Thread: Help with vapor lock

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Just to make sure we are talking the same thing, vapor lock is when fuel in the fuel lines changes from liquid to vapor, as the fuel pump cannot pump vapor, the engine stalls and usually will not restart until the fuel system has cooled enough for the fuel to return to liquid state. The term vapor lock is normally used to refer to the fuel vaporizing on a running engine.

Heat Soak is a condition where the fuel vaporizes in the fuel system after a hot engine is shut off. This will often occur where the fuel line runs very close to hot exhaust system components, engine block etc. It can also occur at the carburator. As the fuel sits stagnate in the fuel bowls it will expand (and in severe cases boil) and overflow into the intake manifold. If the level drops past a certain level the needle valve will open allowing the remaining fuel under pressure between the fuel pump and carburator to enter the fuel bowl causing additional flooding of the engine.

Vapor lock and heat soak problems are more common on carbureted engines using a mechanical engine mounted fuel pump. Vapor lock is normally not a major problem on fuel injected cars as the fuel system in under enough pressure to prevent the fuel from vaporizing.

Several factors can contribute to a vapor lock/heat soak problem.

Outside temperature.

Under hood temperature.

Quality of fuel (fuels containing ethanol have a lower boiling point than straight gasoline).

Fuel system routing.

Altitude (the higher the altitude, the less air pressure and liquids will boil at a lower temperature).

If it’s a vapor lock problem, common cures are:

Re-routing or insulating the fuel lines.

Adding a low pressure electric fuel pump at the tank to pressurize the fuel in the line to the fuel pump.

Adding a return fuel line as close to the carburator as possible to constantly circulate the fuel while the engine is running which will prevent the fuel from being in the lines long enough to pick up enough heat to vaporize.

Heat soak cures run from;

Better fuel

Setting the float level a little lower,

An insulating carburator spacer