How to prevent a fuel vapor lock

Updated April 17, 2017

Vapour lock can affect any kind of engine during normal operation where the outside ambient temperature remains high, or the fuel system becomes overheated because of high engine temperatures or lack of insulation. Fuels that have high volatility can also cause vapour lock. When fuel boils inside a metal line or carburettor, it vaporises, which causes a halt of the fuel delivery. This condition can cause an engine to stall or refuse to restart, even with a perfectly running engine. Preventing vapour lock requires some observation of the existing fuel delivery system, along with some modifications.

Place the transmission in park or neutral, depending upon the type of transmission. Set the emergency brake and raise the hood. For any other engine applications, remove the engine cowl or case cover by unclasping the hood snaps. For boats, unclasp the cowl snaps by hand and pull the cover off. Some boat engine cowls might require you to use a socket and wrench to remove the bottom latch bolts.

Examine the fuel line routing, particularly the metal lines that attach to the carburettor or fuel rail -- injected systems. Any metal fuel line that sits directly over the intake manifold or in proximity to the exhaust manifold will serve as a high heat sink. Use a pipe cutter to remove as much metal line as possible. Leave enough of the original line to attach a hose.

Replace the metal line with neoprene impregnated fabric line or hose, by pushing it onto the cut end of the metal line. In the case of a fuel inlet, use a fuel line wrench to loosen the nut and remove the line completely. Screw an adaptor into the fuel inlet that has a hose nipple on the end. Use wire cutters to cut the appropriate length of hose you need. Attach the hose with hose clamps; tighten the clamps with a screwdriver. Route the hose properly, keeping it up away from heat sources.

Use scissors to cut strips of reflective heat insulation fabric that you can wrap around your fuel lines. You can wrap metal fuel lines or neoprene lines. Cut the lengths required to cover as much line as possible and tightly wrap them around the fuel line. Secure the insulation fabric with heat-resistant tape, by wrapping it around the insulation fabric several times.

Avoid using fuels that have a high volatility rate, which consist of fuels that have extra alcohol content. In the case of a marine engine, find out what your rated Reid Vapor Pressure states in your owner's manual. Drain your gas tank at the petcock valve with a socket, and then refill the tank with a lower RVP rating. This will reduce the volatility of the gas and ward off vapour lock.

Change the fuel filter by removing the filter hose clamps with a screwdriver. Disconnect the filter from the hoses and attach a new filter to each hose end. Tighten the hose clamps with a screwdriver. For metal in-line fuel filters, use a fuel line wrench to disconnect the fitting on both ends. Screw the fittings onto the new filter with a fuel line wrench. A new filter will increase fuel volume. Look for any sharp bends or kinks in the fuel line and straighten them out or replace the line.

Remove the radiator cap and check the engine coolant. Add water or coolant if the radiator is low. The thermostat and cooling fan must be in proper working order -- an engine that runs hot will vapour lock more readily. On marine engines, clean all obstructions from the engine cowl vents with a stiff brush. Use a stiff brush to clean all debris away from the radiator fins on automotive vehicle, such as road debris, plastic, leaves and insects.


In some cases on the older carburettor vehicles, the addition of a base gasket spacer between the intake manifold and carburettor will block radiated heat from the engine. The spacer has heat-absorbing qualities that will protect the carburettor float bowl from fuel percolation. You must check first to see if you have the hood clearance to install such a part. These spacers can be purchased at an auto parts store. Remove the fuel line, linkage attachments and carburettor bolts to the carburettor with a socket and screwdriver. Locate the fuel return line. You can splice in an extra length of fuel return line and wrap a foot or two of excess line around the cold AC evaporator line. This will cool the fuel inside the hose.

Things You'll Need

  • Socket set
  • Ratchet wrench
  • Screwdrivers
  • Fuel line wrenches
  • Fuel inlet adaptors
  • Neoprene or impregnated fabric fuel line
  • Pipe cutter
  • Wire cutters
  • Hose clamps
  • Reflective heat insulation
  • Scissors
  • Heat-resistant tape
  • Boat owner's manual (if applicable)
  • Fuel filter (if applicable)
  • Stiff cleaning brush
  • Carburettor base gasket spacer (optional)
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About the Author

Chris Stevenson has been writing since 1988. His automotive vocation has spanned more than 35 years and he authored the auto repair manual "Auto Repair Shams and Scams" in 1990. Stevenson holds a P.D.S Toyota certificate, ASE brake certification, Clean Air Act certification and a California smog license.