Go With the Flow: BASIC FUEL SYSTEM ANALYSIS

Motor,  Jul 2007  by Thompson, John

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In operation, fuel is pumped from the fuel delivery module inside the fuel tank, through a check valve and fuel filter, pressure transducer, fuel rail and finally through the fuel injectors. The fuel pump pumps only the amount of fuel needed to keep the fuel rail at the desired or set operating pressure.

Understanding how the FRP PID is calculated is crucial to understanding the system strategy. If the PCM desires 40 psi of pressure, 40 psi is the target pressure it sets for the injector nozzles, not for the fuel rail! It's important to note that the FRP PID on a scan tool does not reflect die actual line pressure you'd see with a fuel pressure gauge.

The FRP sensor is not only responsible for calculating pressure in the fuel rail; using a short vacuum hose attached to the intake plenum, it also acts as a vacuum transducer. Using manifold vacuum to extrapolate pressure drop across the injectors, the FRP sends a feedback calculation to the PCM. Negative psi can be calculated by halving vacuum pressure measured in inches of mercury (1 in. Hg. = .5 psi). By sensing manifold vacuum, the current negative pressure at the injector outlets (nozzles) in the plenum is calculated by the FRP and added to the positive fuel rail pressure at the injector inlets.

For example, 30 psi of fuel pressure in the rail added to 20 in. Hg sensed manifold vacuum (-10 psi) would result in an FRP PID reading of 40 psi. That is, 30 psi at the top of the injectors added to -10 psi of pressure present at the injector nozzles equals 40 psi of pressure exiting the injector nozzles.

What would happen if the throttle were snapped from fully closed to wide open? A manifold vacuum drop to O in. Hg would be seen by the FRP as O psi at the injector nozzles while only 30 psi of pressure is present in the fuel rail. In this situation, the PCM would calculate that increased fuel rail pressure would be required to maintain the system target of 40 psi pressure at the injector nozzles. The PCM would immediately send a command to the FPDM to increase the fuel pump duty cycle in order to raise the actual rail pressure.

Flow or current testing of electronic returnless fuel systems requires the pump to run continuously. You may command the pump continuously on using a scan tool and sending a 50% duty cycle command.

Remember, just because you have "good" pressure and are satisfied with current analysis of a fuel pump does not mean that the pump is delivering the volume of ftiel the injectors will require under all operating conditions. Let's discuss the differences among fuel pressure, volume and flow.

Fuel Pressure=Energy/Volume

Extended engine cranking before start-up can indicate a loss of residual fuel pressure, which should remain constant even when an engine has been shut off for several hours. Pressure could be lost on the supply side of the system by a bad fuel pump check valve or a leaking supply line. On the other hand, with a conventional fuel system, it could also be lost by a poorly seated fuel pressure regulator on the return side of the system.