Vapour lock is a problem that mostly affects petrol-fuelled internal combustion engines. It occurs when the liquid fuel changes state from liquid to vapour while still in the fuel delivery system. This disrupts the operation of the fuel pump, causing loss of feed pressure to the carburetor or fuel injection system, resulting in transient loss of power or complete stalling. Restarting the engine from this state may be difficult. The fuel can vaporise due to being heated by the engine, by the local climate or due to a lower boiling point at high altitude. In regions where higher volatility fuels are used during the winter to improve the starting of the engine, the use of "winter" fuels during the summer can cause vapor lock to occur more readily.


Causes and Incidence
Vapour lock was far more common in older petrol fuel systems incorporating a low-pressure mechanical fuel pump driven by the engine, located in the engine compartment and feeding a carburetor. Such pumps were typically located higher than the fuel tank, were directly heated by the engine and fed fuel directly to the float tank inside the carburetor. Fuel was drawn under negative pressure from the feed line, increasing the risk of a vapour lock developing between the tank and pump. A vapour lock being drawn into the fuel pump could disrupt the fuel pressure long enough for the float chamber in the carburetor to partially or completely drain, causing fuel starvation in the engine. Even temporary disruption of fuel supply into the float chamber is not ideal; most carburetors are designed to run at a fixed level of petrol in the float chamber and reducing the level will reduce the air:fuel mixture delivered.

Carburetor units may not effectively deal with fuel vapour being delivered to the float chamber. Most designs incorporate a pressure balance duct linking the top of the float chamber with either the intake to the carburetor or the outside air. Even if the pump can handle vapour locks effectively, fuel vapour entering the float chamber has to be vented. If this is done via the intake system, the mixture is, in-effect, enriched, creating a mixture control and pollution issue. If it is done by venting to the outside, the result is direct hydrocarbon pollution and an effective loss of fuel efficiency and possibly a petrol odour problem. For this reason, some fuel delivery systems allow fuel vapour to be returned to the fuel tank to be condensed back to the liquid phase. This is usually implemented by removing fuel vapour from the fuel line near the engine rather than from the float chamber. Such a system may also divert excess fuel pressure from the pump back to the tank.

Most modern engines are equipped with fuel injection, and have a high pressure electric fuel pump in the fuel tank. Moving the fuel pump to the interior of the tank helps prevent vapour lock, since the entire fuel delivery system is under high pressure and the fuel pump runs cooler than if it is located in the engine compartment. This is the primary reason that vapour lock is rare in modern fuel systems. For the same reason, some carbureted engines are retrofitted with an electric fuel pump near the fuel tank.

Other solutions to vapor lock are rerouting of the fuel lines away from heat generating components, installation of a fuel cooler or cool can, shielding of heat generating components near fuel lines, and insulation of fuel lines.

A vapour lock is more likely to develop when the vehicle is in traffic because the under-bonnet temperature tends to rise. A vapour lock can also develop when the engine is stopped while hot and the vehicle is parked for a short period. The fuel in the line near the engine does not move and can thus heat up sufficiently to form a vapour lock. The problem is more likely in hot weather or high altitude in either case.


Incidence with other fuels
The higher the volatility of the fuel, the more likely it is that vapour lock will occur. Historically, gasoline (petrol) was a more volatile distillate than today and was more prone to vapour lock. Conversely, fuel for diesel engines is far less volatile than petrol and thus these engines hardly ever suffer from vapour lock. However, diesel engine fuel systems are far more susceptible to air locks in their fuel lines as standard diesel fuel injection pumps rely on the fuel being non-compressable. Air locks are caused by air leaking into the fuel delivery line or from the tank rather than the fuel evaporating in them. Eliminating such air locks requires an extended period of turning over the engine using the starter motor or manually bleeding the system.

A vapour lock is more likely to develop when the vehicle is in traffic because the under-bonnet temperature tends to rise. A vapour lock can also develop when the engine is stopped while hot and the vehicle is parked for a short period. The fuel in the line near the engine does not move and can thus heat up sufficiently to form a vapour lock. The problem is more likely in hot weather or high altitude in either case.
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Hallo Yohanv,

Identiek aan probleem wat ik had,
Zie " warmestart probleem 3.0 SC" datum 10 okt j.l.

Hier staat veel info voor je in,
Bij mij was het uiteindelijk een kabelbreuk in de kabel naar CDC Unit, (dat aluminium apparaat met koelribben links in motorcompartiment)

Succes! en ben benieuwd wat bij jou het probleem uiteindelijk is.