Jeep Wrangler/YJ 1987-1995 Repair Guide

General Information


All 1987-90 Wranglers equipped with the 2.5L engines use a Throttle Body Injection (TBI) system. TBI is a single point, pulse time system that injects fuel through an electrically operated fuel injector into the throttle body (above the throttle plate).

The fuel injection pulse width (period of time that the injector is energized-causing fuel to be released into the throttle body) is controlled by the Engine Control Unit (ECU). The ECU accomplishes this by opening and closing the ground path to the injector. By controlling the fuel injector pulse width the ECU is able to meter the amount of fuel to the engine and constantly adjust the air fuel ratio.

The ECU receives inputs from sensors that react to exhaust gas oxygen content, coolant temperature, manifold absolute pressure, engine speed (crankshaft position), throttle position, battery voltage, and air fuel temperature.

Air-fuel mixture calibrations for various driving and atmospheric conditions are pre-programmed into the ECU. The ECU monitors and analyzes its various inputs, computes engine fuel and ignition timing requirements based on these inputs, then controls fuel delivery and ignition timing accordingly.