Issue 16: Fuel Pump Misdiagnosis

Fuel pumps are amongst the most common parts to be replaced within the fuel system but misdiagnosis of the symptoms can often mean the fuel pump is being replaced unnecessarily and ultimately won’t resolve the issues being experienced by the vehicle.

Faulty Relay

Symptoms – Engine stalling or not starting

The relay is the primary switch that controls the flow of power to the  fuel pump, allowing it function.  Any faults with the relay that can range from an intermittent flow of power to no flow of power at all,  will prevent the fuel pump from functioning correctly or at all.


Using a voltmeter, check the main and actuating feeds are present, as well as the complimenting earths at the relay multi-plug. Using an ohmmeter, check the resistance across the relay field coil terminal to ensure continuity.

Failed pressure regulator

Symptoms – Misfires, decrease in power/acceleration and fuel efficiency

The fuel regulator is responsible for controlling the pressure of the fuel being delivered to the injectors. The flow of fuel is dependent on varying engine operating conditions, in which the regulator needs to react to ensure the correct flow of fuel through the system. If the regulator is faulty, the ratio of air-to-fuel mix required for combustion can be thrown off, causing a range of issues.


By attaching a fuel pressure gauge to either the regulator itself, or the fuel line and running the engine at idle speed, you’ll be able to view the operating pressure and compare this with the manufacturer’s specifications.

Clogged Fuel Filter

Symptoms – Engine hesitation, stalling and not starting

Fuel filters are responsible for preventing any dirt or corrosion particles in the fuel from reaching the engine.  Over time, fuel filters become dirty and eventually clogged which will result in poor engine performance.


Fuel filters should be regularly replaced as part of a vehicle’s service and therefore should rarely become blocked.  If symptoms of a blocked filter occur, due to the relatively low cost of the product, it’s recommended that they be replaced.

Issues with the wiring or fuse

Symptoms – Engine Cranks but will not start

If the ignition (petrol) and compression (diesel) systems on the vehicle are performing as expected, failure for the vehicle starting could be due to a lack of fuel. When starting the engine, the pump should make a noise as it begins to build pressure. If no noise is heard, it could be down to a lack of power reaching the pump due to a poor connection or a blown fuse.


Having confirmed that the relay is functioning as expected, check the required positive and negative feeds are being received at the pump. This may include checking the condition and connections of the fuse in the fuse-box and the continuity of the wire from the fuse to the pump with an ohmmeter. A physical check of the wiring should also be carried out to highlight chaffing and corroded terminals.

Cambiare Tips and Tricks issue 16