Why is oil coming out of the lawn mower exhaust?

Have you noticed oil leaking from the lawn mower you have been using? Nothing messes up your lawn more than the mower exhaust leaking oil. That hinders the purpose of the equipment in the first place. So, before your entire yard needs to get redoing from the school, it is essential to find out why exactly there is oil coming out of your lawn mower exhaust. Generally, exhaust starts releasing oil when internal parts are dysfunctional. Four major parts make a mower; piston rings, the reciprocating cylinder, the exhaust pipe, and the exhaust manifold. The deformation or dysfunction of any of these would result in an oil leak from the exhaust.

Why is oil coming out of the lawn mower exhaust?

Overfilled Crankcase:

The crankcase of a combustion engine is quite big and they can hold an optimal amount of oil but lawn mower engines are a smaller version of a full-size combustion engine. And the case can hardly hold a quart of oil, even less. So, it gets overfilled with oil very often. The overflowing oil makes its way from the crankshaft to the cylinder then to the engine. It is the leading cause of the lawn mower blowing white smoke. But when the excess oil moves from cylinder to carburetor, there is no way out other than passing through the exhaust pipe. Therefore, the oil makes its way through the air filters and exhaust pipe and finds its way out from the exhaust manifold. This is the most common reason why you find oil coming out of the exhaust.

Depreciated Piston Ring:

Overworked or old piston rings let oil leak out rather than burning it, even when the machine is on. The depreciated ring forces the oil out and sends it off to the carburetor. And like the overfilled crankcase scenario, the oil finds its way out through the exhaust again. The worn rings are the only reason causing this. Replacing the rings is the easiest solution to fix this.

Blocked Air Filter:

Air filters are a pretty important part of the combustion engine. The oxygen helps the fuel burn out and generate maximum power. But over-used filters often get clogged. The blockage created resists the air supply to the revolving pistons. Thus, the fuel is not optimally burnt. The unburn fuel flows throughout the process and comes out from the end making a mess outside. If the carburetor is adjusted correctly, it is less likely to cause a filter clogging.

Drenched Air Filter:

When air filters encounter oil frequently, the filters get over-saturated with oil that never dries off. The drenched filters allow more oil to come through the carburetor and other parts aligned with it (needle valve, internal jets). The oil ultimately comes out of the exhaust.

Frayed Valves

Valves are the entrances to the combustion engine for the fuel input and exhaust system in the lawn mower. Worn valves cause performance deficiency. Thus, they do not let the cylinders to seal properly. The flaws in the seal allow the oil to leak out before it can be burnt into energy for the engine. Frayed valves are the sole reason for this leak that ends up in your backyard through the exhaust system.

Coolant leakage:

If overheating is a daily occurrence for your mower, then you must be familiar with the liquid engine fluids used to keep the engine cool. Some lawn mowers use these sorts of coolants to keep the engine from overheating, much in the same way as a car. Usually, four-stroke engine-based lawn mowers have coolant tracks. Damaged gaskets and cracks in the cooling system usually intrigue the coolant leak. This happens with very old or overused mowers, especially when used on rough and uneven surfaces every day.

Wrongly used and stored:

If the machine is misused, like tipped over, used in more than 15° tilted positions, or stored on its side, there is a lot of damage done already. The oil can go from breather to carburetor to filters pretty easily this way. All these parts are inter-connected and the wrong positioning of the machinery causes serious dysfunction between them. So, leaking oil from the exhaust has to be the least of your worries if you have been doing these for a long time without knowing.

Disfigured Engine block and Blown Head Gaskets:

These symptoms are usually validated for old mowers. The head gasket works to separate the liquid input outside and away from the engine top. On the other hand, the engine block works to keep every internal part and in-process liquids inside the engine safe and separated. But a disfigured/damaged engine block is unable to do its part. Thus, the liquid leaks from various parts of the engine and comes out through the exhaust. The same goes for when the head gasket is blown. The gasket is unable to deliver the oil to designated areas and flows to other ways. The ultimate escape of the leaked oil is the exhaust manifold. So, it is clear which way the oil would find its way out.


So, we know the main reasons for the very trouble, we should understand the actual reason why it is happening, which is a lack of maintenance and weak operating skills. Yes, this is the main reason behind all the unfortunate occurrences. So, to prevent these kinds of inconveniences from happening, you should learn the basic skills of operating the machinery and rightfully string it. It will also increase the longevity of your lawn mower. Gardening tools like a leaf blower and a lawn mower can go many years only if you do proper maintenance. That’s the only condition.

Bottom line:

So now we know all the possible causes of oil coming out of your lawn mower exhaust. Leaking from the exhaust is a serious issue. So, it would help if you took that as a warning for a more significant inconvenience coming. It will be best if you consult with a professional mechanic or manufacturers. If there is a warranty left for your purchase, you can save yourself a lot of money and effort.