Chapter 15 Fuel and Emission Control Systems 311 Copyright by Goodheart-Willcox Co., Inc. Throttle position sensor Throttle plate Throttle bore Idle adjustment screw Throttle lever Air bypass valve Injector Airflow Fuel pressure regulator Fuel return Fuel supply Throttle position sensor Throttle valve B A Figure 15-9. A—This is a throttle body for a multiport injection system. It is primarily used to control airflow into the engine, which, in turn, controls engine speed. B—The throttle body assembly in a throttle body injection system controls air and fuel. The pressure regulator and injector are mounted inside the throttle body. (Ford) • Throttle body. A throttle body assembly for multiport injection is primarily used to control airflow into the engine. See Figure 15-9A. A throttle body assembly for throttle body injection controls airflow, but also con- tains the fuel injector(s) and pressure regulator. Look at Figure 15-9B. It should not be confused with a throttle body for multiport injection. • Idle speed motor. Sometimes used on throttle body assemblies to control engine idle speed. The computer actuates the positioner to open or close the throttle plates. Engine Sensors An engine sensor is an electronic device that changes circuit resistance or voltage with a change in a condition, Figure 15-10. For example, the resistance of a tempera- ture sensor may decrease as temperature increases. The computer can use the increased current flow through the sensor to calculate any needed change in injector opening. Typical sensors for an EFI system include: • Oxygen sensor. Senses the amount of oxygen in engine exhaust, Figure 15-11. A high oxygen content indicates a lean mixture. • Engine coolant temperature sensor. Senses the tem- perature of the engine coolant. • Mass airflow sensor. Monitors the mass or volume of air flowing into the intake manifold. • Intake air temperature sensor. Senses the temperature of the outside air entering the engine. • Throttle position sensor. Monitors the position of the throttle plates. • Manifold absolute pressure sensor. Senses vacuum in the engine intake manifold. • Crankshaft position sensor. Monitors the rotation (speed) of the engine crankshaft. Other sensors may also be used. Refer to the service manual for specific details as needed. Gasoline Injection Service To diagnose problems in the fuel injection system, you must use your knowledge of system operation, basic trouble- shooting skills, and the service manual. As you try to locate problems, visualize the operation of the system. Relate the function of each component to the problem. This will let you eliminate several possible sources and concentrate on others. Figure 15-12 shows several possible problems with the fuel injection system. Check the condition of all hoses, wires, and other parts. Look for fuel leaks, vacuum leaks, kinked lines, loose electrical connections, and other troubles. You may need to disconnect and check the terminals of the wir- ing harness. Inspect them for rust, corrosion, or burning. High resistance at terminal connections is a frequent cause of problems. On-Board Diagnostics The first thing a technician often does when diagnos- ing a problem in a computerized system is check for vehicle diagnostic trouble codes with a scan tool. Modern computer control systems test themselves and can indicate the loca- tion of a problem. There are two types of computer control systems in use today: • On-board diagnostics generation one (OBD I). • On-board diagnostics generation two (OBD II). The first system has been in use for years and is referred to as on-board diagnostics generation one or OBD I. The second system was implemented on all new vehicles starting with the 1996 model year. This system is referred to as on-board diagnostics generation two or OBD II. It can detect sensor malfunctions before they become noticeable to the driver or technician. OBD II is