How to Clean a Portable Boat Fuel Tank Full of Ethanol Gunk

My 6-gallon portable tank has brown sludge in the bottom — how do I clean it out?

That brown sludge is almost always old, oxidized E10 gasoline plus water and the gummy varnish/phase-separation residue ethanol leaves behind when fuel sits for months. The good news: a plastic 6-gallon portable tank is the easiest fuel container on the boat to fix — you can usually drain it, rinse it, and get it clean in an afternoon for the cost of fresh fuel. The hard part is doing it safely (gasoline vapor is explosive) and not just pushing the gunk downstream into your fuel line, filter, and carburetor.

ℹ️ Reference only: For general reference only. This guide does not guarantee any result — every home is different. Verify against your local building codes and a licensed professional before acting, especially for electrical, gas, plumbing, structural, or roof work.

💵 $20-$60 DIY (fresh fuel for rinsing, stabilizer, new primer bulb/fuel line, a marine filter, gaskets). A new 6-gallon marine portable tank is about $50-$120 if it's not worth saving. A marine shop charging to clean the tank and clear a fouled fuel system typically runs $150-$400+. ⏱ 1-3 hours, plus drying time before refilling. ● Use caution

Safety: Gasoline vapor is explosive and heavier than air — work outdoors with strong ventilation, never in a closed garage or below deck, and keep all flames, sparks, electronics, and running engines away. Static electricity can ignite gasoline vapor: set portable plastic tanks on bare ground (not a truck bed, dock, or boat) when filling or rinsing, and keep the fill nozzle in contact with the neck. Keep a Class B marine fire extinguisher (UL-rated 5-B or larger) at hand. Old/contaminated gasoline and sludge are hazardous waste — dispose of them at a household hazardous-waste facility, never on the ground, down a drain, or into the water. Use only marine fuel-rated hose (USCG/SAE J1527) on a boat, and only ignition-protected electrical components in the engine space; automotive parts are not approved and can ignite vapors. If you do this near the water, mind footing and the risk of falling overboard, and never run the engine in an enclosed space because of carbon monoxide.

Common causes

Ethanol (E10) fuel sat for months and oxidized — ethanol absorbs water from humid air, then 'phase separates' into a watery layer at the bottom that, combined with degraded gum/varnish, looks like brown sludge. (most common)
Quick check:
No fuel stabilizer was added before storage, so the gasoline broke down into sticky resin and the lighter fractions evaporated, leaving a thick brown residue. (common)
Quick check:
Water intrusion past a loose or damaged tank cap / fuel gauge gasket, accelerating phase separation and corrosion of any metal fittings. (common)
Quick check:
Rust or scale flaking off internal metal parts (pickup tube, fittings) tinting the residue brown — more likely if any part of the system is metal rather than all-plastic. (less common)
Quick check:
Microbial growth ('diesel bug'-type slime) — rare in gasoline tanks but possible where standing water has persisted a long time. (rare)
Quick check:

How to fix it

Work outdoors, away from any ignition source. Do this on land, not in the boat, with no flames, sparks, cigarettes, running motors, or live electronics nearby. Have a Class B marine fire extinguisher (UL-rated 5-B or larger) within reach, wear nitrile gloves and eye protection, and keep kids and pets clear. Gasoline vapor is heavier than air and pools low — never do this in a closed garage or below deck.
Disconnect the fuel line at the tank and remove the tank from the boat entirely. Bring it to an open, ventilated spot. Have an approved gasoline container ready to catch the old fuel and a sealed pail for the sludge.
Drain all the old fuel and sludge out. Pour through a fine paint strainer or a water-separating funnel so you can see the brown layer and any free water. Old/contaminated gasoline is hazardous waste — never dump it on the ground, down a drain, or in the water. Take it to a municipal household hazardous-waste site or an auto parts store that accepts used fuel.
Inspect the inside with a flashlight. Check the pickup tube, the cap/vent gasket, and the fuel gauge sender seal for corrosion or cracking. If the tank is plastic and the gunk is just varnish, it will clean up. If you see heavy rust or a cracked tank, replace the tank — a leaking portable fuel tank is not worth saving.
Rinse and agitate. For a plastic tank, add a gallon or two of fresh gasoline (or a dedicated carb/fuel-system cleaner mixed per its label), put the cap on, and slosh it hard in all directions to lift the varnish off the walls and bottom. Set the tank on bare ground while doing this to dissipate static charge, and crack the cap slowly afterward to vent vapor. Drain again through the strainer. Repeat until what comes out is clear and bright with no brown film. Avoid water for rinsing — any water left behind restarts the problem and won't mix out of gasoline.
Let it dry, then reassemble. Make sure the tank is fully dry inside before refilling. Reinstall the pickup, cap, and any sender with intact gaskets. Replace the cap O-ring/vent gasket if it's hard or cracked so water can't get back in.
Protect everything downstream. The fuel line and primer bulb must be marine fuel-rated (USCG Type A1-15 / SAE J1527, fire-resistant) — automotive fuel hose is not approved in a boat. If gunk made it down the line, replace the fuel-line assembly and primer bulb and install or change the engine's inline/water-separating fuel filter (e.g., a 10-micron marine separator). Note that 'ignition protection' is a separate requirement that applies to electrical and ignition components in the gasoline engine space (pumps, switches, alternators), not to the fuel hose itself.
Refill with fresh, ethanol-free fuel if you can get it, or fresh E10, and add a marine fuel stabilizer (and an ethanol treatment if running E10) at the label dose. When refilling a portable plastic tank, set it on the ground — never in a truck bed or on the dock/boat — to avoid a static spark, and keep the nozzle in contact with the fill neck. Run the engine afterward to confirm clean fuel delivery, and watch the filter — a freshly cleaned tank often sheds a little remaining residue into the first filter, so check it after the first outing.
Going forward: keep the tank full during storage (less air space = less condensation), always stabilize fuel you'll store more than a month, store the tank out of direct sun and temperature swings, and seal the vent when not in use to keep humid air out.

DIY or call a pro?

Strongly DIY. A portable plastic tank is the one fuel job most competent owners can do themselves safely outdoors — drain, rinse with fresh fuel, refit good gaskets. Call a pro only if the contamination has reached the engine (rough running, clogged carb), if you have a permanently installed (built-in) tank instead of a portable one, or if you find a cracked/leaking tank or corroded metal you're not comfortable judging. Carburetor cleaning or built-in tank/fuel-system service is shop territory.

Tools & parts

Nitrile gloves and safety glasses
Class B marine fire extinguisher (UL-rated 5-B or larger)
Approved gasoline/jerry container for old fuel
Sealed pail or bag for sludge disposal
Fine paint strainer or water-separating funnel
Flashlight or inspection light
Fresh gasoline (preferably ethanol-free) for rinsing
Marine/automotive fuel-system or carb cleaner (label-rated for use in tanks)
Marine fuel stabilizer and ethanol treatment (e.g., STA-BIL Marine, Star Tron)
Marine fuel-rated (USCG Type A1-15 / SAE J1527) fuel line and primer bulb if replacing
Marine inline/water-separating fuel filter (e.g., 10-micron)
Replacement cap O-ring / vent and sender gaskets

Keep a record of every fix you make — what broke, what it cost, how you solved it.

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Based on: BoatUS / BoatUS Foundation; American Boat & Yacht Council (ABYC); U.S. Coast Guard / USCG Auxiliary; National Marine Manufacturers Association (NMMA); National Fire Protection Association (NFPA); Mercury Marine service guidance; Yamaha Marine service guidance

General marine-maintenance guidance, not a substitute for a qualified marine technician or surveyor. Boats and conditions vary; for fuel, electrical, fire, or structural issues — or anything safety-critical — consult a professional. Always follow your engine and equipment manuals.