Marine Fuel Polishing: How to Clean Water, Sludge & Bugs from Your Tank

When your boat's engine starts running rough or you notice water or sludge in your fuel, it’s time to consider marine fuel polishing. This process is a safe and effective way to clean contaminated fuel in your vessel’s tank without dumping good fuel, saving you money and preventing engine damage. Fuel contamination can occur from water ingress, algae growth (commonly called diesel bug), or phase separation of ethanol-blended fuels like E10. These issues are common in NZ marinas such as Westhaven, Gulf Harbour, Tauranga, and Waikawa, especially after long periods of inactivity or storage.

Fuel polishing involves circulating the fuel through specialised equipment that removes water, sludge, and microbial growth. The process starts with a certified marine technician draining the tank’s contents, then flushing and filtering the fuel to separate contaminants. The technician will also check for phase separation, which happens when ethanol-rich water separates from the diesel, creating a cloudy or milky appearance. This can cause engine issues if not addressed. The key benefit of fuel polishing is that it cleans the entire fuel system without wasting the good fuel, which is especially important for vessels with limited fuel capacity or expensive fuel costs.

During the process, the technician uses high-quality filters and water separators to remove water and sludge. They may also add biocides or stabilisers if microbial growth is detected. Once the fuel is cleaned, it is returned to the tank, ready for safe use. This method is ideal for boats that have been stored for a while, have experienced water ingress, or show signs of diesel bug. It’s a practical solution that helps maintain engine performance and prolongs the life of your fuel system.

What to do right now if you suspect fuel contamination? First, do not start your engine. Running contaminated fuel can cause serious damage. Contact a certified marine technician who can assess the situation and perform fuel polishing if needed. Regular fuel checks and proper storage can prevent many issues, but when contamination occurs, professional cleaning is the safest and most effective approach.

If your vessel is experiencing fuel problems, don’t hesitate to call us at 0800 769 000. We send a certified marine technician to your berth, mooring, or ramp 24/7 to drain, flush, and polish your fuel system. Our team ensures your boat’s fuel is clean and ready for smooth sailing in the Hauraki Gulf or NZ’s other prime boating areas. Trust EEK Mechanical to keep your vessel running reliably, no matter the time of day or night.