Parting Thoughts

By Robert Van Brunt , Chief Petty Officer U.S.G.G. ret

ETHANOL AND VAPOR LOCK

Short description:
When the engine compartment becomes hot either by climate or idling, and you use ethanol-blend gasoline it can cause excessive vapors in your fuel line and starve the engine of fuel.  The engine can run poorly or stop and will not run until the fuel condenses.

THE PROBLEMS

Vapor Lock

Fuel containing 10% ethanol is called E10.  If you have ethanol in your gas, you run the risk of creating vapor lock because of excess vapors.

Ethanol “boils” at 87ºF (at normal atmospheric pressure) and turns from a liquid to a gaseous state.  By comparison, most automobiles have their fuel pump in the gas tank, so the whole system remains under pressure unlike boats whose fuel tanks are vented.  In a closed system, the higher pressure raises the flash point of the ethanol reducing the amount of vapor that is produced.  In addition, most automobile fuel lines are outside of the vehicle allowing them to stay cooler.

Since most boat fuel lines are in the enclosed space (sometimes even insulated) of the engine compartment, normal ventilation will not cool the fuel significantly enough to avoid the potential problems of vapor lock.  Furthermore, since the fuel pump in a boat is mounted on the engine (versus a car where the pump resides in the tank) the action of the pump can reduce pressure in the tank to below atmospheric pressure and further reduce the flash point.

Boat engineers are aware of this problem and are reducing the likelihood of this occurring by reducing the suction required by the fuel pump, minimizing hose fittings and bends, and including a quality anti-siphon valve.  In existing boats, fuel lines and filters should be kept as low in the boat as possible and tank vents should be cleaned and open.

Heat Soak

Most boats have “forced” ventilation.  Air moves through the engine compartment when the boat moves forward.  Heat soak happens after you have been at high RPM and then stop or drift on idle for a while.  Because of heat soak the engine compartment will rise to a point where the ethanol will boil

THE CURE

To prevent vapor lock (i.e. boiling ethanol):

1. Make sure the engine compartment has adequate ventilation.
2. Relocate fuel lines to be low in the bilge.  (The bilge is cooler because it is in direct contact with the water.)
3. Monitor the engine compartment temperature.
4. Add (or turn on) engine room blowers.
5. Keep the tank vent clean and unobstructed.

ETHANOL, WATER AND YOUR FUEL SYSTEM

PROBLEMS

Ethanol is a solvent and can damage fiberglass tanks. It can dissolve old sludge in tanks which mixes with the gasoline and clogs filters.  This can cause the engine to run very poorly.

Since ethanol molecules bond with water molecules the fuel can be contaminated by water.  Fuel/water separators (filters) are supposed to separate out this water, but the amount of water trapped by the ethanol can quickly overwhelm the filter and allow water to pass through to the engine.  Moist air enters the fuel tank by the tank vent (remember automobiles do not have this problem due to their closed system designed to help reduce emissions).  Water can also settle to the bottom of the tank and cause the engine to run poorly and corrode fuel system components especially if the tank is not kept close to full.

E10 fuel is not as stable as past formulas.  Older formulations would stay “fresh” for about 6 months.  E10 can go stale in about 2 – 3 weeks.  This is not a problem for an automobile with a 15 gallon tank, but if you have a boat with a 250 gallon tank, watch out!

THE CURE

1. The best way to keep water from the engine is to install a good water separator with a 10 micron filter to remove sludge and other contaminants.  It is also crucial to drain the filter and check it more frequently than you may have in the past.
2. Volvo Penta recommends adding Sta-bil ™ to the fuel system.  This must be done with new fuel.  The Sta-bil will not treat stale fuel.Purchase 32 oz bottle
   Purchase 8 oz bottle