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Vapor Lock | MG Classics of Jacksonville

Vapor Lock: The Cause, Prevention, and Cure

Help Your MG Fight the Summer Heat

Vapor lock is often a problem with older carbureted cars, but not an issue with a 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 ingest 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 in stop-go-traffic makes your engine work harder, causing it to run hotter and mechanically driven radiator cooling fans are not very effective when the engine runs at low rpm. Excess heat causes the fuel in the carburetors or feed lines to vaporize, keeping 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 heat source of the engine, as with some MG T-types, causes the fuel in the line to become very hot. When heated, fuel turns from a liquid to a vapor, as water turns to steam when boiled. Sucking fuel into the engine creates a vacuum, thus hastening vaporization and vapor lock.

When the fuel vaporizes, 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 dies. Meanwhile, the car may 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 ventilation in the engine bay 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 to kick-in when needed. Thus, engines in modern cars are less likely to overheat.

Some Preventative Measures That Can Be Taken.

  • Install an Electric Fuel Pump near the Fuel Tank at the rear of the car. The pump will keep the fuel moving through the lines even if the engine compartment heats up. Moss sells a pump that can replace or work in conjunction with the original pump. If the faulty pump remains, the replacement pump near the fuel-tank pumps the fuel through the original pump, thus preserving the original look.

  • Install the Carburetor-to-c

  • Carburetor Fuel Line Away from the Manifold. Using a longer line may help.

  • Install an Electric Fan. You connect these fans 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 exhaust Manifold, as Possible.

  • Insulate the Fuel Lines. (Google “Fuel line Insulation”)

  • 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 two separate shields that mount the same way but keep the heat from getting to the float bowls. Burlen Ltd ( is the sole manufacturer of SU carburetors. They also sell heat shields.

  • 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 a􀆩empt to address the heat transfer between the intake manifold and the carburetor body. You can buy thinner ones (1/4”) from Burlen Ltd (

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 expedient is to strap a bag of ice to the problem area like MG Classics member, John Lovejoy, a TD driver, did a year or so ago. The bag of ice will quickly cool the system components. Consequently, the fuel will change from a vapor to a liquid, thus, eliminating the vapor lock for an extended period.

  • If you are in no hurry open the bonnet and allow the engine to cool due to natural convection. This may take significant time on a hot, sunny day. ​​

  • Gently Start the Engine

1. Turn the key and press the ignition switch to start the vehicle while at the same time slightly depressing the accelerator.

2. Do not press the accelerator all the way to the floor. Doing so will send too much fuel through the system, thus preventing the vehicle from starting.

3. Depress accelerator, as needed, when the vehicle starts until the engine is running smoothly. Initially, the engine will splutter until there is no vaporized fuel in the system. Voila, you’re back on the road!

* When MG upgraded the Fuel pump location from the engine bay to the rear chassis on the later model TFs they changed the fuel pump from a L-Type to the L-Type HP with a slightly higher pressure rating to allow for extra line loses. However. there are many solid state fuel pump options available to fit onto the rear chassis in a location that is not visible to the casual observer.

Article originally published by James Nolan on 10/31/2014 and updated 04/01/2020

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