Chapter 10 Emission Control and Exhaust System Fundamentals 195 Copyright by Goodheart-Willcox Co., Inc. motor–driven systems are controlled by the vehicle ECM. A trouble code will set if a motor-driven system develops problems. Flexible hoses and steel tubing connect the air pump to the exhaust manifolds and to the catalytic converter. The tubes at the exhaust manifold are made of heat-resistant steel, threaded into the exhaust manifold. Several vacuum-operated control valves direct the output of the air-injection-pump system. One-way check valves are used to prevent exhaust gas from backing up into the smog pump if pressure in the exhaust manifold becomes higher than normal, Figure 10-22. High exhaust pressure could be caused by high engine loads or exhaust restrictions, such as a plugged catalytic converter or muffler or a collapsed pipe. The check valves are usually installed where the flexible hoses attach to the exhaust manifold pipes. Diverter valves are installed to divert the pump out- put to the atmosphere during deceleration. This is done to prevent backfiring in the exhaust system by removing the oxygen supply. However, some systems do not use a diverter valve since the fuel supply to the fuel injectors is cut off dur- ing deceleration. When used, the diverter valve is mounted on the air pump or in the hoses between the pump and check valves. See Figure 10-23. Most engines with a three-way catalytic converter and air pump are equipped with a switching valve. This valve directs air to the exhaust manifold when the engine is cold. When the engine heats up, the switching valve redirects the air to the catalytic converter. This allows the incoming oxy- gen to combine most efficiently with the unburned gasoline. The air pump valves are controlled by vacuum diaphragms, usually operated by the ECM using solenoids. Air-injection valves on OBD II vehicles are monitored for function and the presence of airflow in the exhaust stream. Figure 10-21. Types of air-injection pumps. A—This belt-driven pump is installed on an inline four-cylinder engine. B—This is an electric pump. It is installed under the fender. (Cardone) A B Figure 10-22. This check valve opens to allow air into the exhaust manifold, but does not allow exhaust gases to back up into the air pump. Figure 10-23. The diverter valve is installed on the air pump outlet piping and is controlled by intake manifold vacuum. It prevents backfiring when the engine is decelerating. The valve switches air between the exhaust manifold and the atmosphere.