Why Does Your Mower Oil Smell Like Gas?

There are several reasons your lawn mower might smell of gas, and sometimes it has nothing to do with the oil. You might have tipped it over and spilled some. Maybe the fuel filter is clogged (which can be why your lawnmower won't start). There might be a leak in the tank or fuel pipe. These are relatively easy to identify and fix. More serious problems can arise if the actual oil smells. This indicates that gas has somehow found its way into the crankcase (the bottom of the engine where the oil pan is).

We'll look at the three possible scenarios in a moment. First though, it's worth understanding that this doesn't happen to all mowers. If you have a 2-stroke model (sometimes called 2-cycle), the oil and gas are mixed before use and go into a single tank. It's not a problem. In fact, it's vital for proper cooling. Four-stroke mowers (or 4-cycle) are different. These have separate tanks for oil and gas, and the two should never mix. If they do, something is going wrong.

There are three likely reasons why your mower oil smells like gas: The first could be that the mower was only run for a short time. The second reason could be a carburetor issue. Lastly, there could be a problem with the piston rings. In each case, you should stop the motor and find a solution. Continuing to run it will only make things worse. The first two are relatively easy to fix, the third is more challenging.

When you start your mower, the choke feeds it extra gas to get it going. If you only mow for a minute or two, some of this gas may not burn and will dribble down into the oil, causing it to smell. It's not a serious issue, and running the engine for longer should prevent it. You can also change the oil, which should be done annually anyway, but is one of those lawnmower maintenance tasks people forget.

A sticking carburetor is another culprit, or more precisely a sticking float or needle. Gunk builds up in the gas tank and works its way into the carburetor. As a result, the small parts get stuck. So too much gasoline gets into the engine, and an unburned portion runs into the crankcase as before. Black smoke from the exhaust and uneven running are both signs of a dirty carburetor. Although you may need to take it apart, there are several carb cleaner sprays on the market that are worth trying first.

The final cause, and the most difficult to fix, is a problem with the piston ring. Piston rings wear eventually and, in older mowers, can allow unburned fuel to seep past. The same can happen in newer engines if the piston ring gets damaged. As well as excessive smoke, there will usually be a drop in performance. If you don't have the required skills, this is one of the times your lawnmower needs to go to a professional to be fixed.