Will There Be Permanent Damage if You Accidentally Added Oil to the Gas Tank of Lawnmower?

Do No Harm

• Some lawnmowers use a two-stroke engine. A two-stroke engine has to have oil mixed in the gasoline, usually in a 50-to-1 ratio – 50 parts gas to one part oil. Because two-stroke gasoline engines don’t have a separate system to circulate oil, it’s introduced through the gasoline. If you don't know whether your lawnmower has a two-stroke engine or not, look under "Specifications" in the operator's manual. If the manual tells you your lawnmower has a two-stroke engine, determine how much oil you should mix with the gas before you use the lawnmower. If you do not have the operator’s manual, look on the metal plate on the top or side of the engine.

Gravity as a Cleaning Tool

• If your lawnmower doesn’t have a two-stroke engine, and if the tank was empty, ascertain how much oil was poured into the fuel tank. If the tank was empty, and you haven’t tried to prime the engine’s carburetor with the primer pump, gravity can do much of the clean-up for you. All you need to do is disconnect the fuel line from the fuel tank – thus trapping the oil in the fuel tank – remove the tank from the lawnmower and invert the open tank over an oil absorbent pad. Mix some degreasing liquid dish detergent with water. Slosh it around inside the tank after draining as much oil from it as possible and blow part of a can of compressed air into the tank to dry it out. After you reinstall the tank you can begin mowing.

New Lines and Starting Fluid

• If you’ve tried to start the mower and it started, don’t worry about it. Sadly, if you’ve tried to start the lawnmower's four-stroke engine with a mixture containing a substantial amount of oil, the oil is in the fuel line from the tank to the primer pump. You can remove the line and wash it, or you can simply replace the line. If you tried to start the mower and it wouldn’t start, clean out the fuel tank, replace the gas lines and reconnect them. Pour new gas in the tank, spray some starting fluid -- available at auto parts stores -- into the carburetor, start the mower and mow the lawn.

From Soup to Nuts

• In the very worst case, you’ll have to clean out the carburetor as well as clean out the gas lines and the fuel tank. It's a matter of taking the carburetor off the motor, removing the fuel bowl and float, and soaking its metal parts in carburetor cleaner. The gaskets and other fiber parts should be replaced. Then, you can put it back together, put it back on the lawnmower, fill the tank with fresh gas and mow the lawn.

An Afterthought on Stale Gas

• While you might be interested in saving a few dollars by using the gasoline left over from the last mowing season, don't. Gasoline begins to go stale after about two weeks of sitting still in a gas can. It starts to break down into a variety of undesirable products including a varnish that can plug your carburetor faster than the oil you poured in. Even if the gas can was full when you stored it, there was some open space in it, somewhere. Although you stopped mowing the lawn at the end of September, it was still hot enough that moisture began to condense out of the air trapped at the top of the can. The water then flowed down the sides of the can and, since water is heavier than gasoline, made its way through the gas to the bottom of the can. This means that, if you use stale gas, you're pouring some water into the tank, too.