Explain to students that the evaporative emissions control (EVAP) system prevents toxic fuel system vapors from entering the atmosphere. Let students know that gasoline and many of its additives evaporate easily, especially if exposed to the atmosphere. Inform students that Pre-emission-control vehicles used vented gas tank caps. Carburetor bowls were also vented to the atmosphere, which caused a considerable amount of hydrocarbon emissions from unburned fuel. Explain that modern vehicles commonly use an evaporative emissions control system to prevent this source of air pollution.
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Inform students about the major components of the evaporative emission system. All systems should contain the following: * Charcoal canister in which to store the HC vapors until burned. * Filler cap with vacuum and pressure relief valves. * Fuel tank with a domed area to provide for expansion of fuel. The tank cannot be completely filled so it can store vapors. * Purge valve or solenoid to allow for the drawing of the fuel vapor into the engine * Fuel tank pressure control valve (TPCV) * Hoses and tubing to connect the system components
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Newer, enhanced systems (OBD II) Should also contain these: * Vent valve to control the flow of fresh air into the system * Vacuum pressure sensor mounted on the tank * Small filler neck that contains a check valve (ORVR-equipped vehicles) * Check and one-way valves to keep liquid and vapor separated (Figure 14-9) * Fuel level sensor * An EVAP pump, used on some vehicles to pressurize the system while the fuel tank pressure sensor monitors for leaks
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Instruct students that the EVAP (Evaporative Emission Control System) system diagnosis varies depending on the vehicle make and model year. Tell students to always follow the service and diagnostic procedure in the vehicle manufacturer's service manual. Explain to students that if the EVAP system is purging vapors from the charcoal canister when the engine is idling or operating at very low speed, rough engine operation will occur, especially at higher atmospheric temperatures. Also, cracked hoses or a canister saturated with gasoline may allow gasoline vapors to escape to the atmosphere, resulting in gasoline odor in and around the vehicle.
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Inform students that all of the hoses in the EVAP system should be checked for leaks, restrictions, and loose connections. The electrical connections in the EVAP system should be checked for looseness, corroded terminals, and worn insulation. Let students know that when a defect occurs in the canister purge solenoid and related circuit, a DTC is usually set in the PCM memory. If a DTC related to the EVAP system is set in the PCM memory, tell students to always correct the cause of this code before further EVAP system diagnosis.
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Teach students about Component Diagnosis: Tell students to check the canister to make sure that it is not cracked or otherwise damaged. Also have them make certain that the canister filter is not completely saturated. Let them know that a saturated charcoal filter can cause symptoms that can be mistaken for fuel system problems. Rough idle, flooding, and other conditions can indicate a canister problem. Explain that a canister filled with liquid or water causes backpressure in the fuel tank. It can also cause richness and flooding symptoms during purge or start-up. To test for saturation, have students unplug the canister momentarily during a diagnosis procedure and observe the engine's operation. If the canister is saturated, let them know that either it or the filter must be replaced depending on its design. Some models have a replaceable filter, others do not.
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Inform students that a vacuum leak in any of the evaporative emission components or hoses can cause starting and performance problems as can any engine vacuum leak. It can also cause complaints of fuel odor. Let them know that incorrect connection of the components can cause rich stumble or lack of purging, resulting in fuel odor. To conduct a vacuum-on, valve-open test, tell students to consult the vehicle's service manual.
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The canister purge solenoid winding may be checked with an ohmmeter. Tell students to refer to the manufacturers shop manual or information for specs.
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Instruct students that the fuel tank pressure control valve is designed to control fuel tank pressure while the vehicle is sitting still. Vapors are stored in the tank with the valve closed. If tank pressure builds too high, the valve opens and lets the vapor into the canister. When the engine is running, the valve has vacuum applied, opening the valve and allowing vapors to be stored in the canister until purging. With the fuel tank pressure control valve removed, tell students to try to blow air through the valve with their mouths from the tank side of the valve. Let them know that some restriction to airflow should be felt until the air pressure opens the valve, Have them connect a vacuum hand pump to the vacuum fitting on the valve and apply 10 in. Hg to the valve. Tell them to now try and blow air through the valve from the tank side. Under this condition, there should be no restriction to airflow. If the tank pressure control valve does not operate properly, tell students to replace the valve.
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Inform students that if the fuel tank has a pressure and vacuum valve in the filler cap, check these valves for dirt contamination and damage. The cap may be washed in clean solvent. When the valves are sticking or damaged, tell students to replace the cap.
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Instruct students that there are two different EVAP systems they may encounter on fuel injected vehicles. The first is just a basic EVAP system used during OBD I. Explain that the computer controls the purge valve by pulse width modulation of the purge solenoid whenever conditions are appropriate. The basic EVAP system does not use computerized monitoring for leaks in the system.
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The second and more recent EVAP system is the enhanced EVAP used with OBD II. Let them know that the vent valve and tank pressure sensor were added for diagnostic purposes, and the fuel level sensor was added to the list of input sensors to the PCM. With enhanced EVAP, explain that the PCM conducts several tests to determine if the system is operational and there are no leaks. Modern (OBD II) evaporative emission systems must be able to detect a leak as small as 0.020 inch in diameter and use tests to confirm that there are no leaks present.
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