Quick answer: water can get into a fuel tank through condensation, a damaged fuel cap, contaminated fuel or flood exposure. It causes rough running, misfires, stalling and, over time, corrosion. Diesels are especially vulnerable. If you suspect water, avoid driving and get the fuel tested and drained.
How water gets into fuel
- Condensation: temperature changes can cause moisture to form inside a partly empty tank, especially in vehicles that sit unused.
- Damaged or missing fuel cap: rain or wash water can get in through a poor seal.
- Contaminated supply: occasionally, fuel from a storage tank or drum contains water.
- Flood exposure: vehicles caught in floodwater can take water into the tank and fuel system.
- Jerry cans and drums: containers left open or stored outside can collect moisture.
Why water is a problem
Water doesn’t burn and it doesn’t lubricate. In a diesel system, which relies on fuel for lubrication and runs at very high pressure, water can quickly damage the pump and injectors. Water also encourages corrosion and, in diesel, microbial growth (sometimes called “diesel bug”) that blocks filters.
Warning signs
| Symptom | Possible cause |
|---|---|
| Sputtering or hesitation | Water reaching the injectors |
| Hard starting | Water in the lines or filter |
| Stalling, especially after refuelling | A slug of water picked up from the tank |
| Water-in-fuel warning light (many diesels) | The filter’s water separator is full |
| Black sludge in the filter | Microbial growth in diesel |
What to do
- Stop driving if symptoms appear suddenly after refuelling.
- Note where and when you last filled up, and keep the receipt.
- Don’t keep restarting the engine, as that draws more water through.
- Get a professional to test and drain the fuel, replace filters and check the system.
If you think a service station supplied contaminated fuel, see what to do if the fuel from the servo was contaminated.
Water in petrol versus water in diesel
Petrol cars: E10 can absorb a small amount of water, but too much causes “phase separation”, where a water and ethanol layer settles at the bottom of the tank. That layer can stop the engine running.
Diesel vehicles: diesel doesn’t mix with water, so it sinks to the bottom and collects. Many diesels have a water separator with a warning light. If it comes on, stop and get it checked rather than draining it repeatedly.
In both cases, a professional drain removes the contaminated fuel and the water, and the filter is usually replaced. See diesel bug for what happens when water sits in diesel for a long time.
Water and microbial growth often go together in diesel. RACQ explains how water feeds diesel fuel bugs and why tanks on vehicles that sit unused are most at risk.
Prevention
- Keep the fuel cap in good condition and fully closed.
- Avoid leaving vehicles and machinery sitting with a near-empty tank for long periods.
- Store fuel in sealed, approved containers out of the weather.
- Drain the water separator on diesels as the manufacturer recommends.
Read more in Preventing fuel contamination.
Frequently asked questions
Can I drive with water in my fuel tank?
It’s not a good idea. Water can cause misfires and stalling, and in diesel engines it can damage the high-pressure pump and injectors.
How does water get into a fuel tank?
Condensation in a part-empty tank, a damaged fuel cap seal, flood water, or contaminated fuel from a service station tank.
Will a fuel additive fix water in the tank?
Additives can deal with tiny traces of moisture, but they won’t fix a real water problem. That needs a drain.
