All 1991-95 Wranglers employ a Chrysler sequential Multi-Point Fuel Injection (MFI) System on both the 2.5L and 4.0L engines. The system is controlled by a Single Board Engine Controller II (SBEC II) for 1991-92 vehicles or a Powertrain Control Module (PCM) for 1993-95 vehicles. The SBEC II or PCM is a pre-programmed, dual microprocessor digital computer. It regulates ignition timing, air-fuel ratio, emission control devices, charging system, speed control and idle speed.
Fuel is injected into the intake ports directly above the intake valves in precise metered amounts through electrically operated injectors. The injectors are fired in a specific sequence by the SBEC II or PCM. The control module attempts to maintain an air/fuel ratio of 14.7/1 by constantly adjusting injector pulse width (the length of time the injector is turned on and therefore open).
The SBEC II or PCM adjusts ignition timing by controlling the ignition coil. Base ignition timing is not adjustable.