Why Is There Oil In My Lawn Mower Air Filter? (Explained)

Person opening side of lawn mower

For such a relatively small and low-cost item, the air filter on your lawn mower does a very important job.

It provides a defense against dirt and grime getting into your engine and potentially causing serious damage.

If you check the air filter regularly, good job!

If you are here because you have just checked it and you are asking, why is there oil in my lawn mower air filter, then good job again! 

Because that is the question we are going to answer today…

Why Is There Oil In My Lawn Mower Air Filter?

The most common cause of oil in a lawn more air filter is human error. Tipping the mower up when cleaning or maintaining it, or accidentally overfilling the oil sump. Sometimes though it can indicate an internal engine problem or a stuck needle valve in the carburetor. Always investigate the problem thoroughly and keep an eye on the filter to see if the problem reoccurs.

Reason #1: You Have Tipped It Over to Clean It

The most regular cause of oil coming out of an air filter on a lawn mower is simply because the mower has been tipped onto its side for cleaning or maintenance.

If you tip your mower with the air filter side to the ground it is quite simple, oil will overflow into the air filter.

To stop this happening always tip your mower with the air filter/carburetor side up.

Similarly be careful when you store your mower.

Many years ago I inadvertently stored my mower for the winter standing up on its two front wheels.

Guess what?

A few months later when I opened up the storage unit to get it out in the spring, the floor was covered in oil that had come out of the air filter.

If you have to store a mower standing up, do it with the front wheels in the air. Then the oil will drain to the back of the engine where there are no holes.

Reason #2: You Have Overfilled It With Oil

Another common cause of oil in the air filter of a lawn mower is that the oil crankcase has been overfilled.

Too much oil means extra pressure on the crankcase and the additional oil has to go somewhere.

When you do top up the oil sump it is important to use the oil recommended by the manufacturer, never use anything lighter than is suggested in a small engine.

Also, check the mower for any signs of an oil leak and check the oil filter itself, as that could need replacing.

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Reason #3: There Could Be An Internal Engine Problem

Person fixing lawn mower

Oil leaking through the air filter can be a sign of a problem with the engine.

Unfortunately, this is something that isn’t always easy to diagnose.

It could be a blown head gasket, a damaged cylinder or worn cylinder rings.

Sometimes, it could be wear and tear on the engine and a sign of blow-by past the rings

Blow-by is caused by worn valve guides and sees a mixture of air/fuel or gas bypassing the pistons and getting into the oil, which could leak out through the air filter.

A small engine specialist should be able to diagnose the problem and remedy it accordingly.

Reason #4: You Might Need a New Breather Valve

On the subject of valves, the problem might be that the crankcase breather valve needs replacing.

Before we quite rightly got more serious about emissions and their effect on the environment, the breather valve of a crankcase would release oil vapor into the atmosphere.

But now the crankcase breather valve must be vented back into the ‘clean air’ side of the engine.

If the valve is leaking then it will often leave an oily residue on the inside of the air filter.

So if you are noticing oil just on the inside of your air filter, that might be the issue.

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Reason #5: It Might Be An Issue With the Carburetor

If the carburetor float needle is not closing properly fuel will run straight through the carburetor and into the engine, where it will spill out via the air intake.

Dirt behind the needle valve can often be the problem, stopping it from closing properly.

Check your oil level and, if it is overfull and smells of petrol, this is almost certainly the problem.

You will either need to thoroughly clean the carburetor or rebuild/replace it and then change the oil.

Reason #6: The Air Filter Might Need Replacing

Person mowing lawn in sun

A bad or blocked air filter sees the carburetor trying to pull air from anywhere it can to compensate for the lack of air coming via the air filter.

Sometimes it will try and pull air from the tube that connects the carburetor to the crankcase.

If it does that oil will come with it, which further reduces air flow and makes the problem worse.

If this is the case a new air filter is in order.

Reason #7: It Might Be Normal

A light coating of oil is applied to foam air filters to help trap dust and particles.

Also, many small engines use the air filter to vent fumes containing oil vapor from the crankcase.

Over time these will accumulate with use, especially if the mower is regularly operated on a sloping surface, the weather has been changeable (ie going from cold to hot in a short space of time) or if the engine is starting to wear.

If it is just a splatter of oil, replace or clean the filter and keep an eye on it.

What Kind of Oil Is Best To Oil a Lawn Mower Air Filter?

Many of the foam air filters (note – not paper air filters), found in lawn mowers are designed to have oil in them to better capture dust and grass clippings.

There are special oils you can buy just for this job, but the truth is that any light oil will do when applied sparingly.

From my experience, most foam air filters suggest that 30-weight oil is used.

And when it comes to the air pre-filter follow your manufacturer’s guidelines. Some come oiled and some don’t.

I usually just wash the foam pre-filter often in standard dish detergent and water at regular intervals.

RELATED ===> What Happens If A Lawn Mower Has No Oil?

How To Clean Lawn Mower Paper Air Filter

If oil gets into a paper air filter, it is permanently clogged and needs to be replaced, it is as simple as that.

When it comes to cleaning the paper air filter found in many lawn mowers, then any cleaning that is done needs to be dry cleaning.

Even then I would recommend that the extent of your dry cleaning is lightly tapping it to knock any loose material off.

Some people recommend the use of compressed air, but even low air pressure can open up holes in the filter that allows bigger chunks of dirt or dust into the engine.

Trying to clean a paper air filter just to save a few bucks can end up costing you a lot more in the long-run.

Final Thoughts

Why Is There Oil In My Lawn Mower Filter infographic

There are a whole number of reasons that oil could be coming out of your lawn mower’s air filter.

Usually the most common causes are good old human error, typing the mower on its side to clean it or overfilling the crankcase with oil.

Sometimes it can hint at a more complicated problem though.

In the grand scheme of things air filters are relatively inexpensive, and I would always recommend simply replacing an oily lawn mower air filter rather than trying to clean it, especially if it is a paper air filter.

The air filter is an important part of your lawn mower and should be treated as such.

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