Some late model vehicles covered by this guide are equipped with a Throttle Body Injection (TBI) system instead of a carburetor. The system uses a throttle body mounted centrally on the intake manifold where a carburetor would normally be found on older vehicles. The throttle body assembly is equipped with 2 electronic fuel injectors in order to supply fuel to regulate the air/fuel mixture. All fuel injection and ignition functions are controlled by the Electronic Control Module (ECM), which is sometimes also referred to as the Engine Control Module. It accepts inputs from various sensors and switches, calculates the optimum air/fuel mixture and operates the various output devices to provide peak performance within specific emissions limits. The ECM will attempt to maintain the ideal air/fuel mixture of 14.7:1 in order to optimize catalytic converter operation. If a system failure occurs that is not serious enough to stop the engine, the ECM will illuminate the SERVICE ENGINE SOON light and will continue to operate the engine, although it may need to operate in a backup or fail-safe mode.
Fuel is supplied to the injectors through an electric fuel pump assembly which is mounted in the vehicle's fuel tank. The ECM provides a signal to operate the fuel pump though the fuel pump relay and oil pressure switch.
Other system components include a pressure regulator, an Idle Air Control (IAC) valve, a Throttle Position Sensor (TPS), Manifold Air Temperature (MAT) sensor, Coolant Temperature Sensor (CTS), a Manifold Absolute Pressure (MAP) sensor and an oxygen sensor. The fuel injectors are solenoid valves that the ECM pulses on and off many times per second to promote proper fuel atomization. The pulse width determines how long an injector is ON each cycle and this regulates the amount of fuel supplied to the engine.
The system pressure regulator is part of the throttle body fuel meter cover which is designed to keep fuel pressure constant at the injector regardless of engine rpm. This is accomplished by controlling the flow in the return line (a calibrated bypass).
The idle air control valve is a stepper motor that controls the amount of air allowed to bypass the throttle plate. With this valve the ECM can closely control idle speed even when the engine is cold or when there is a high engine load at idle.
The ECM used on EFI vehicles has a learning capability which is used to provide corrections for a particular engine's condition. If the battery is disconnected to clear diagnostic codes, or for safety during a repair, the learning process must start all over again. A change may be noted in vehicle performance. In order to "teach" the vehicle, make sure the vehicle is at normal operating temperature, then drive at part throttle, under moderate acceleration and idle conditions, until normal performance returns.
When the ignition switch is first turned ON, the fuel pump relay is energized by the ECM for 2 seconds in order to build system pressure. In the start mode, the ECM checks the CTS, TPS and crank signal or MAP sensor (later model vehicles) in order to determine the best air/fuel ratio for starting. Ratios could range from 1.5:1 at -33°F (-36°C), to 14.7:1 at 201°F (94°C).Clear Flood Mode
If the engine becomes flooded, it can be cleared by opening the accelerator to the full throttle position. When the throttle is open all the way and engine rpm is less than 600, the ECM will pulse the fuel injector at an air/fuel ratio of 20:1 while the engine is turning over in order to clear the engine of excess fuel. If throttle position is reduced below 80 percent, the ECM will return to the start mode.Open Loop Mode
When the engine first starts and engine speed rises above 400 rpm, the ECM operates in the Open Loop mode until specific parameters are met. In Open Loop mode, the fuel requirements are calculated based on information from the MAP sensor and the CTS. The oxygen sensor signal is ignored during initial engine operation because it needs time to warm up.Closed Loop Mode
When the correct parameters are met, the ECM will use O2;s sensor output and adjust the air/fuel mixture in order to maintain a narrow band of exhaust gas oxygen concentration. When the ECM is correcting and adjusting fuel mixture based on the oxygen sensor signal along with the other sensors, this is known as feedback air/fuel ratio control. The ECM will shift into this Closed Loop mode when:
If the throttle position and manifold pressure is quickly increased, the ECM will provide extra fuel for smooth acceleration.Deceleration Mode
As the throttle closes and the manifold pressure changes, fuel flow is reduced by the ECM. If both conditions remain for a specific number of engine revolutions indicating a very fast deceleration, the ECM decides fuel flow is not needed and stops the flow by temporarily shutting off the injectors.Battery Low Mode
If the ECM detects a low battery, it will increase injector pulse width to compensate for the low voltage and provide proper fuel delivery. It will also increase idle speed and, in some cases, ignition dwell time to increase alternator output and allow for proper engine operation.Field Service Mode
When the diagnostic terminal of the test connector is grounded with the engine running, the ECM will enter the Field Service Mode. If the engine is running in Open Loop Mode, the SERVICE ENGINE SOON light will flash quickly, about 2 1 / 2 times per second. When the engine is in Closed Loop Mode, the light will flash only about once per second. If the light stays OFF most of the time in Close Loop, the engine is running lean. If the light is ON most of the time, the engine is running rich.
While the engine continues to operate in Field Service Mode certain conditions will apply: