Will Too Much Oil REALLY Hurt A Lawn Mower Engine? (Explained)

Man holding oil tube

Oil and gas are two essentials for every lawn mower.

A mower won’t function without either.

We are often reminded of the dangers of too little oil in a mower, but what about the opposite?

Will too much oil hurt a lawn mower engine?

Well, let’s find out…

Will Too Much Oil Hurt A Lawn Mower Engine?

Yes too much oil can hurt a lawn mower engine. It all depends on how much extra oil you have added to the tank, but the more you overfill a mower with oil the more likely it is to cause permanent or even irreparable damage to the engine.

What Happens If There Is Too Much Oil In A Lawn Mower?

Using too much oil will have negative effects on your mower. Damage could range from mild and temporary to significant and permanent.

It all depends on how much extra oil you have put in and how long you use the mower without realizing it.

I have overfilled a mower with oil before and got away without it causing any real damage.

Mainly because I only overfilled it fractionally, so the mower ran without any major issues.

Adding too much oil to a 2-stroke mower engine is less of an issue than adding too much oil to a 4-stroke mower engine.

You will most probably see a white plume of smoke coming from the exhaust and end up with a spark plug that short circuits.

It is easier to resolve too. Usually, you can just add non-mixed fuel into the tank to dilute it.

But most mowers these days have 4-stroke engines.

So what happens if you put too much oil in a 4-stroke engine?

Slightly overfilling a 4-stroke engine with oil? You should be ok.

Excessively overfilling a 4-stroke engine with oil? You could see some very serious problems…

And as ever we like to look at the worst-case scenario here at Suave Yards!

So let’s spotlight what some of these problems could be…

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#1: It Will Stop Vital Parts of the Mower from Being Lubricated

Lawn Mower viewed from behind with shed in background

It might seem counter-intuitive that one of the main problems of overfilling your mower with oil might be reduced lubrication inside the engine, but that is exactly what could happen.

Four-stroke mower engines have an oil pan at the bottom of the engine along with an oil pump that lubricates the various parts of the engine.

The pump simply takes oil from the bottom of the pan and circulates it around the essential parts of the engine.

Then gravity comes into play and the oil trickles its way back to the pan at the bottom of the engine.

But when the oil level is too high the crankshaft will dip too much into the oil and the oil will be whipped up into a foam.

Any liquid oil that is pumped from the bottom of the pan will trickle down back onto the top of the foamy oil.

Eventually, the pump has no liquid oil to circulate, only the air-filled foamy oil.

So sufficient oil will not be dispersed around the engine, and it won’t be properly lubricated.

This can ruin crankshaft bearings and, at worst, lead to terminal engine failure.

#2: It Could Blow the Gaskets and Seals

Oil will heat up and expand as your engine runs.

So if you have too much oil in your lawn mower, as it expands it will naturally need somewhere to go.

Pressure could build up on the crankcase, and oil could leak into the carburetor, destroy your pistons and push out the gasket and seals.

The end result is either a rather large repair bill or a mower damaged completely beyond repair.

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#3: It Could Hydrolock the Engine

Too much oil could also cause a hydraulic lock.

This is where oil seeps into the air space in the combustion chamber. So instead of this area only having air and a small amount of fuel in it, it now has oil as well.

The pistons in the chamber cannot compress oil as they do air, and it denies them sufficient space to move in.

The engine may still turn over when you try to start it, but it is hydro-locked and won’t start.

Continually attempting to start it will make any internal damage worse and could cause rings to break and gaskets to blow.

#4: It Could Foul the Spark Plug

In the above instance, oil in the combustion chamber can also have another unwanted effect.

The poor combustion as a result of excess oil in the chamber will see your little lawn mower belching a lot of smoke.

More than you might imagine possible for such a small engine.

Apart from your mower not working properly the other result is a fouled spark plug.

#5: The Engine Will Overheat With (Potentially) Drastic Consequences

With too much oil you are effectively overloading many different parts of the mower.

This can lead to the engine getting hotter and hotter as the crankshaft and rod attempt to push through more oil.

The excessive loads and the heat can then even bend and break the rods and crankshaft.

At the same time, all of the internal parts of the engine will be under extra pressure, so a whole host of damage could be caused.

Symptoms of Too Much Oil in a Lawn Mower

If you have put far too much oil in your lawn mower, it will usually become obvious pretty quickly.

Usually you will notice your mower is either very difficult to start or, it starts up but feels very underpowered or like it is misfiring.

Once it is going, white smoke being emitted from the exhaust is a sure sign something is wrong! Often this could be too much oil.

If you notice oil in the carburetor, or on the air filter, or leaking from other parts of the machine then you undoubtedly have too much oil in the mower.

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What Should You Do If You Have Overfilled Your Mower With Oil?

Quite simply, remove the oil!

Let me clarify, if it is only a marginal overfill it shouldn’t be a huge problem, but if you have filled the tank way beyond the recommended level you need to rectify it as soon as possible.

Or you could well experience some of the problems above!

If your mower has a drain plug, just open that and remove the required amount of oil.

If there is no drain plug, the oil has to go out the way it came in. So carefully put your engine on its side and let it drain out slowly.

Another inexpensive solution is to use a cheap turkey baster to suck out the oil.

But the main thing is to remove excessive oil overfills before you start the mower.

If you can do that, then just clean the mower up and it should be good to go again.

Final Thoughts

Will Too Much Oil Hurt a A Lawn Mower Engine infographic

Too much oil can and will hurt a lawn mower engine, although it depends on how much you have overfilled your mower.

Overfilling it by just a few ounces most probably won’t cause any problems. I say probably as I can’t say for definite everything will be ok.

The more you have overfilled your mower, the more likely it is to suffer severe damage, as shown in the infographic above.

Large overfills could cause irreparable damage to your mower’s engine, meaning you might need to shell out on a brand-new lawn mower.

Or you might get lucky and just have to pay an excessive repair bill…

Either way, a lawn mower can definitely have too much oil, so please be careful when adding oil to your mower and only fill it to the recommended level.

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