Fuel Pump Follies

Basic tests

Is the vehicle out of gas? Fuel gauges can be stuck or wildly inaccurate, so the first step is often to add 8-10 litres of fuel and crank the engine. Listen for fuel pump operation with KOEO. If it’s silent, go to a voltage drop test. If it runs, use a fuel pressure test.

Fuel Pressure Test

Install a gauge on the fuel rail Schrader valve, or if necessary “tee” it into the system between the filter and the pressure regulator. Make sure that mechanical gauges are rated for the expected pressure and remember that mechanical gauges are most accurate in the middle of their scales, not at the top or bottom. For returnless systems, low pressure points toward the pump. For a return-type system, check the return line and pressure regulator first before going into the tank. Pinching off the return line and watching for pressure climb is a useful pump test. A filter change can sometimes bring pressure up to spec, but if it’s that clogged and has been changed regularly, the tank is probably contaminated.

Pressure above spec

Returnless system: Faulty pump

Return-type system: Defective regulator or return line

Pressure in spec

Returnless: O.K.

Return:Pinch return line

Pressure rises: Pump OK

Pressure doesn’t rise: Check fuel filter

Pressure below spec:

Check fuel filter

The voltage drop test follows the pressure testing procedure. It’s important to check both positive and negative sides of the circuit. Perform the test with the original wiring connected and energized. Inspect connectors and wiring for corrosion, chafed insulation and melted plastic at connectors. Loose connectors, especially at the hangar assembly are another possible source.

Sum of all voltage drops (positive and negative) cannot exceed 1.5 volts. If it exceeds 1.5 volts, check across each component in the system; fuse, relay, connectors and wiring on the power and ground sides of the circuit. The idea is to test as much of the circuit as you can reach with the tank in place. If voltage drop is less than 1.5 volts, remove the pump and test the remainder of the circuit. Check for faulty hoses and pulse dampers while you’re there. If the electrical circuit is O.K., replace the pump, but consider a replacement connector assembly or harness too, if the original is marginal. They’re available for many models with known connector wear issues.

Common Mistakes

* Re-using old strainer

* Pounding strainer onto pump

* Failure to flush/clean old tank

* Failure to strain fuel returned to tank after job

A point about time and money

Most repair databases quote fuel pump service time based on a simple remove and replace procedure. Fuel tank cleaning or flushing is billable time, so don’t forget to include it in the work order. Also remember that a considerable amount of additional fresh gasoline added for diagnostic purposes is a legitimate shop expense, especially at over a buck a litre!