The impeller of the belt-driven water pump is made of plastic, and if the blades break apart, there is nothing left to circulate the coolant. The belt keeps driving the pump, but the engine gets no cooling and heats up quickly.

This fault is dangerous because it gives no advance sign. There is no warning signal from a disintegrating impeller, and the engine temperature runs up faster than the gauge can follow; by the time it shows, the cylinder head can already be damaged. The breaking impeller is a fault of its own on the six-cylinder built between 1992 and 1995, which is why the maker went back to a metal impeller, and then used a composite impeller in the later engines, which proved more durable. A pump with a metal impeller is still available today, as a durability replacement.

There is, however, one sign that can be caught before the failure: play developing in the shaft of the pump. That means the bearing is wearing, and because the impeller sits on this shaft, play in the shaft signals the coming end of the pump. With the belt off it can be felt by hand.

On the M54 it appears at around 120 000 and on the N62 at around 150 000 kilometres. Once the engine has overheated, the consequence stays: a warped cylinder head and a damaged gasket are no longer put right by replacing the pump.