Fuel Systems

Fuel Systems

Overview
The fuel system supplies the sequential multiport fuel injection (SFI) fuel injectors with clean fuel at a controlled pressure. The PCM controls the fuel pump and monitors the fuel pump circuit. The PCM controls the fuel injector on/off cycle duration and determines the correct timing and amount of fuel delivered. When a new fuel injector is installed it is necessary to reset the learned values contained in the keep alive memory (KAM) in the PCM. See RESETTING THE KEEP ALIVE MEMORY (KAM).

Electronic Returnless Fuel System

Electronic Returnless System (ERFS):

Electronic Returnless Fuel System

The fuel system consists of a fuel tank with reservoir, fuel pump, fuel filter, fuel supply line, fuel rail, fuel rail pulse damper, fuel injectors, and schrader/pressure test point. Operation of the system is as follows:
1. The fuel delivery system is enabled during key ON, engine OFF for 1 second and during crank or running mode once the PCM receives a crankshaft position (CKP) sensor signal.
2. The fuel pump logic is defined in the fuel system control strategy and is executed by the PCM.
3. The PCM commands a duty cycle to the fuel pump driver module (FPDM).
4. The FPDM modulates the voltage to the fuel pump (FP) required to achieve the correct fuel pressure. Voltage for the fuel pump is supplied by the power relay or FPDM power supply relay. For additional information see, FUEL PUMP CONTROL and FUEL PUMP MONITOR.
5. The FRP or FRPT sensor provides the PCM with the current fuel rail pressure. The PCM uses this information to vary the duty cycle output to the FPDM to compensate for varying loads.
6. The FRPT sensor measures the current fuel temperatures in the fuel rail. This information is used to vary the fuel pressure and avoid fuel system vaporization.
7. The fuel injector is a solenoid-operated valve that meters the fuel flow to each combustion cylinder. The fuel injector is opened and closed a constant number of times per crankshaft revolution. The amount of fuel is controlled by the length of time the fuel injector is held open. The fuel injector is normally closed, and is operated by a 12-volt source from either the electronic engine control (EEC) power relay or the fuel pump relay. The ground signal is controlled by the PCM.
8. There are 3 filtering or screening devices in the fuel delivery system. The intake filter is a fine, nylon mesh screen mounted on the intake side of the fuel pump. There is a fuel filter screen located at the fuel rail side of the fuel injector. The fuel filter assembly is located between the fuel pump and the fuel rail.
9. The fuel pump (FP) module is a device that contains the fuel pump and the fuel sender assembly. The fuel pump is located inside the reservoir and supplies fuel through the fuel pump module manifold to the engine and the fuel pump module jet pump.
10. The inertia fuel shut-off (IFS) switch is used to de-energize the fuel delivery secondary circuit in the event of a collision. The IFS switch is a safety device that should only be reset after a thorough inspection of the vehicle following a collision.

Typical Electronic Returnless System Schematic:

Fuel Pump Control - ERFS
The FP signal is a duty cycle command sent from the PCM to the FPDM. The FPDM uses the FP command to operate the fuel pump at the speed requested by the PCM or to turn the pump off. When the key is turned on, the electric fuel pump runs for about 1 second and is requested off by the PCM if engine rotation is not detected.

Fuel Pump Duty Cycle Output From PCM:

For additional information, see POWERTRAIN CONTROL HARDWARE, Fuel Pump Driver Module (FPDM).

Fuel Pump Monitor (FPM) - ERFS
The FPDM communicates diagnostic information to the PCM through the FPM circuit. This information is sent by the FPDM as a duty cycle signal. The 3 duty cycle signals that may be sent are listed in the following table.

Fuel Pump Driver Module Duty Cycle Signals

Fuel Pump Duty Cycle From PCM:

a. If a duty cycle meter and breakout box is used, be aware that these values may be reversed depending on the trigger setting of the specific meter (for example, 25% from FPDM may read as 75% on duty cycle meter depending on trigger setting).
b. DTC: Diagnostic trouble code, retrieved using scan tool. Some scan tools display the FP_M PID as the duty cycle in column 1. Other scan tools display the FP_M PID as a value shown in the FP_M PID column. This value fluctuates randomly. It is OK for the value to briefly go outside this range, then returns.

For additional information, see POWERTRAIN CONTROL HARDWARE, Fuel Pump Driver Module (FPDM).

Fuel Filters
The system contains 4 filtering or screening devices. Refer to the individual component illustrations for locations.
3. The fuel intake filter or screen is a fine nylon mesh filter mounted on the intake side of the fuel pump. It is part of the assembly and cannot be repaired separately.
4. The filter/screen at the fuel rail port of the injectors is part of the fuel injector assembly and cannot be repaired separately.
5. The filter/screen at fuel inlet side of the fuel pressure regulator is part of the regulator assembly and cannot be repaired separately.
6. The fuel filter assembly is located between the fuel pump (tank) and the pressure test point (Schrader valve) or injectors. A new filter may be installed.

Pressure Test Point
On some applications there is a pressure test point with a Schrader fitting in the fuel rail that relieves the fuel pressure and measures the fuel injector supply pressure for repair and diagnostic procedures. Before repairing or diagnosing the fuel system, read any WARNING, CAUTION, and HANDLING information. On vehicles not equipped with a Schrader valve, use the Mazda Fuel Pressure Test Set 49N0-13-1A0C or equivalent.