My Mower Is Running Bad With Moisture in the Gas

How a Four-Stroke Lawnmower Engine Works

• A gasoline engine has a fuel line and an air intake. When the accelerator mechanism is engaged, a mixture of gasoline fuel and air is pulled into the engine cylinder through the intake valve. The valve is closed and the fuel/air mixture is compressed by the piston. The compressed mixture is ignited by an electric spark that is transmitted into the cylinder by a spark plug. The compressed mixture is ignited, and the piston is forced downward; this power stroke turns the crankshaft, and the crankshaft turns the lawnmower blade.

How Water Affects Fuel Combustion

• Water is a stable chemical compound whose molecules are made up of two hydrogen atoms and one oxygen atom. Water is not flammable, and if water vapor is introduced into the ignition cycle of a four-stroke engine, the water can interfere with the combustion process.

How Water May Enter the Fuel Tank

• Much of the gasoline sold in the United States since 2007 contains ethanol. Ethanol can absorb water, whereas gasoline and water do not mix. When the temperature or ambient humidity increases, the ethanol additive in each tank of gasoline absorbs some of the water vapor in the air. When the air cools or the humidity drops, the water vapor condenses into water droplets which can form on the inside walls of a fuel tank. If a partially empty fuel tank is left unattended for a long period of time – such as several weeks or months – significant amounts of water can accumulate in the tank.

Problems Related to Water in a Gasoline Tank

• Water is heavier than gasoline. If water is present in a gasoline tank, this water will settle to the bottom of the tank. If the tank is made of steel or iron, the water will rust the tank and contaminate the fuel in the tank as well as damage the tank. As noted above, water in a fuel tank will also inhibit the combustion process if water and fuel pass from the tank to the engine’s combustion cylinder.

What To Do if Water Is Present in the Tank

• Since water in a tank is often a symptom of old fuel, it is often necessary to drain the tank completely and refill the tank with fresh fuel. Small amounts of water in the fuel may also be absorbed by adding a “fuel-drying” additive that contains methanol to the tank; however, not all engines will run properly if the alcohol-to-gasoline ratio is increased. If you choose to use an additive, check the lawnmower owner’s manual to see if the engine can safely use such an additive.