Vapor Lock: The Cause, Prevention, and Cure

October 27, 2018

Vapor lock is often a problem with older carbureted cars, but not an issue with modern electronic fuel injected cars. Fuel injection uses a computer to tell the injectors how much gasoline to squirt into the engine.  A carburetor is a mechanical device that uses the engine’s natural vacuum to suck the required amounts of fuel into the combustion chambers.


Vapor lock causes a car to stop running when the fuel in the system overheats. It is most likely to happen when driving on hot days and in stop-and-go traffic. Constant acceleration and deceleration make your engine work harder, causing it to run hotter. Excess heat causes the fuel to vaporize. This keeps the fuel from reaching the engine.


Many carbureted engines have fuel pumps located near or next to the engine. The pump’s closeness to the engine, as with some T-types, causes the fuel in the line to become very hot. When heated, fuel turns to a vapor, like water turns to steam when boiled. This process is hastened by the vacuum created in the line as the fuel is sucked into the engine.


When the fuel turns to vapor, the fuel pump can no longer move it through the system. As a result, the fuel doesn’t get to the combustion chambers; the car runs roughly; or it dies. Meanwhile, the car will not restart or will continue to have problems if it does.


Cooling fans in older cars run off of the momentum of the engine. They are somewhat inefficient while idling in traffic. Moreover, the lack of motion means less air flowing through the engine compartment, and the fan at the idling speed of the engine is unable to cool the engine sufficiently. Modern cars typically have remotely located fuel pumps, pressurized fuel lines, and electric cooling fans that detect the engine’s temperature causing them kick-in when needed. Thus, engines in modern cars are less likely to overheat.


Some Preventive Measures That Can Be Taken.
  • Install a Low-pressure Electric Fuel Pump near the Fuel Tank. This will keep the fuel moving through the lines even if the engine compartment heats up. Moss sells a solid-state electronic fuel pump that can be used in place of or in conjunction with, the original pump. It is easily hidden and can be installed near the gas tank away from engine heat. It can be switched on and off, as needed/desired. Some of these pumps are installed to replace a faulty SU Fuel pump. They  usually pump the fuel through the faulty pump which is not removed, thus preserving the original look.

  • Install the Carburetor-to-carburetor Fuel Line Away from the Manifold. Using a longer line may help, also.

  • Install an Electric Fan. These fans can be connected to an engine temperature sensor like in modern cars. Moss sells Hayden Electrical fans designed to run as either “pusher” (in front of the radiator) or “puller” configuration.

  • Locate the Carburetor Float Bowls as Far from the Manifold, as Possible.

  • Insulate the Fuel Lines.

  • Install a Heat Shield. There are shields available for some MGs similar to the factory ones on MGBs. There is another type of shield that is actually two separate shields that mount the same way but keep the heat from getting to the float bowls. They are sold by the company that is now manufacturing SU carburetors.

  • Install Bakelite Spacers. These are like those used on MG TF carburetors. Neil Nelson says he believes this was the MG Car Company’s first attempt to address the heat transfer between the intake manifold and the carburetor body. You can buy thinner ones (1/4") from the Company now making the SU carburetors.


Action to Take When Experiencing Vapor Lock.




  • Cool the System Down.  With the ignition off, pour cold water over the fuel pump, carburetors, and fuel lines. A longer-lasting solution is to strap a bag of ice at the scene of the problem like John Lovejoy did, recently. This will be quickly cool down the fuel pump and condense the fuel from vapor back to liquid, eliminating the vapor lock in the fuel system for an extend period of time.

  • Gently Start the Engine. Turn the key in the ignition to start the vehicle while at the same time slightly depressing the accelerator. Do not press the accelerator all the way to the floor, as this will send too much fuel through the system and prevent the vehicle from starting. Hold the accelerator down when the vehicle starts until the engine is running smoothly. The engine will sputter for a few seconds until the vapor lock is cleared from the lines.



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