oil come out of the lawnmower exhaust.
  6. A pressure below 70Psi will indicate a damaged piston ring if you have a compression gauge.

How Does A Lawnmower Use Oil?

The oil keeps the internal moving engine parts lubricated.

A lawnmower internal combustion engine works through 4 cycles.

  1. As the piston rises, the intake valve opens, and the carburetor sends the correct fuel/air mixture into the piston chamber via the intake valve.
  2. The valves close and seal the chamber, and the fuel/air mixture is compressed by the piston rising in the chamber.
  3. As the piston reaches the top, the coil sends a 20,000-to-30,000-volt impulse, making the spark plug ignite the fuel /air mixture, which causes an explosion, and the resulting gas expands and forces the piston down.
  4. The exhaust valve opens and forces the burnt exhaust gasses out.

The moving piston turns the lawnmower’s crankshaft, which rotates the mower blades.

This sequence happens at very high speeds and for long durations. The piston of a typical walk-behind lawnmower moves up and down more than 48 times every second.

The tolerances of the valves, piston rings, and piston chamber are measured in 0.001-inch increments.

Although removing the exhaust gasses via the exhaust valve reduces the temperature, without a continuous oil supply, the heat caused by the friction of the piston (with rings installed) moving up and down the cylinder 96 (48 up and 48 down) times per second, would cause the piston and rings to expand and seize against the piston wall.

How Does A Lawnmower Engine Move Oil Around?

To counter this part of the cycle involves the crankshaft collecting oil and sending it onto the walls of the piston chamber.

Modern lawnmowers use one of the following methods to distribute oil into the piston chamber.

  1. Splash
  2. Pressure
  3. Full Pressure

A splash system involves a spinning gear with paddles installed, immersed in oil that slings lubricant throughout the crankcase.

A pressure oil distribution system uses a more controlled oil pump to collect the oil and distribute it to the crankcase mechanically.

A full pressure system is only found in high-end, ride-on machines. The system works in the same way an automobile engine does, in that a controlled pump constantly distributes oil to the crankcase.

How Do You Stop Oil Coming Out Of A Lawnmower Exhaust? [Solution]

The following is the remedy for each of the three reasons oil may be coming out of the lawnmower’s exhaust.

The Oil Level Is Too High

If oil is coming out of the lawnmower exhaust because it is too high, it is essential to reduce the oil level immediately.

Because the change in temperature and pressures may have changed the oil’s viscosity, the best method is to fully drain the oil tank and refill it with the correct quantity of fresh oil.

The Operator The Lawnmower The Wrong Way

In most instances, leaving the lawnmower standing for a few hours will be sufficient time to drain the oil back into the crankcase where it originally came from.

Apart from a short period where the lawnmower produces clouds of smoke, the lawnmower is not damaged, and you can carry on.

The Piston Ring Is Worn

If the piston rings are worn, they will need to be changed sooner or later.

It is not hard to replace the piston ring yourself; however, if you are not confident of your ability, this is best left to a trained lawnmower technician.


Most gardeners have noticed oil coming out of their lawnmower’s exhausts at some time. Often it is caused by tilting the lawnmower, so the carburetor and air filter face down, or the oil tank has been overfilled.

If the lawnmower has been operated in very dusty environments and it is an old machine that has worked for many hours, it is possible that the piston ring is worn and needs to be replaced.

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