Fuel pump and accessories for BMW Serie 3 - E46

Mechanical or electrical: two different fuel supply systems

• Mechanical diaphragm pump: driven by an eccentric on the camshaft or distributor, it draws in fuel and delivers it to the carburettor at low pressure, in the region of a few tenths of a bar. It only delivers fuel whilst the engine is running, which provides a safety feature in the event of an impact.
• Low-pressure electric pump: fitted in-line or in the fuel tank, it is suitable for carburettor set-ups and resolves hot-start issues. It requires a relay and a cut-off when the engine is switched off.
• High-pressure pump: reserved for direct-injection engines, it operates at pressures far higher than those of the previous types and must be replaced together with its push rod.

Fitting an electric fuel pump with an unsuitable pressure rating to a carburettor will flood the engine: the float cannot withstand excessively high pressure. This is the most common mistake made with this type of conversion.

Repair the pump rather than replace it

On a mechanical pump, the part that fails is almost always the diaphragm – which has hardened due to modern petrol – or the mounting seal. The pump body itself is generally in good condition and is sometimes impossible to find as a new part. Diaphragms, seals and intermediate flanges are therefore listed separately, including for older set-ups. A new diaphragm restores the flow rate at a fraction of the cost of a complete assembly, whilst allowing the original part to remain on the engine.

A useful tip: a punctured diaphragm sends fuel into the engine oil. If the oil level rises and smells of petrol, it is the diaphragm, not the cylinder head gasket.

Diagnose before ordering

• The engine stalls when hot and starts again once it has cooled down: this is caused by vapour lock, which is common when the pump or fuel line runs close to a heat source. The pump may be in good working order.
• The flow rate drops under load, when revving or going uphill: a worn diaphragm or clogged valves.
• No fuel is reaching the carburettor: before blaming the pump, check the suction pipe and the filter in the tank, which can become clogged with deposits.
• Smell of petrol in the oil: a punctured diaphragm; this must be dealt with immediately as diluted oil no longer lubricates properly.

Fuel pump for classic cars: a different requirement

A generic search for a fuel pump nowadays tends to yield submersible units for modern vehicles, sold by original equipment manufacturers and general retailers. This is not what a restorer is looking for: they need a mechanical diaphragm pump, a replacement diaphragm or a mounting gasket for a carburettor-equipped engine. This is what this range covers, and that is why it lists parts that generalist catalogues no longer stock.