Water in diesel fuel causes rough running, loss of power, hard starting, stalling, white smoke, and a water-in-fuel warning light on trucks that have one. Water does not burn, does not lubricate, and causes corrosion, so it can quickly damage high-pressure fuel pumps and injectors. Draining the water separator and replacing the fuel filters fixes small amounts; larger amounts mean draining the tank and checking the injection system.
Symptoms of water in diesel fuel
- Water-in-fuel light or message
- Rough idle, misfires and surging
- Loss of power and stalling; see diesel losing power
- Hard starting or no start
- White smoke or steam-like exhaust; see diesel white smoke
- Low fuel pressure codes such as P0087
What happens if water gets in diesel fuel?
Modern common-rail injection systems run at very high pressure with tight clearances lubricated by the fuel itself. Water washes away that lubrication and causes rust and pitting, which can lead to pump and injector failures. In the worst cases, a failing pump sends debris through the entire fuel system; see 6.4 Power Stroke problems for an example of fuel system contamination.
Where water comes from
- Contaminated fuel from a station or storage tank
- Condensation in a tank left partly empty, especially in humid or cold weather
- A damaged fuel cap seal or filler neck
- Water entering during pressure washing near the cap
What to do
- Stop and drain the water separator until clean fuel runs out.
- Replace the fuel filters.
- If water keeps appearing, drain the tank and refill with fresh fuel.
- Bleed the system; see how to bleed a diesel fuel system.
- If the engine ran rough for a long time, have the injection system checked.
Cold weather makes it worse
Water in the fuel can freeze in lines and filters in cold weather, causing the same symptoms as gelled fuel. A truck that sits outside with a partly empty tank collects condensation through the season, so keep it full and drain the separator before winter; see diesel hard to start when cold.
Water in the fuel filter
Most diesel fuel filters include a water separator with a drain valve and, on many trucks, a sensor. Drain it at each oil change or whenever the light comes on; see fuel filter.
Prevention
- Buy fuel from busy stations.
- Keep the tank full when parked for long periods.
- Drain the separator regularly and change filters on schedule.
Related: diesel fuel gelling and bad diesel injector symptoms.
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