Pontiac Fiero 1984-1988 Repair Guide

General Information


On 1985 and later models, equipped with the V6 engine, a new Multi-Port Fuel Injection (MPFI) system is available. The MPFI system is controlled by an Electronic Control Module (ECM) which monitors engine operations and generates output signals to provide the correct air/fuel mixture, ignition timing and engine idle speed control. Input to the control unit is provided by an oxygen sensor, coolant temperature sensor, detonation sensor, hot film air mass sensor and throttle position sensor. The ECM also receives information concerning engine rpm, road speed, transmission gear position and air conditioning.

The main control sensor is the Oxygen (O 2 ) sensor, which is located in the exhaust manifold. The O 2 sensor tells the ECM how much oxygen is in the exhaust gas and the ECM changes the Air/Fuel mixture to the engine by controlling the fuel injectors. The best mixture for the engine to operate properly is a 14.7 to 1 ratio (air to fuel).

The ECM looks at the voltages from several sensors to determine how much fuel to give to each injector. The fuel is delivered under one of several conditions called "modes". All of the modes are controlled by the ECM. The modes consist of starting, clear flood, run, acceleration and deceleration.

The system uses Bosch injectors, one at each intake port, rather than the single injector found on the earlier throttle body system. The injectors are mounted on a fuel rail and are activated by a signal from the electronic control module. The injector is a solenoid-operated valve which remains open depending on the width of the electronic pulses (length of the signal) from the ECM; the longer the open time, the more fuel is injected. In this manner, the air/fuel mixture can be precisely controlled for maximum performance with minimum emissions.

Fuel is pumped from the tank by a high pressure fuel pump, located inside the fuel tank. It is a positive displacement roller vane pump. The impeller serves as a vapor separator and pre-charges the high pressure assembly. A pressure regulator maintains 28-36 psi in the fuel line to the injectors and the excess fuel is fed back to the tank. On MPFI systems, a fuel accumulator is used to dampen the hydraulic line hammer in the system created when all injectors open simultaneously.

The Mass Air Flow (MAF) Sensor is used to measure the mass of air that is drawn into the engine cylinders. It is located just ahead of the air throttle in the intake system and consists of a heated film which measures the mass of air, rather than just the volume. A resistor is used to measure the temperature of the incoming air and the air mass sensor maintains the temperature of the film at 75° above ambient temperature. As the ambient (outside) air temperature rises, more energy is required to maintain the heated film at the higher temperature and the control unit uses this difference in required energy to calculate the mass of the incoming air. The control unit uses this information to determine the duration of fuel injection pulse, timing and EGR.

The throttle body incorporates an Idle Air Control (IAC) that provides for a bypass channel through which air can flow. It consists of an orifice and pintle which is controlled by the ECM through a stopper motor. The IAC provides air flow for idle and allows additional air during cold start until the engine reaches operating temperature. As the engine temperature rises, the opening through which air passes is slowly closed.

The Throttle Position Sensor (TPS) provides the control unit with information on throttle position, in order to determine injector pulse width and hence correct mixture. The TPS is connected to the throttle shaft on the throttle body and consists of a potentiometer with one end connected to a 5 volt source from the ECM and the other to ground. A third wire is connected to the ECM to measure the voltage output from the TPS which changes as the throttle valve angle is changed (accelerator pedal moves). At the closed throttle position, the output is low (approximately .4 volts); as the throttle valve opens, the output increases to a maximum 5 volts at Wide Open Throttle (WOT). The TPS can be misadjusted open, shorted, or loose and, if it is out of adjustment, the idle quality or WOT performance may be poor. A loose TPS can cause intermittent bursts of fuel from the injectors and an unstable idle because the ECM thinks the throttle is moving. Once a trouble code is set, the ECM will use a preset value for TPS and some vehicle performance may return. A small amount of engine coolant is routed through the throttle assembly to prevent freezing inside the throttle bore during cold operation.