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Water In The Fuel
Water is the root of almost every marine fuel problem. It corrodes the tank, feeds microbial growth, and destroys the injection system — which relies on diesel itself for lubrication and gets none from water.
Is it safe to keep going?
If the separator is catching it, yes, cautiously, while you sort out the source. If water is reaching the injection system, no — every hour of running is wear on a pump and injectors that cost far more than the fix.
How it gets in
| Route | How common |
|---|---|
| Condensation in a part-full tank | Very common |
| Leaking or perished deck fill seal | Very common |
| Failed or submerged tank vent | Common |
| Contaminated fuel at the dock | Common |
| Water taken over the deck into a poor fill | Common |
| Corroded tank | Occasional |
Marine tanks condense far more than road tanks because they sit part-full for months in humid air. A full tank over a layup is the single most effective prevention, because there is no air space to condense.
What it causes
- Filters blocking, and the separator filling
- Microbial growth at the fuel-water interface Diesel bug in the fuel tank
- High-pressure pump and injector wear, from loss of lubrication
- Corrosion inside the tank
- Rough running, power loss, hard starting, white smoke
What you can check yourself
- Drain the water separator. Routinely, and act on the first water you see. This is the cheapest sixty seconds on the boat.
- Take a tank-bottom sample and let it settle in a clear jar — water sits below the diesel
- Check the deck fill seal — squeeze it, look for perishing
- Check the tank vent is clear and not taking spray
- If a water-in-fuel warning is fitted, never ignore it
Fuel filter and water separator service
When to call a mechanic
If there is significant water in the tank, if it keeps returning after draining, or if the engine has been running rough. Draining the separator is a stopgap; the tank is the problem.
Fuel polishing and tank cleaning
What marine engine work costs in Australia
Common on these engines
Modern high-pressure common rail systems are the least tolerant; older mechanically injected engines survive more but still suffer.
Marine injector testing and replacement · Marine fuel pump repair
Frequently asked questions
How much water is too much? Any reaching the injection system. That is exactly what the separator exists to prevent, which is why draining it matters.
Should I store the tank full or empty? Full. Air space condenses moisture, and moisture is what feeds every other fuel problem.
The separator keeps filling — is that the separator’s fault? No, it is doing its job. Something is putting water in the tank; find it.
Can water damage injectors? Yes. Diesel lubricates the injection system; water does not, and it corrodes.
Practical summary
Use this fault guide safely
Stop now
Shut down for oil-pressure alarms, overheating, fuel spray, smoke in the engine space or loss of cooling-water flow.
Check safely
Start with visible leaks, strainers, fluid levels, alarms and loose connections—only when the machinery is safe and isolated.
Record evidence
Note rpm, temperature, smoke colour, alarm text, sea state and exactly when the symptom began.
Escalate
Give the evidence and engine model to a qualified marine mechanic; avoid repeated restart attempts.
Sources and verification
This page provides general Australian guidance. The current manual for the exact engine, gearbox or drive remains the authority for procedures, specifications and service intervals.
- 1Tank pickup
- 2Primary filter
- 3Lift pump
- 4Secondary filter
- 5Injection system