Chapter 10 Emission Control and Exhaust System Fundamentals 193 Copyright by Goodheart-Willcox Co., Inc. The service port allows the technician to remove any carbon released into the system by the canister. Most ser- vice ports have a Schrader valve and a cap to keep the port free of dirt and grease, Figure 10-17. The fuel-level sensor is monitored by the ECM to determine if the fuel level is sufficient for system diagnostics. The fuel tank–pressure sensor is installed on the fuel tank or in one of the fuel lines. Sometimes, the sensor is part of the fuel pump assembly, as shown in Figure 10-18. This sensor measures the difference in air pressure between the fuel tank and outside air. The vent valve allows outside air into the system as the fuel level drops. The valve is designed to let air to flow into the tank, but not out of it. Figure 10-19 shows a vent valve located in the filler cap. Some newer vehicles do not have a filler cap and use flaps in the filler neck to seal the neck. These are called capless filler necks. Enhanced Evaporative Emission Control System Operation The purge solenoid on the enhanced system is operated by the ECM based on readings from the MAF or MAP and intake air temperature sensors, along with the current fuel trim. However, purge valve operation will not begin until the following conditions are met. • The engine must be in operation for about two to three minutes on a cold start or 30 seconds to one minute on a hot start. • The engine must be at operating temperature. • The computer system must be in closed loop. Figure 10-16. Vehicles equipped with OBD II use an enhanced evaporative emission control system. The EVAP purge solenoid is controlled by the ECM and operates only when venting the fuel vapor into the engine will not affect exhaust emissions. (University of Toyota/Toyota Motor Sales USA) VSV (for EVAP) Intake air chamber Fuel tank overfill check valve Fuel inlet pipe Fuel Fuel pump Vapor ECM Vapor pressure sensor Charcoal canister VSV (for vapor pressure sensor) Figure 10-17. This figure shows a typical purge valve. The caps of most purge valves are green to simplify identification. Figure 10-18. Fuel tank–pressure sensors are often placed in the fuel tank sending and pump unit. Fuel tank– pressure sensor Figure 10-19. The fuel tank inlet vent is often in the filler cap. The cap may also contain a pressure-relief valve. Some systems control the vent with a solenoid that is energized for operation and testing. Vent valve