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Adding water to your car’s gas tank can cause a variety of problems. What’s more, it’s easier for water to get into the fuel system than you might think.

Water can seep into the underground fuel storage tanks at gas stations, then make its way into your gas tank while you’re refueling. Although the problem doesn’t happen all that often, it’s still something that you’ll want to be aware of.

water in gas
Filling up with gasoline that’s mixed with water can result in engine performance problems, such as rough running and stalling.

What Happens if You Put Water in a Gas Tank?

Unless you’re itching to get a hefty repair bill from your mechanic, you probably won’t be pouring water directly into your car’s gas tank anytime soon.

Because most modern cars have a fuel door release button inside of the cabin, it’s also unlikely that someone will pour water into your gas tank as an act of vandalism (sugar is more popular for that, anyhow).

But it’s not entirely uncommon to get water-contaminated gas at a commercial fueling station. The water can get into the facility’s underground storage tanks through seepage, condensation, and other means.

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Filling up with gasoline that’s mixed with water can result in engine performance problems, such as rough running and stalling. What’s more, the contamination can eventually cause costly fuel system components to corrode and fail.

The video below outlines what can happen when you refuel your car with gasoline that’s mixed with a significant amount of H2O:

Note: Water-contaminated diesel fuel can be an even bigger problem that can quickly damage major fuel system components, such as the high-pressure fuel pump. Diesel vehicles usually have a water-fuel separator to defend against normal water accumulation.

Symptoms of Water in a Gas Tank

When a significant amount of water has been added to your car’s gas tank, you’ll likely notice one or more of the following symptoms:

Engine Runs Rough Shortly After Refueling

Water is a non-combustible liquid that, when drawn into the fuel system, can cause the engine to run rough and misfire. The problem will usually begin shortly after refueling.

Vehicle Stalls Shortly After Refueling

Because a gasoline engine cannot combust water, the vehicle will eventually stall if there’s enough H2O in the fuel system.

The Check Engine Light Illuminates Shortly After Refueling

Your car’s engine computer looks for problems, such as a misfire, that could lead to an increase in tailpipe emissions. If the device notices the engine is misfiring from water-contaminated gasoline, it will turn on the check engine light and store a diagnostic trouble code in memory.

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illuminated check engine light icon
The Check Engine Light will turn on if your PCM notices the engine is misfiring from water-contaminated gasoline.

How Much Water in a Gas Tank is Bad?

Water in your gas tank is never a good thing. But if there are only trace amounts of water mixed in with the fuel, you may be able to combat the problem by adding dry gas (isopropyl) to your gas tank. Dry gas is a product that can help restore combustive properties to gasoline-contaminated water.

Meanwhile, a significant amount of water will usually require draining the fuel tank. You may also end up having to service other parts of the fuel system, especially if you continue driving with water-contaminated fuel.

When in doubt, it’s best to have your vehicle looked at by a professional mechanic right away.

How to Check for Water in Your Gas Tank

Gasoline weighs approximately six pounds per gallon, whereas water weighs about seven pounds per gallon. Water will visibly separate from gasoline when the two are mixed together.

There are commercial test kits available that allow you to check for water contamination in gasoline. Another less expensive method is to take a fuel sample and place it in a clear container. If there’s water in the gasoline, you’ll likely see the two liquids separate after sitting for a while, as demonstrated in the video below.

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Here is another video that demonstrates a different take on the same basic test method:

How to Protect Yourself From Water-Contaminated Gasoline

There are steps that you can take to protect your car and your wallet against water-contaminated gasoline.

First of all, it’s a good idea to purchase your gasoline from busy fueling stations with a high turnover rate. The gasoline at these facilities is more likely to be fresh and free from water contamination.

You’ll also want to avoid purchasing gasoline when the tanker is refilling the fueling station’s storage tanks. The refilling procedure often stirs up water and other contaminants lingering in the bottom of the underground containers.

Finally, always get a receipt from the fueling station after you purchase gasoline. Doing so will provide you with a path of recourse if you get stuck with water-contaminated fuel.

woman refuelling in a gas station
Purchase your gasoline from busy fueling stations with a high turnover rate as they are more likely to be fresh and free from water contamination.
About The Author
CarParts Research Team Research Team

Automotive and Tech Writers

The Research Team is composed of experienced automotive and tech writers working with (ASE)-certified automobile technicians and automotive journalists to bring up-to-date, helpful information to car owners in the US. Guided by's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

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