Fuel, tanks and diesel bug
Contaminated fuel, microbial growth, blocked filters and water in the tank — on a boat the tank causes more trouble than the engine.
Will a biocide alone fix it?
It kills the organisms but leaves the dead biomass and the water. Both need removing or the problem returns — and the dead material itself blocks filters.
Full guide: Diesel Bug Treatment and Removal
Why did my filters get worse right after treatment?
Because the treatment killed a large colony and released it into the fuel as sludge. This is expected, and it is why polishing and filter changes follow treatment.
Can I prevent it entirely?
Keep water out of the tank and it cannot establish. Full tanks over layup, a sound deck fill and vent, and routine separator draining do most of the work.
Is diesel bug worse in Australia?
Warm humid conditions favour it, so northern Australia is a higher-risk environment than the south.
How do I know it's the bug and not just dirty fuel?
Sample the tank bottom and use a test kit. Slime and jelly indicate biological growth; dry particulate suggests sediment or rust instead.
How do I know if my fuel is contaminated?
Take a sample from the tank bottom. Dark sludge, a jelly-like layer, or free water settling out are all clear indicators — and cheaper to find deliberately than during a passage.
Full guide: Fuel Polishing and Tank Cleaning
Is polishing enough, or do I need the tank cleaned?
Polishing handles the fuel; cleaning handles the tank. If there is established growth on the walls and baffles, polishing alone gives a temporary result.
Why do my filters only block when it's rough?
Because motion stirs settled sludge off the tank bottom and walls and sends it to the filters. Calm-water running never disturbs it.
Can I polish fuel myself?
Portable systems exist and work for maintenance. An established contamination problem needs the tank opened, which is specialist work.
Should I keep the tank full or empty over winter?
Full. Air space condenses moisture, and water in the tank is what feeds microbial growth.
How often should marine fuel filters be changed?
At least annually, and more often if the bowl shows contamination. Interval by hours misses the point — tank condition drives it.
Full guide: Fuel Filter and Water Separator Service
Can I fit a finer filter for better protection?
No. A primary finer than specified restricts flow and starves the engine at high revs. Use the rating the engine maker specifies.
Why does my engine run rough but the filters look clean?
Very often an air leak on the suction side — a perished bowl seal or a loose fitting. It mimics an injection fault and costs a fraction to fix.
Should I carry spares?
Yes, and know how to fit and bleed them. It is the most likely repair you will ever need to make under way.
What is the vacuum gauge on my filter for?
It shows how hard the engine is having to suck through the filter. A rising reading means the element is loading up, giving you warning before it blocks.
My engine dies after an hour and then restarts — what is that?
Classic blocked tank vent. A vacuum builds as fuel is drawn out, starving the engine; opening the filler releases it. Check the vent before anything expensive.
Full guide: Marine Fuel Pump Repair
Why is the quote so much more than the price of a pump?
Because a pump that shed metal contaminated the injectors, rail and lines. Replacing only the pump on a contaminated system fails again quickly.
Can a marine injection pump be rebuilt?
Mechanical pumps very often can, by a diesel injection specialist with a calibration bench. Ask for the test certificate.
How do I stop it happening again?
Address the tank. Water and particulate are what killed it, and they are still there after the pump is replaced.
My engine smokes and won't reach full revs — is it the injectors?
Possibly, but check the bottom and the propeller first. A fouled hull or an over-pitched prop overloads the engine and produces identical symptoms for a fraction of the cost.
Full guide: Marine Injector Testing and Replacement
Can marine injectors be reconditioned?
Frequently yes, especially older mechanical types. Insist on test data showing opening pressure and spray pattern against specification.
Should all injectors be done at once?
Usually, because they wear together and the labour is shared. Get them all tested before deciding.
Why did my injectors fail?
On a boat, almost always fuel. Water and microbial contamination are the leading causes, and they will damage the new ones too if the tank is not addressed.
How long should marine injectors last?
On clean fuel, a long time — thousands of hours. Fuel quality determines it far more than hours do.