Jeep Wrangler/YJ 1987-1995 Repair Guide

Components and Operation


See Figure 1

The throttle body fuel injection is used on 1987-90 Wranglers equipped with the 2.5L engine and is a "pulse time'' system that uses a single solenoid-type injector to meter fuel into the throttle body above the throttle plate. Fuel is metered to the engine by an Electronic Control Unit (ECU), which controls the amount of fuel delivery according to input from various engine sensors that monitor exhaust gas oxygen content, coolant temperature, manifold absolute pressure, crankshaft position and throttle position. These sensors provide an electronic signal by varying resistance within the sensor itself. By reading the difference in resistance, the ECU can determine engine operating conditions and calculate the correct air/fuel mixture, as well as ignition timing under varying engine loads and temperatures. In addition, the ECU controls fuel pump operation, idle speed, emissions, the upshift indicator lamp and the A/C compressor clutch.

TBI fuel injection has two main subsystems; a fuel subsystem and a control subsystem. The fuel subsystem consists of an electric fuel pump (mounted in the fuel tank), a fuel filter, a pressure regulator and the fuel injector. The control subsystem consists of a Manifold Air/fuel Temperature (MAT) sensor, a Coolant Temperature Sensor (CTS), a Manifold Absolute Pressure (MAP) sensor, a knock sensor, an exhaust gas oxygen (O 2 ) sensor, an Electronic Control Unit (ECU), a gear position indicator (automatic transmission only), a Throttle Position Sensor (TPS) and power steering pressure switch with a load swap relay. In addition to these sensors which send signals to the ECU, there are various devices which receive signals from the ECU to control different functions such as exhaust gas recirculation, idle speed control, air conditioner operation, etc.

Before performing any system tests, first determine that the problem is not being caused by a component other than the fuel injection system, such as spark plugs, distributor, ignition timing, etc. Also make sure that no air is entering the intake and exhaust system above the catalytic converter and that fuel is reaching the injector under normal pressure.

The diagnostic connectors D1 and D2 are located on the dash panel in the engine compartment. For complete diagnostic testing, please refer to Emission Controls of this guide.

Click image to see an enlarged view

Fig. Fig. 1: Throttle body assembly-1987-90 with 2.5L engine