My friend was told by his mechanic that drivers should avoid running cars on very low levels of petrol, and certainly not to a point when the low-fuel warning comes on. He could not say why. Is this true and, if so, what is the reason?

Fuel pumps used to be mounted along the fuel line between the tank and carburettor, and were mostly mechanically driven.

With early fuel-injection systems which required higher pressures and constant flow rates, engine designers switched to electrically powered fuel pumps. These pumps were mounted just outside the fuel tank or in the engine compartment.

In almost all cars these days, the pump is part of an assembly comprising the electrical motor, filter, pressure regulator and level sensor. This "fuel-delivery module" is installed in the fuel tank.

Electric fuel pumps are generally very reliable. A little known but important fact to bear in mind, however, is that the petrol it pumps acts as a fluid to provide both lubrication and cooling.

When the tank level is very low, there is a possibility of intermittent fuel starvation that could lead to increased bearing wear and temperature build-up. A pump that runs dry - even briefly - is likely to fail prematurely.

Also, with a reduced amount of fluid, the pump needs to work harder.

Therefore, it is prudent to avoid running the car till the low-fuel warning comes on.