GM Grand Am/Achieva/Calais/Skylark/Somerset 1985-98

General information


See Figures 1 and 2

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Fig. Fig. 1: Schematic of the fuel supply system

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Fig. Fig. 2: View of the Alternating Synchronous Double Fire (ASDF), found on the 2.3L engine only

Multi-port fuel injection is found on vehicles equipped with 2.0L, 2.3L, 3.0L and 3.3L engines. The Sequential fuel injection system is found on the 2.4L and 3.1L engines.

The Sequential/Multi-Port Fuel Injection (MPI) system is controlled by an computer control module (ECM or PCM depending upon vehicle application) 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 (O2) sensor, Engine Coolant Temperature (ECT) sensor, Knock (KS) sensor (if equipped), Mass Air Flow (MAF) sensor or Manifold Absolute Pressure (MAP) sensor and Throttle Position Sensor (TPS). The ECM/PCM also receives information concerning engine rpm, vehicle speed, transaxle selector position, power steering and air conditioning.

There are injectors are at each cylinder's intake port, rather than the single injector found on the throttle body system. The injectors are mounted on a fuel rail and are activated by a signal from the computer 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/PCM; the longer the injector is open, the greater the amount of fuel is injected. In this manner, the air/fuel mixture can be precisely controlled for maximum performance with minimum emissions. On all multi-port fuel injection systems, except the 2.3L engine, the computer control module fires all of the injectors at once.

The system on 2.3L engines is slightly different in that the injectors are paired by companion cylinders and are fired in pairs. This system is called Alternating Synchronous Double Fire (ASDF) fuel injection. It is similar in operation to the other injection systems except that each pair of cylinders fires once per crankshaft revolution. This means that cylinders 1 and 4, as well as 2 and 3, fire once per crankshaft revolution. In this way, a cylinder's injector fires on the intake stroke and exhaust stroke. Firing the injector on the exhaust stroke (when the intake valve is closed) helps provide better fuel vaporization.

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 fuel pressure regulator maintains 28-36 psi (193-248 kPa), (28-50 psi 193-345 kPa on turbocharged engines) in the fuel line to the injectors and the excess fuel is fed back to the tank. A fuel accumulator is used to dampen the hydraulic pulses in the system created when all the injectors open simultaneously.

The throttle body incorporates an Idle Air Control (IAC) valve that provides for a bypass channel through which air can flow. It consists of an orifice and pintle which is controlled by the ECM/PCM through a step motor. The IAC valve is operated continuously by the ECM/PCM to maintain idle speed.

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 as potentiometer with on end connected to a 5 volt source from the computer control module 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 0.4 volts); as the throttle valve opens, the output increases to a maximum 5 volts at Wide Open Throttle (WOT). The TPS can be misadjusted (3.0L and 3.3L engines) open, shorted, or loose. On the 3.0L and 3.3L engines if the sensor 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 computer thinks the throttle is moving. This should cause a trouble code to be set. Once a trouble code is set, the computer control module 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.

The non-adjustable Throttle Position (TP) sensor is mounted on the side of the throttle body opposite the throttle lever. It senses the throttle valve angle and relays that information to the PCM. This information is required by the PCM to generate the required injector control signals (pulses).