Fuel Metering System

The fuel metering system consists of the following:
- Fuel supply components (fuel tank, pump, and lines).
- Fuel pump electrical circuit.
- Throttle Body Fuel Injection (TBI) unit.

Fuel Supply System
The fuel supply is stored in the fuel tank. An electric fuel pump, located in the fuel tank with the gauge sending unit, pumps fuel through an in-line fuel filter to the TBI unit. The pump provides fuel at a pressure greater than is needed by the injectors. The TBI pressure regulator keeps fuel available to the injectors at a regulated pressure. Unused fuel is returned to the fuel tank by a separate line.

Fuel Pump Electrical Circuit
When the ignition switch is turned to the "ON" position (engine not running), the Powertrain Control Module (PCM) energizes the fuel pump relay for two seconds, causing the fuel pump to pressurize the fuel system. If the PCM does not receive ignition reference pulses (engine cranking or running) within two seconds, it shuts "OFF" the fuel pump relay, causing the fuel pump to stop.

As a backup system to the fuel pump relay, the fuel pump also can be energized by the fuel pump and engine oil pressure indicator switch. The switch has two internal circuits. One operates the oil pressure indicator or gauge in the instrument cluster, and the other is a normally open switch which closes when oil pressure reaches about 28 kPa (4 psi). If the fuel pump relay fails, a Diagnostic Trouble Code (DTC) 54 is set in the PCM, and the fuel pump and engine oil pressure indicator switch runs the fuel pump.

An inoperative fuel pump relay can result in long cranking times, particularly if the engine is cold. The fuel pump and engine oil pressure indicator switch energizes the fuel pump as soon as oil pressure reaches about 28 kPa (4 psi).

Throttle Body Fuel Injection (TBI) Unit
The Model 220 TBI unit consists of three assemblies:
- Fuel meter cover.
- Pressure regulator.
- Fuel meter body.
- TBI fuel injector assemblies.
- Throttle body.
- Two throttle valves to control air flow into the engine.
- Idle Air Control (IAC) valve.
- Throttle Position (TP) sensor.
- Vacuum ports located at, above, or below the throttle valves, to generate vacuum signals to various controls and accessories.