Water and gasoline don’t truly mix. Gasoline repels water, so when the two come together, the water sinks to the bottom of the tank and sits there, ready to cause problems ranging from rough engine performance to serious mechanical damage. How much trouble depends on the amount of water involved and how long it stays in the system.
Why Water and Gasoline Separate
Pure gasoline can dissolve an almost negligible amount of water: about 0.15 teaspoons per gallon at 60°F, according to EPA data. That’s practically nothing. Most gasoline sold today contains 10% ethanol (E10), which absorbs more moisture because ethanol is attracted to water. E10 fuel can hold roughly 3.8 teaspoons of water per gallon before problems begin. That sounds like a small buffer, and it is.
Once the fuel absorbs all the water it can hold, the excess water separates out and drops to the bottom of the tank. This process is called phase separation. In ethanol-blended fuel, it’s worse than simple separation: the ethanol actually pulls away from the gasoline to bond with the water, creating a corrosive layer of ethanol and water sitting beneath a layer of now lower-octane fuel. Cold temperatures accelerate this process because fuel holds even less dissolved water when it’s cold.
What You’ll Notice While Driving
A small amount of water in the tank causes subtle but noticeable symptoms. The engine idles roughly, RPMs bounce around, and you may feel hesitation or a surging sensation, almost like the engine has hiccups. This happens because your fuel system intermittently pulls in water instead of gasoline, and water doesn’t ignite the way fuel does. The result is uneven combustion: some cylinders fire normally while others misfire.
As contamination gets worse, the symptoms escalate. Hard starts, frequent stalling (especially at stoplights or during acceleration), white steam from the exhaust, and a noticeable drop in power and fuel efficiency are all common signs. Stalling during acceleration is particularly dangerous in traffic and typically means the fuel system is drawing in more water than gasoline.
Damage to the Fuel System
Water sitting in a fuel system causes corrosion. Fuel injectors, fuel pumps, and the inside of the tank itself are all vulnerable. These components are designed to handle petroleum-based fluids, not water, and prolonged exposure leads to rust, pitting, and eventual failure. Water also strips away the lubricating properties of the fuel, increasing friction and wear on precision components like high-pressure fuel pumps and injectors that rely on the fuel itself for lubrication.
If enough water reaches the engine’s combustion chambers all at once, you risk a condition called hydrolock. Pistons compress air and fuel vapor easily, but water doesn’t compress. When a cylinder fills with water instead of fuel-air mixture, the piston can’t complete its stroke. The result can be bent connecting rods, cracked cylinder heads, or a completely destroyed engine. Hydrolock from fuel contamination is rare in everyday driving, but it’s the worst-case scenario with significant water intrusion.
Microbial Growth at the Water Line
One of the less obvious consequences is biological. The boundary where water meets fuel creates an ideal environment for bacteria and fungi to thrive. These microorganisms feed on the fuel itself, particularly its paraffin compounds, and produce acidic byproducts and a sludge that accumulates at the bottom of the tank. Researchers have identified numerous species that colonize contaminated fuel tanks, including various bacteria and molds. The sludge they produce clogs fuel filters, restricts fuel lines, and accelerates corrosion. This is a bigger concern in vehicles or equipment that sit unused for long periods, giving microbial colonies time to establish.
How Water Gets Into a Gas Tank
Condensation is the most common source. Temperature swings cause moisture in the air inside a partially filled tank to condense on the tank walls and drip down into the fuel. The emptier the tank, the more air space available for condensation. This is why vehicles that sit for weeks with a near-empty tank are especially prone to water accumulation.
Other sources include a loose or damaged gas cap, a cracked filler neck, or pumping fuel from a gas station with a contaminated underground storage tank (more likely after heavy rain or flooding). Occasionally, water enters through a deteriorated seal in the fuel system itself.
Fixing Water-Contaminated Fuel
For minor contamination (a few ounces of water), commercial fuel additives designed to absorb small amounts of water can help. These products bond with water molecules and allow them to pass through the combustion process without causing harm. They work best as a preventive measure or for very light contamination.
For anything beyond that, the contaminated fuel needs to be drained. The process involves shutting down the vehicle, accessing the tank’s drain point or disconnecting a fuel line, and pumping out all the old fuel until clean gasoline flows through. After draining, the fuel filter should be replaced and the lines flushed before refilling with fresh fuel. On newer vehicles, a mechanic can use diagnostic tools to activate the fuel pump and push contaminated fuel out through a disconnected line without removing the tank. Professional fuel system draining typically costs around $300, though the price climbs significantly if injectors, pumps, or other components have already been damaged by corrosion.
If the vehicle has been sitting with water in the tank for an extended period, a mechanic will also inspect for microbial contamination and corrosion inside the tank. Severe cases may require tank replacement or professional fuel polishing, a process that filters and cleans the fuel in place without draining.
Preventing Water Buildup
Keeping your tank at least half full reduces the air space available for condensation, which is the simplest preventive step. Make sure your gas cap seals tightly and replace it if the seal looks cracked or worn. If you’re storing a vehicle, boat, or piece of equipment for an extended period, fill the tank as close to full as practical and consider adding a fuel stabilizer that includes a water-dispersing agent. Avoid filling up at gas stations immediately after heavy rainfall, since underground storage tanks can take on water through compromised seals, and that water gets pumped right into your vehicle.