192 Auto Engine Performance and Driveability Copyright by Goodheart-Willcox Co., Inc. Note: Some older V-type engines with carburetors or TBI had a valve in one exhaust manifold that closed to block exhaust gas flow on that manifold. The gases were forced to exit through a passage under the intake manifold, warming the air-fuel mixture. Throttle Body Coolant Passages Many vehicles prevent icing of the throttle valve by diverting some of the engine coolant through the throttle body. Figure 10-14 shows a typical coolant chamber located under the throttle body. Since the cylinder heads warm more quickly than the rest of the engine, coolant is usually pumped directly from the cylinder head passages to the throttle body. The coolant usually travels through the heater core on its way back to the engine. Evaporative Emission Control System The evaporative emission control system prevents gasoline vapors in the fuel system from escaping into the atmosphere. The system consists of a charcoal-filled vapor canister with purge valves and hoses connected to the fuel tank and intake manifold, Figure 10-15. The pressure relief vents in the gas tank are connected by hoses to the canister. The canister contains charcoal, which is able to absorb gaso- line vapors. When the engine is turned off, any gasoline that evaporates from the tank or fuel system enters the canister. Whenever the engine is running, manifold vacuum draws air through the canister. Airflow through the canis- ter draws gasoline vapors from the charcoal into the engine for burning. Flow is controlled by a valve that allows the engine to briefly operate before vapors can be drawn in. This reduces surging or stalling on a hot engine. The control valve can be operated by a solenoid or vacuum. Enhanced Evaporative Emission Control System Vehicles equipped with OBD II use an enhanced evaporative emissions system, Figure 10-16. In addition to the control valve, an enhanced evaporative emission system also has a service port, fuel-level sensor, fuel tank–pressure sensor, and vent valve. Figure 10-13. A thermostatically controlled valve closes the passage to the outside air and opens the passage to air heated by the exhaust manifold. Valve is closed Figure 10-14. Most throttle bodies have the throttle valve area warmed with engine coolant. The coolant circulates through the chamber under the valve whenever the engine is running. Throttle valve From cooling system Coolant passage Figure 10-15. The evaporative emission control system absorbs evaporated fuel and stores it in a charcoal-filled canister. The fuel is later drawn into the engine and burned. Charcoal-filled canister