Chapter 15 Fuel and Emission Control Systems 319 Copyright by Goodheart-Willcox Co., Inc. Signal tube Fuel fill cap Vapor recirculation tube Vapor recirculation valve Fuel tank Evap two-way valve Evap bypass solenoid valve Evap control canister Evap control canister filter Vent shut valve Fuel tank pressure sensor Evap control canister vent shut valve Evap purge control solenoid valve Blk/ yel Red/ yel Grn/ blk Yel/ blu Lt/ grn Blue Blk Lt grn/ wht ECM Various sensors Figure 15-21. The evaporative emissions control system collects and stores fuel vapors in a charcoal canister (EVAP control canister). When the engine is started, vapors are pulled out of the charcoal canister through the purge line and into the intake manifold. (Honda) condenses on the walls of the liquid-vapor separator. The liquid fuel then flows back into the fuel tank. A rollover valve is sometimes used in the vent line from the fuel tank. It keeps liquid fuel from entering the vent line after an accident where the vehicle rolls upside-down. A purge line is used for removing or cleaning the stored vapors out of the charcoal canister. It is connected to the canister and the engine intake manifold. When the engine is running, engine vacuum draws the vapors out of the canister and through the purge line. A purge valve controls the flow of fuel vapor stored in the canister to the intake manifold. This vacuum- or computer-operated valve is located on the top of the canister or in the purge line. Purge valves generally allow flow when the engine reaches operating temperature and is operating above idle speed. This helps minimize emissions when the engine is cold and also prevents rough idle. Exhaust Gas Recirculation System The exhaust gas recirculation (EGR) system injects burned exhaust gases into the engine intake manifold to lower the combustion temperature and reduce NO X pollu- tion. The EGR valve is usually bolted to the engine intake manifold or an adapter plate. It consists of a vacuum dia- phragm, spring, plunger, exhaust gas valve, and a diaphragm housing. Exhaust gases are routed through the cylinder head and manifold to the EGR valve. At idle, the throttle plate in the throttle body is closed. This blocks engine vacuum so it cannot act on the EGR valve. The EGR spring holds the valve shut and exhaust gases do not enter the intake manifold, Figure 15-22A. If the EGR valve opens at idle, the air-fuel mixture will be upset and the engine may stall. When the throttle plate is opened to increase engine speed, engine vacuum is applied to the EGR diaphragm, which is pulled up. In turn, the diaphragm pulls the valve open. Engine exhaust can then enter the intake manifold and combustion chambers, Figure 15-22B. Note: Some EGR valves are equipped with position and flow sensors that monitor system performance. Also, some EGR valves are completely electronic and do not use engine vacuum. Air Injection System An air injection system uses an air pump to force fresh air at low pressure into the exhaust ports of the engine, Figure 15-23. This helps burn any fuel present and reduce HC and CO emissions. The exhaust gases leaving an engine can contain unburned and partially burned fuel. Oxygen in the air pumped in by the air injection systems causes this fuel to continue to burn.