194 Auto Engine Performance and Driveability Copyright by Goodheart-Willcox Co., Inc. Once all conditions are met, the ECM opens the purge valve and allows fuel vapors to be drawn into the intake manifold. The ECM also reduces the amount of fuel injected into the engine to compensate for the vapors entering the intake manifold. Leak Diagnosis An important part of the operation of the enhanced evaporative emission control system is a self-check for leaks in the tank and fuel system. The ECM performs leak detec- tion diagnosis by one of the following methods. • When the engine is running, the vent valve is closed and a vacuum is applied to the enhanced evaporative emission control system. • When the engine is not running, pressure is applied to the enhanced evaporative emission control system. Pressure is supplied by a small pump mounted on the tank or in one of the fuel lines. • With the engine off, the tank is sealed and the contrac- tion caused by the cooling of the air and liquid in the tank is allowed to create a vacuum. All of these leak-detection methods use one or more pressure sensors to detect leaks. If the ECM reads an out-of-specifica- tion value, it sets a trouble code. Enhanced emission control system diagnosis and repair is discussed in Chapter 18. Air-Injection Systems Since the combustion process in the cylinder is not perfect, some unburned fuel escapes into the exhaust sys- tem. If not reduced in some way, this fuel would overwhelm the catalytic converter and escape into the atmosphere. To burn the gasoline before it reaches the catalytic converter and atmosphere, outside air is pumped into the exhaust manifold. The high temperature of the exhaust gas com- bined with the oxygen content of the outside air causes the unburned gasoline to burn. The two types of air-injection systems are the pulse-air-injection system and the air- injection-pump system. Pulse-Air-Injection System The pulse-air-injection system uses the pressure pulses in the exhaust manifold to draw outside air into the manifold. It consists of check valves placed in fittings at carefully calculated positions on the exhaust manifold. As each exhaust valve closes, the sudden loss of exhaust gas flow causes a pulse of negative pressure, or vacuum, in the exhaust manifold. The check valve in that section of the exhaust manifold opens when a vacuum pulse occurs. The check valve closes when the exhaust valve opens and that section of the manifold is again pressurized. Therefore, the opening and closing of the check valves draws air into the exhaust manifold without allowing exhaust gases to escape. Outside air is drawn into the pulse-air-injection sys- tem through a hose attached to the air cleaner housing. This ensures that the incoming air is filtered. Also, the intake pulses of the system are silenced by the air-induction system. The pulse-air-injection system is inexpensive to produce and takes up little space on the engine. See Figure 10-20. Another advantage is that the system does not consume any engine power because it makes use of the natural pressure variations in the exhaust system. This system is satisfactory for smaller engines. On small engines, the airflow does not have to be closely controlled and power losses must be kept to a minimum. However, this system cannot be controlled as well as the belt-driven type, which is discussed in the next section. Air-Injection-Pump Systems The air-injection-pump system uses a centrifugal-type pump, sometimes referred to as a smog pump, to inject out- side air into the exhaust manifold. Air pumps may be driven by an engine belt or electric motor, Figure 10-21. The pump develops pressure of no more than 10 to 15 psi. This is suffi- cient pressure to force outside air into the exhaust manifold. Usually belt-driven pumps operate whenever the engine is running, and motor-driven pumps operate only when the engine is started cold and is in open loop. Electric Figure 10-20. A pulse-air-injection system uses the scavenging effect of exhaust pulses to draw air through a check valve into the exhaust manifold. (Ford) Air cleaner Silencer Check valve Check valve Front of vehicle