Chapter 10 Emission Control and Exhaust System Fundamentals 197 Copyright by Goodheart-Willcox Co., Inc. (difference in pressure) on each side of the EGR valve. In some systems, changes in the MAP sensor are monitored, which monitors manifold vacuum, over several EGR valve actuations. With either setup, a trouble code is set when an abnormally low or high flow rate or change in manifold pressure is detected. Note: Some dual overhead cam (DOHC) engines use variable valve timing to reduce NO X . When the ECM decides that operating conditions will result in excessive NO X levels, it commands the variable timing mechanism to open the exhaust valves for a longer period. Holding the exhaust valves open longer as the piston moves down on the intake stroke draws exhaust gases into the combustion chamber. This has the same effect as the EGR valve and eliminates the need for a separate valve. Exhaust System The exhaust system is a set of pipes and sound- deadening devices that remove exhaust gases and heat from the engine, Figure 10-27. The exhaust system contains a catalytic converter and, therefore, plays a role in the emis- sion control system. The exhaust system includes: • Exhaust manifold. • Exhaust pipe. • Catalytic converter. • Muffler. • Tailpipe. Some models are equipped with more than one muffler. Many exhaust system components are assembled with flanged connections. Other exhaust system components have straight-pipe ends. The pipes slip into each other and are tightened by muffler clamps. Figure 10-27. Exploded view of a typical exhaust system. Note the two oxygen sensors, one mounted before and one after the cata- lytic converter. (Toyota) Rear floor heat shield Rear floor heat shield Oil cooler hose heat shield Support bracket Support bracket Oxygen sensor Muffler Insulator bushing Insulator bushing Oxygen sensor Sensor cover Gasket Gasket Gasket Gasket Nut Bolt Catalytic converter Catalytic converter bracket Support bracket Exhaust tailpipe Exhaust pipe support Exhaust pipe bushing Support bracket (hanger) Support bracket (hanger) Support bracket (hanger)