Weak fuel pump symptoms: misfires and power loss under load

What it means

On most petrol cars an electric pump sits submerged in the tank and sends fuel forward to the engine. On direct injection that low-pressure pump feeds a mechanical high-pressure pump on the engine, which raises the pressure for the injectors.

A pump that is wearing out can still hold enough pressure at idle, when the engine needs little, and then fall away when the engine asks for flow. A weak in-tank pump can also starve the high-pressure pump, and the code that results names the high-pressure side.

Common causes

• A misfire only under load
• A lean fuel trim code
• A fuel trim problem that shows most under load and less at idle
• On direct injection, a high-pressure fuel code that is really caused by the low-pressure pump
• A restricted serviceable fuel filter, which reduces flow and pressure under load

What to do now

1. Note whether the problem appears under load, such as accelerating or climbing, or at idle.
2. Ask for fuel pressure to be measured under load, then the ignition checked under load.
3. Ask for the full set of pressure tests: static pressure, running pressure, volume and residual pressure.
4. On direct injection, ask for the low-pressure supply to be tested first, and which pump was found at fault.
5. If your car has a serviceable fuel filter, follow the replacement interval in the manual. Many in-tank modules have no routine interval.

Pump, or a look-alike?

• A misfire only under load: a weak pump, low rail pressure or a restricted injector, but also a coil breaking down under load.
• Lean at idle, fine at speed: more likely an air leak than the pump.
• Power loss on a turbo engine: a boost leak or an exhaust restriction can look the same.
• A high-pressure fuel code on direct injection: test the low-pressure supply first.

What people often get wrong

That a high-pressure fuel code means the high-pressure pump. On direct injection the most common cause is a weak low-pressure pump in the tank starving it, which is why the low-pressure side is tested first.

When the car must go to a workshop

Opening any fuel line is a workshop job. Even the low-pressure side can spray fuel across a hot engine, and a direct injection system holds extremely high pressure. The manufacturer's procedure for releasing pressure comes first.

Read next

• P0171: system too lean
• Turbo losing power: what to check before the turbo
• Clogged fuel injector symptoms, and the faults that look the same

This page is general information and does not replace a professional assessment. Symbols, codes and behaviour vary between makes. Always check the handbook for your own car.