FUEL SYSTEM PRESSURE
49.2 psi +/- 2 psi (339 kPa +/- 34 kPa).
The fuel pump module assembly is located on the top of the fuel tank. The complete assembly contains the following components:
- A combination fuel filter/fuel pressure regulator
- Electric fuel pump
- Fuel pump reservoir
- A separate in-tank fuel pick-up filter (strainer)
- Pressure relief/rollover valve
- Fuel gauge sending unit (fuel level sensor)
- Fuel supply line connection
- Auxiliary non-pressurized fuel supply fitting (not all engines)
The fuel gauge sending unit may be serviced separately. On many trucks, if the electrical fuel pump, primary inlet filter, fuel filter or fuel pressure regulator require service, the entire fuel pump module must be replaced (until there is an aftermarket source for individual component parts).
All gasoline engines have a fuel filter inside the fuel tank at the bottom of the fuel pump's pickup. There is also a combination fuel filter and fuel pressure regulator located on the top of the fuel pump module. A replaceable in-tank filter is attached to the bottom of the fuel pump module on some 1994-1998 trucks. A separate frame or engine mounted fuel filter is not used with any engine. Both fuel filters (at bottom of fuel pump module and within fuel pressure regulator) are designed for extended service and do not require normal scheduled maintenance unless a diagnostic procedure indicates that one or both of these filters require service.
In Tank Fuel Filter Replacement 1. Drain fuel tank and remove tank. Refer to Fuel Tank Removal/Installation. Note: Some people report success with partially dropping the tank to gain access to the tank top. YMMV! 2. Remove the Fuel Pump Assembly fromr the tank (see service manual!) 3. Pry the mounting tabs back and remove the intank fuel filter. 4. Install new fuel filter into retaining tabs. 5. Install the Fuel Pump Assembly in the tank (see service manual!) 6. Install fuel tank and check for fuel leaks.
Regulator / Filter:
Two different types of filter/regulators are used. If a rubber grommet is located between the bottom of filter/regulator and top of fuel pump module, it can be serviced separately. If this grommet is not used, the entire Fuel Pump Assembly must be replaced.
Fuel Filter / Regulator Replacement 1. Drain fuel tank and remove tank. Refer to Fuel Tank Removal/Installation. Note: Some people report success with partially dropping the tank to gain access to the tank top. YMMV! 2. The fuel filter/regulator is pressed into a rubber grommet. Remove by twisting and pulling straight up. CAUTION: Do not pull the filter/regulator more than three inches from the fuel pump module. Damage to the coiled fuel tube (line) may result. 3. Remove the snap ring retaining the convoluted tube (cover) to the filter/regulator. Slide the plastic tube down the fuel tube to clear the fuel tube clamp. 4. Gently cut the old fuel tube (line) clamp taking care not to damage the plastic fuel tube. Remove and discard the old fuel tube clamp. 5. Remove the plastic fuel tube from the filter/regulator by gently pulling downward. Remove filter/regulator from fuel pump module. 6. Install a new clamp over the plastic fuel tube. 7. Install filter/regulator to fuel tube. Rotate the filter/regulator in the fuel tube (line) until it is pointed toward the drivers side of vehicle. 8. Tighten line clamp to fuel line using special Hose Clamp Pliers number C-4124 or equivalent. Do not use conventional side cutters to tighten this type of clamp. 9. Slide the convoluted plastic tube (cover) up to the bottom of filter/regulator and install snap ring 10. Press the filter/regulator (by hand) into the rubber grommet. The assembly should be pointed to the drivers side of vehicle 11. Install fuel tank and check for fuel leaks.
Fuel Pressure Regulator Operation:
The pressure regulator is a mechanical device containing a diaphragm, calibrated springs and a fuel return valve. The regulator is not controlled by engine vacuum or the Powertrain Control Module (PCM), and it is calibrated to maintain fuel system operating pressure of approximately 49.2 psi ± 5 psi (339 kPa ± 34 kPa) at the fuel injectors. If fuel pressure at the pressure regulator exceeds 49.2 psi, an internal diaphragm opens and excess fuel is returned the tank through the bottom of pressure regulator. A separate fuel return line from the engine to the tank is not used. The internal fuel filter is also part of the assembly.
To help when starting the engine, the regulator acts as a check valve to maintain some fuel pressure while the engine is off. Fuel is supplied to the filter/regulator by the electric fuel pump inside the fuel tank below the filter/regulator. A second check valve is located at the the electric fuel pump outlet.
The fuel gauge sending unit is attached to the side of the fuel pump module. The sending unit consists of a float, an arm, and a variable resistor. The PCM supplies constant current to the variable resistor and monitors the voltage drop accross the resistor. Using this voltage returned from the fuel level sensor, the PCM calculates the fuel level and supplies the level as digital information to the instrument panel for the fuel gauge.
The fuel pump module has 4 different circuits (wires). Two of these circuits are used for the fuel gauge sending unit for fuel gauge operation, and for certain OBD II emission requirements. The other 2 wires are used for electric fuel pump operation. from fuel pump module.
For diagnostic purposes, the 12V constant current source can only be verified with the circuit opened (fuel pump module electrical connector unplugged). With the connectors attached, fuel level output voltages will vary from about 0.6 volts at FULL, to about 7.0 volts at EMPTY. The variable resistor track is used to vary the voltage (resistance) depending on fuel tank float level. As fuel level increases, the float and arm move up, which decreases voltage. As fuel level decreases, the float and arm move down, which increases voltage. The varied voltage signal is returned back to the PCM through the sensor return circuit.
Fuel level and OBDII diagnostic codes:
If the fuel level is less than approximately 15 percent of the tank capacity, the OBD II system will not record/set a misfire code. The Leak Detection Pump EVAP system monitor (if equipped) will be activated if the fuel level in the tank is more than approximately 85 percent of tank capacity.
Auxiliary Fuel Fitting:
An auxiliary (capped) fuel supply fitting is located on the top of the fuel pump module (Fig. 1). This fitting will be available only on models with either a 5.9L V-8 heavy duty or 8.0L V-10 engine and all cab-chassis models. This fitting supplies a non-pressurized auxiliary fuel source.
The electric fuel pump is located inside the fuel pump module, which installs though the top of the fuel tank. A 12 volt, permanent magnet, electric motor powers the fuel pump. Voltage to operate the electric pump is supplied through the fuel pump relay. Fuel is drawn in through a filter at the bottom of the module and pushed through the electric motor gearset to the pump outlet.
The bottom section of the fuel pump module contains a one-way check valve to prevent fuel flow back into the tank. When pump is not running, this check valve maintains fuel supply line pressure when the engine is warm, and keeps the fuel supply line full of gasoline. After the vehicle has cooled down, fuel pressure may drop to 0 psi as the gasoline cools and contracts, but liquid gasoline will remain in fuel supply line between the check valve and fuel injectors. Fuel pressure that has dropped to 0 psi on a cooled down vehicle (engine off) is a normal condition.
The electric fuel pump is not a separate, serviceable component. See the Service Manual for fuel system component removal and replacement instructions.
WARNING: THE FUEL SYSTEM MAY BE UNDER A CONSTANT PRESSURE (EVEN WITH THE ENGINE OFF). BEFORE SERVICING ANY FUEL SYSTEM HOSES, FITTINGS, LINES, OR MOST COMPONENTS, FUEL SYSTEM PRESSURE MUST BE RELEASED. REFER TO FUEL SYSTEM PRESSURE RELEASE PROCEDURE IN THE FACTORY SERVICE MANUAL.
CAUTION: The interior components (o-rings, spacers) of some types of quick-connect fitting are not serviced separately. If service parts are not available, do not attempt to repair a damaged fitting or fuel line. If repair is necessary, replace complete fuel line assembly.
This page was edited on: May 3, 2004