Fuel Filter

Injection systems have very tight tolerances, so fuel must be clean. On many petrol cars the filter sits in the tank as part of the pump module and is rarely replaced. On diesels it is usually a separate filter in the engine bay with a water separator. Diesel filters are changed at set intervals and some need draining of water. After a change, the system may need bleeding so no air reaches the high-pressure pump.

On many petrol cars the filter is part of the fuel pump module inside the tank and is considered a lifetime part. On diesels it is a regular service item.

Fuel passes through a fine filter medium that traps particles. In diesel filters, water is heavier than fuel and collects at the bottom, sometimes with a sensor that warns when it needs draining. Diesel filters often include a heater, so that fuel wax does not block the filter in cold weather, and a water drain or water-in-fuel sensor.

A blocked filter causes loss of power under load, hesitation, hard starting and low fuel pressure codes. Water reaching common-rail components can cause expensive damage. A blocked filter can cause hesitation, power loss under load and hard starting. Use the filter specified for your engine; a cheap filter with poor filtration can damage high-pressure injection parts.