Chrysler Full-Size Trucks 1997-2000 Repair Guide

General Information


A Multi-port Fuel Injection (MFI) system is used on all engines. The MFI system is controlled by a pre-programmed digital computer known as the Powertrain Control Module (PCM). The PCM controls ignition timing, air/fuel ratio, emission control devices, charging system and idle speed. The PCM constantly varies timing, fuel delivery and idle speed to meet changing engine operating conditions.

Various sensors provide the input necessary for the PCM to correctly regulate the fuel flow at the fuel injectors. These include the manifold absolute pressure, throttle position, oxygen sensor, coolant temperature, intake air temperature, and camshaft and crankshaft position sensors. In addition to the sensors, various switches also provide important information. These include the neutral safety, air conditioning, air conditioning clutch, and brake light switches.

All inputs to the PCM are converted into signals that are used to calculate and adjust the fuel flow at the injectors or ignition timing or both. The PCM accomplishes this by varying the pulse width of the injectors to adjust the fuel/air ratio, or advancing or retarding timing. The PCM tests many of its own input and output circuits. If a fault is found in a major system, this information is stored in the PCM as a Diagnostic Trouble Code (DTC). Information on this fault can be displayed to a technician by means of the grounding a terminal and reading the check engine lamp flashes or by connecting a scan tool and reading the DTCs (see "Driveability And Emission Controls" for a more complete procedure).

The primary variables that the engine controller uses to determine pulse width are manifold absolute pressure (air density) and engine rpm (speed). In addition to manifold absolute pressure (MAP) and engine speed (rpm), the engine controller also considers input from the following sensors to determine the pulse width:

Exhaust gas content
Coolant temperature
Throttle position
Battery voltage
Air conditioning selection
Transmission gear selection
Speed control

The fuel injection delivery system consists of the following major components:

Fuel tank
Fuel pump
Pressure regulator
Fuel injectors
Fuel rails and lines

Click image to see an enlarged view

Fig. Fuel tank pump module components-typical

The fuel tank incorporates a fuel pump assembly and may have one or more rollover valves.

The fuel pump module is located in the top of the fuel tank and incorporates a pressure regulator. Two fuel filters are also used. One is incorporated into the pump/regulator assembly and the other is at the bottom of the module. The filters do NOT need routine maintenance.

The pressure regulator is a mechanical device. It maintains a constant pressure of about 49 psi (339 kPa).

Fuel is delivered through the supply line to the metal fuel rails. Injector nozzles (one for each cylinder) are installed on the rails. The injectors are electrical solenoids and are fired by the PCM. When activated, voltage is applied across the injector terminals. The injector opens, releasing the fuel built up behind the injector. The pulse width, or amount of time the injector remains open, is determined by the inputs to the PCM and in modified depending on the needs of the engine, air pollution standards and so on.