Should you worry about water in your fuel? | Fuel Facts

“With ethanol-blended fuels, they can carry a little bit of water in them,” notes Zachary J. Santner, senior specialist of quality at Sunoco. When the water enters the combustion chamber, it’s simply turned into steam and pushed out through the exhaust, along with the water that’s a byproduct of combustion. (Remember, perfect combustion produces CO2 and H2O).

“As long as the water is in the fuel,” he continues, “the engine will take care of it.”

Fresh fuel contains very little water–maybe 0.1% by volume. The problems start when too much water becomes present–north of about 2%, depending on temperature, Santner notes. This is when phase separation occurs: The two liquids no longer mix, meaning fuel sits on top of the water. This can lead to rust in the tank, a poor-running engine or worse. 

How does water get into the fuel? Blame the atmosphere, as our air contains water. It’s just the price of living on Earth. Blame the weather, too, as warmer fuel can hold more water than colder fuel. “So fuel absorbs water during summer and then it gets cold and then phase separation happens,” Santner continues. This is why many issues are discovered after colder weather has started.

How can you tell if your fuel is watery? A sample of fuel should look clear; any cloudiness could indicate a contaminant like water.

If a car has sat for a while and now has water in the tank, what do you do? If it’s a lot of water, Santner recommends draining the tank. “We need to get everything in the tank out.” Carburetors act as fantastic water traps, he adds. 

An easier solution, assuming there’s room in the tank and things haven’t gotten too bad, is to top off with fresh fuel to dilute the problem. Then use up that tank. 

Additives that promise to remove water from gasoline could be a solution, Santner notes. Most contain methanol, which, thanks to miscibility, reblends the fuel and water. Methanol, he cautions, can be hard on the engine as it also acts as a terrific degreaser. Use it sparingly, he continues, and then, ideally, burn through that tank of fuel so the methanol isn’t sitting in the tank. 

Best practice, he says, is to avoid this problem in the first place by regularly driving the car.

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