Fuel Delivery Systems

How to pin point the failure

Mark Lemay

All fuel systems fail in one of 2 ways. Either too much fuel or too little fuel is being delivered. The most common failure is too little fuel resulting in the engine running lean or not starting.

The fuel delivery system consists of the fuel pump, fuel filter, fuel rail and injectors, fuel pressure regulator and the electrical components including the wiring, fuel pump module and the fuel pump relay. Fuel delivery systems fall into 2 categories. Variable speed fuel pumps and non variable speed fuel pumps.

Excessive fuel supply

For complaints of too much fuel, the most common culprit is the fuel pressure regulator or the vacuum hose to the regulator. If this is the case, the engine will usually be rich at idle but will be close to normal under load. Install a fuel pressure gauge at the test port and compare the measured fuel pressure to the manufacturer’s specification. If the pressure is too high, check the vacuum hose to the fuel pressure regulator and repair as required. Note that all tests are done with the engine cranking or running.

Lack of fuel supply

For problems involving too little fuel or no fuel pressure use the diagram in Figure 1.  Start by measuring for battery voltage (B+) at test point A. If there is B+ at test point A, check for B+ at test point D. If both these readings check out, check the fuel pump ground by measuring the fuel pump ground circuit at test point E. The voltage on the ground circuit must be under 0.5 volts. It the voltage and the ground at the fuel pump test normal, then the fuel pump is defective.

Circuit check up

However, if the fuel pump is not being powered up and the voltage at the fuel pump relay test point B test normal, then the control circuit from the PCM must be checked. Start by measuring the voltage at test point C while cranking the engine. If B+ is measured at test point C, measure the relay ground circuit at test point F. If the measurements at C and D are normal then the fuel pump relay is defective. If there is no voltage from the PCM, then the wiring between test points F and G must be checked for opens and shorts. If this circuit test good, then the problem lies with the PCM.

Variable speed fuel pumps

For vehicles with variable speed fuel pumps (Figure 2), the initial testing remains the same as previously described. However, if there is no power to the pump, make sure that you have B+ and proper ground at test points J and K. If these test normal, check the fuel pump control circuit at test point H. Set you voltmeter to measure frequency (frequency will vary with load and system) and crank the engine. If a frequency is measured and the module power and ground check out, then the driver module is bad.

If there is no frequency measured, check the control circuit back to the PCM test point I. If the control circuit tests normal, then the problem lies with the PCM.

For additional information on troubleshooting these systems, consider attending a training class in your area or taking the class online.