BMW Fuel System: Fuel Pumps, Injectors and Common Faults

A BMW fuel system has one job: get fuel from the tank to the injectors at the right pressure. On a fuel-injected BMW petrol engine without direct injection that is a low-pressure system, with an electric pump in the tank feeding the injectors. On BMW's direct-injection petrol engines there is a second, high-pressure pump on the engine that raises the pressure much further before the fuel reaches the injectors. Knowing which system your car has is the first step to finding the fault.

This guide covers both systems, the symptoms of each kind of failure, why fuel pumps are so often replaced for nothing, and which fuel parts are worth buying used.

The two kinds of BMW fuel system

Low-pressure: the electric pump in the tank

A fuel-injected BMW petrol engine starts with an electric fuel pump in the tank. It draws fuel through a filter and sends it forward to the engine. On a direct-injection engine this low-pressure circuit feeds the high-pressure pump, and its pressure is set by the engine computer at up to 6.0 bar, depending on the system (HELLA Tech World, checking a high-pressure fuel pump).

High-pressure: BMW High Precision Injection

BMW calls its petrol direct injection "High Precision Injection". In its model-year 2008 announcement, BMW said that "in addition to the in-line 6-cylinder engines", its four-cylinder petrol engines now also had petrol direct injection, with the fuel delivered by piezo injectors placed between the valves, right next to the spark plug (BMW Group PressClub UK, 2007). On BMW's 2.0-litre TwinPower Turbo four-cylinder, BMW gives the maximum injection pressure as 200 bar (BMW of North America press information, April 2011).

That 200 bar is generated by a high-pressure pump driven by the engine. HELLA lists the parts of a direct-injection high-pressure system as the high-pressure pump, a pressure control device, the fuel rail, the high-pressure line, a fuel pressure sensor and the injectors.

Symptoms: which pump is failing?

In-tank fuel pump

HELLA's product information for fuel pumps lists the signs that the in-tank pump may need replacing: difficult or no starting, extended cranking, immediate or intermittent stalling, rough idle, poor acceleration and severe engine lag (HELLA brief information, fuel pump unit and fuel pump).

High-pressure pump (direct-injection engines)

For a high-pressure pump, HELLA lists erratic running, no power in the upper speed range, poor starting, the engine cutting out with the engine warning light on, and oil dilution. Because the engine computer monitors the high-pressure system, a fault here usually leaves a stored fault code, which makes a diagnostic scan the sensible first step.

Why BMW fuel pumps fail

For the in-tank pump, HELLA names contamination as one of the top causes: dirt and debris that get in when refuelling, or poor-quality fuel. Overheating and wear also cause failures, and driving often on a low tank speeds up wear because it stirs up debris from the bottom of the tank and draws it into the pump.

High-pressure pumps wear for different reasons. HELLA lists mechanical damage to the drive, roller tappet or pump element, leaks, poor lubrication from oil dilution or poor oil, service intervals that were not kept, and contamination coming from the low-pressure side.

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Before you buy a pump: the misdiagnosis problem

Which BMW fuel parts to buy used

• Good used buys: fuel rails, fuel tanks and filler necks. None of these has a motor or moving pump element, so a used one from a car that was hit elsewhere is usually a sound buy.
• Buy used with care: high-pressure pumps and injectors on direct-injection engines. They are costly new, but they must match your engine exactly, and an injector should be tested before it is fitted.
• Usually better new: fuel filters and the in-tank filter sock, which are service parts, plus any seals and O-rings.

If your problem looks more like overheating than fuel, our BMW cooling system guide covers that side of the engine.

What to send us for a fuel part

Fuel parts differ between engines within one model, so we need more than the model name:

1. The year and model, as they appear on your registration papers.
2. The engine code, which is stamped on the engine and recorded in the service book.
3. The VIN from the licence disc or the windscreen.
4. A photo of the old part and its plug.

For the 3 Series, the E90 parts page and the F30 3 Series spares page cover the common parts. For any other model, start with used BMW parts.