Chrysler Full-Size Trucks 1989-1996 Repair Guide

General Information


The sequential Multi-port Fuel Injection (MFI) System was introduced on all 3.9L and 5.2L engine-equipped models in 1992. As of the 1993 model year, all gasoline powered engines except the 2.5L were so equipped. The 2.5L engine was MFI-equipped for the 1996 model year.

The MFI system is controlled by a pre-programmed digital computer known as the Control Module on 1992 models, or the Powertrain Control Module on 1993-96 models. As with the TBI system it replaced, the MFI system computer controls ignition timing, air/fuel ratio, emission control devices, charging system and idle speed. The computer constantly varies timing, fuel delivery and idle speed to meet changing engine operating conditions.

As its name would imply, the chief difference of the MFI system is the utilization of individual fuel injectors at each cylinder. These injectors spray fuel in precisely metered amounts into the intake port directly above the intake valve. They are fired in the correct sequence by the engine controller which constantly reads conditions in order to maintain the ideal air/fuel ratio (stoichiometric) of 14.7:1. By adjusting the length of time the injector opens and sprays fuel into the chamber (pulse width), the proper air/fuel ratio is maintained which greatly helps increase power, driveability and emissions.

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