All 1991-95 Wranglers employ a Chrysler sequential Multi-Point Fuel Injection (MFI) System in both the 2.5L and 4.0L engines. The system is controlled by a Single Board Engine Controller II (SBECII) (1991-92) or Powertrain Control Module (PCM) (1993-95). The SBECII 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 port directly above the intake valve in precise metered amounts through electrically operated injectors. The injectors are fired in a specific sequence by the SBECII or PCM. The SBECII or PCM maintains an air/fuel ratio of 14.7:1 by constantly adjusting injector pulse width. Injector pulse width is the length of time the injector is open.
The SBECII or PCM adjusts ignition timing by controlling the ignition coil. Base ignition timing is not adjustable.
For complete diagnostic procedures, please refer to Emission Controls .