Lawn Mower Blowing Oil Out of Dipstick

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Lawn mower blowing oil out of the dipstick may happen as a result of blown head gasket, bad PCV valve, and improper gas/oil mixture. 

This will affect the functionality and operation of your mowing machine. This article shares 6 reasons why oil might blow out from your mower’s dipstick and troubleshooting tips to fix them.

Ready? Let’s dive right into it:

6 Reasons Why Oil Might Blow Out of Your Mower’s Dipstick


1.  Blown Head Gasket


This is a common problem that occurs in several lawn mower engines. This issue usually occurs between the cylinder and valve area, causing oil to blow out of the dipstick. 

Bad head gaskets also cause oil leaks, preventing the mower from operating with full power.

Fix: Run a cylinder leak-down test to help identify the area of the leak. You can also lap the head and block the area of the leak or else, this problem might occur again.

Signs and Symptoms of a Blown Lawn Mower Head Gasket

Here are some signs of a blown lawn mower head gasket:

  • Leaking oil

The location of the head gasket might vary based on the style and brand of the mower. To examine any signs of oil leak from the gasket, there will be a line of oil around where the two parts are held if it is leaking or blown.

  • Low Pressure

In some cases, your gasket might be faulty if it is not strong enough to keep the pressure of the combustion process.

  • Smoke From the Exhaust

This is one of the common problems that you may have with the combustion system. You will notice smoke from the exhaust because oil is filling the chamber where the fuel is supposed to fire.

2. Dysfunctional PCV Valve

If the PCV valve of your machine isn’t functioning properly, it will cause pressure build-up. This will make the oil blow out of the dipstick.

Fix: Replace the dysfunctional PCV valve with a new one.

3.  Blowby

If you have excessive air forcefully coming out of your dipstick, it is referred to as blowby. 

There are two reasons for this problem- worn rings on one or more cylinders and valves (intake and/or exhaust) not properly seated on the cylinder head. This will result in air/pneumatic pressure into the crankcase.

Fix: The best thing to do is to run a cylinder pressure check. It is also advisable to contact an expert to help you fix it.

4.  Improper Mixture of Gas and Oil

An improper mixture of gas and oil can make oil blow out your lawn mower’s dipstick. 

A grossly overfilled oil may make you experience the following conditions:

  • Poor Crankcase Ventilation

The PCV system usually makes provision to suck crankcase gasses into the intake and then re-burn them in the combustion chambers with the fuel/air mixture. Restricted ventilation in the crankcase will affect the operation of the lawn mower engine.

Fix: Inspect valve, hoses, and other components associated with the Positive Crankcase Ventilation.

  • Excessive Piston Ring Blowby

As we have explained earlier, excessive piston ring blowby will pressurize the crankcase, causing oil to blow out of the dipstick.

Fix: Replace the worn piston ring with a new one.

5. Clogged Breather

If your mower’s breather is clogged, it will cause oil to blow out of the dipstick. You need to fix this as soon as possible because it can blow out other gaskets that can be expensive to replace.

Fix: Check for a clogged breather and replace it with a new one.

6. Loose Cap

A lawn owner using John Deere 316 complained that this problem occurred on his machine due to a loose cap.

Fix: Check and tighten the dipstick cap to prevent this issue from occurring.

What Happens If You Overfill the Crankcase on a Lawn Mower?

The crankcase of lawn mowers is typically small. They often require less than one quart of oil. An overfilled crankcase pushes the excess oil in any direction the liquid can move.

In some cases, it may end up in the cylinder and it can also move into the carburetor and out through the air filter.

How Do I Avoid Overfilling My Lawn Mower with Oil?

The best way to prevent overfilling your mowing machine with oil is to add a little oil at a time. Inspect the oil dipstick to confirm whether the level of oil falls within the appropriate range or not.

Make sure that it is above the add mark and not above the full mark on the dipstick. After that, discard and replace the oil-soaked paper air filter.

Will a Lawn Mower Start with a Blown Head Gasket?

Weakened or blown head gaskets will prevent your mowing machine from operating at full power.

What is the Most Common Cause of an Oil Leak?

The common causes of oil leaks include worn out or improper seals, degraded engine gaskets, and leaks from the oil pan. 

In addition, this problem can also occur as a result of deteriorated valve cover gaskets or missing/loose oil pan drain plugs.

How Do I Know If My Crankcase is Bad?

To know if the crankcase is bad, here are some symptoms to take note of:

  • Decrease in engine performance
  • High Idle
  • Oil leaks.


We have outlined the common reasons why oil might blow out from the dipstick of your lawn mower.

We hope that our recommendations guide and help you solve the issues. If these problems persist, it is advisable to take the machine to a professional to help you fix it. It is also important to maintain your garden equipment properly.