How to Inspect Boat Fuel for Water Intrusion

When water mixes with your boat’s fuel, it quickly leads to poor engine performance that stresses the motor. 

Water in boat fuel

Water suspended in fuel also clogs carburetors and fuel injectors, and can lead to “phase separation”, which is the forming of a gelatinous layer of water/ethanol within the fuel tank that needs to be removed.

Checking for Water in Boat Fuel

Before checking your boat’s fuel, make sure you’re in a well-ventilated area with no naked flames or other flammable sources of ignition.

Check for water in boat fuel

Check the Fuel Line 

Disconnect the line running from the fuel tank at the fuel filter, and drain out a couple of pints of fuel into a clean glass jar. Leave the fuel to sit for half an hour so any water within it has time to settle.

Siphon the Fuel Tank

Use a hand pump to siphon fuel from the very bottom of the tank directly into a clean glass jar. Leave the fuel to sit for half an hour to give any water in the fuel time to settle.

  • Water mixed in oilUncontaminated: Fuel will be amber in color.
  • Water in the fuel: The water will form a clear bubble or layer at the bottom of the jar because it’s denser than gasoline.
  • Phase separation: Fuel will have an opaque gelatinous layer at the bottom of the jar.

NOTE: Photo is meant to illustrate what bubbly or gelatinous oil in water may look like. Don’t pour the fuel into water when doing the test.

Use Water-Finding Paste

Apply water-finding paste to a wooden dowel, then insert it down to the bottom of the fuel tank and remove it. The paste will change color if comes into contact with water. 

NOTE: Water-finding paste is not particularly accurate and won’t tell you whether if it’s water or phase separation at the bottom of the fuel tank.

Removing Water From a Fuel Tank

Portable fuel tank. If your boat’s fuel tank is portable, remove it from the boat and take it to a marina or boatyard that offers fuel and oil disposal services, or take it to a local hazardous waste disposal site.

Boat fuel tank water check

Small fuel tank. If your boat’s fuel tank is small enough (approximately 5 to 8 gallons), pump out the old gas into plastic cans and take them to a marina or boatyard that offers fuel and oil disposal services, or to your local hazardous waste disposal site.

Large fuel tank. If your boat’s fuel tank is over 5-8 gallons, disposing of such a large quantity of gasoline should be carried out by a marina or boatyard that offers fuel and oil disposal services.

Inspect for water in boat fuel

Preventing Water from Getting Into Boat Fuel

Use ethanol-free gas. The ethanol in blended fuels attracts and absorbs water molecules. Using E0 fuel (ethanol-free gas) prevents water absorption.

Boat fueling in water

Consume the fuel. Don’t leave fuel in the tank any longer than necessary. The less time fuel is in the tank, the less likely it is to get contaminated with water. Fill your tank with enough fuel for your next trip, then refill it for the one after that to keep the fuel clean and fresh.

Use fuel stabilizer. If you can’t find E0 fuel and have to use E10 fuel (gasoline containing 10% ethanol), add marine-specific fuel stabilizer to the fuel when you fill up. Fuel stabilizer reduces the water absorption rate of the ethanol and keeps E10 fuel fresh for longer.

Sta-bil fuel stabilizer

Yamaha outboard fuel filter

Replace fuel filters. Regularly inspect, clean or replace your boat’s fuel filters. Refer to your boat engine’s owner’s manual for how often this should be done.

Winterize the fuel tank. Condensation forming within the fuel tank is the #1 cause of water getting into fuel. When winterizing or storing your boat, fill the fuel tank to capacity so there’s no room for condensation to form inside the tank while it’s in storage.

 

 

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