Gasoline fuel is a major source of pollution, before and after it is burned in the automobile engine. From the time the fuel is refined, stored, pumped and transported, again stored until it is pumped into the fuel tank of the vehicle, the gasoline gives off unburned hydrocarbons (HC) into the atmosphere. Through the redesign of storage areas and venting systems, the pollution factor was diminished, but not eliminated, from the refinery standpoint. However, the automobile still remained the primary source of vaporized, unburned hydrocarbon (HC) emissions.
Fuel pumped from an underground storage tank is cool but when exposed to a warmer ambient temperature, will expand. Before controls were mandated, an owner might fill the fuel tank with fuel from an underground storage tank and park the vehicle for some time in warm area, such as a parking lot. As the fuel would warm, it would expand and should no provisions or area be provided for the expansion, the fuel would spill out of the filler neck and onto the ground, causing hydrocarbon (HC) pollution and creating a severe fire hazard. To correct this condition, the vehicle manufacturers added overflow plumbing and/or gasoline tanks with built in expansion areas or domes.
However, this did not control the fuel vapor emission from the fuel tank. It was determined that most of the fuel evaporation occurred when the vehicle was stationary and the engine not operating. Most vehicles carry 5-25 gallons (19-95 liters) of gasoline. Should a large concentration of vehicles be parked in one area, such as a large parking lot, excessive fuel vapor emissions would take place, increasing as the temperature increases.
To prevent the vapor emission from escaping into the atmosphere, the fuel systems were designed to trap the vapors while the vehicle is stationary, by sealing the system from the atmosphere. A storage system is used to collect and hold the fuel vapors from the carburetor (if equipped) and the fuel tank when the engine is not operating. When the engine is started, the storage system is then purged of the fuel vapors, which are drawn into the engine and burned with the air/fuel mixture.
Changes in atmospheric temperature cause fuel tanks to breathe, that is, the air within the tank expands and contracts with outside temperature changes. If an unsealed system was used, when the temperature rises, air would escape through the tank vent tube or the vent in the tank cap. The air that escapes contains gasoline vapors.
The Evaporative Emission Control System provides a sealed fuel system with the capability to store and condense fuel vapors. When the fuel evaporates in the fuel tank, the vapor passes through the EVAP emission valve, through vent hoses or tubes to a carbon filled evaporative canister. When the engine is operating the vapors are drawn into the intake manifold and burned during combustion.
A sealed, maintenance free evaporative canister is used. The canister is filled with granules of an activated carbon mixture. Fuel vapors entering the canister are absorbed by the charcoal granules. A vent cap is located on the top of the canister to provide fresh air to the canister when it is being purged. The vent cap opens to provide fresh air into the canister, which circulates through the charcoal, releasing trapped vapors and carrying them to the engine to be burned.
Fuel tank pressure vents fuel vapors into the canister. They are held in the canister until they can be drawn into the intake manifold. The canister purge valve allows the canister to be purged at a pre-determined time and engine operating conditions.
Vacuum to the canister is controlled by the canister purge valve. The valve is operated by the PCM. The PCM regulates the valve by switching the ground circuit on and off based on engine operating conditions. When energized, the valve prevents vacuum from reaching the canister. When not energized the valve allows vacuum to purge the vapors from the canister.
During warm up and for a specified time during hot starts, the PCM energizes the valve; preventing vacuum from reaching the canister. The EVAP purge control solenoid begins to operate when the engine coolant temperature reaches a predetermined operating temperature.
Once the proper coolant temperature is achieved, the PCM controls the ground circuit to the valve as necessary. When the PCM opens the ground, this allows vacuum to flow through the canister and vapors are purged from the canister into the throttle body. During certain idle conditions, the PCM may energize the purge valve to control fuel mixture calibrations.
The fuel tank is sealed with a pressure-vacuum relief filler cap. The relief valve in the cap is a safety feature, preventing excessive pressure or vacuum in the fuel tank. If the cap is malfunctioning, and needs to be replaced, ensure that the replacement is the identical cap to ensure correct system operation.
The following components are part of and affect the operation of the EVAP (Evaporative Emission) Control system:
Removal & Installation
Evaporative Emission Canister
- If the venting hose is not a slide-on type connection, open the connecting nipple using tool 16 1 040 or equivalent, then pull off the tank venting pipe.
- Remove the hose from the canister.
- Unclip the filter vent.
- Remove the mounting screws and remove the canister from the vehicle.
- Position the canister, then install and tighten the mounting screws.
- Position the filter vent in the clip.
- Install the hose to the canister, then install the tank venting pipe. The hose will push on and clip into place.
- Remove the right rear wheel. Remove the wheel well rear trim.
- Disconnect the hoses from the tank.
- Remove the mounting bolts and remove the expansion tank.
- Install the expansion tank and connect the hoses.
- Replace the wheel well trim and the wheel.
- Disconnect the negative battery cable.
- For the E36 3 Series with M50 engine, open the hose clamps, then remove the intake bellows and air mass sensor to provide access.
- Lift off or compress the retainer, then unplug the valve electrical connector.
- Except for the E36 3 Series with M50 engine, loosen and remove the retaining bolt.
- Disconnect the hoses from the valve, then remove the valve from the vehicle.
- Install the valve and connect the vent hoses.
- If removed, install the retaining bolt.
- Install the electrical connector and retainer to the valve.
- If removed, install the air mass sensor and intake bellows, then tighten the hose clamps.
- Connect the negative battery cable.
The fuel evaporation system consists of an expansion tank, carbon canister, fuel filler cap, solenoid activated purge valve and connecting lines and hoses. The filler cap is a check valve that allows air into the tank but will not allow vapors out. No scheduled maintenance is required, though visually checking the system for damaged hoses and leaks is recommended.
The Evaporative Emission (EVAP) Controls are monitored by the Powertrain Control Module (PCM) and if found to be malfunctioning the PCM records the problem in the fault memory as a Diagnostic Trouble Code (DTC). If the problem persists or compromises the vehicle's emissions, the Malfunction Indicator Light (MIL) will be activated.
The system is considered to be malfunctioning when:
Before the Evaporative System can be tested, the following conditions must be met:
No faults currently present for the following:
- Start the engine and allow it to warm. The engine coolant temperature must be at least 176 °F (80°C), at an altitude of less than 8,202 feet (2502 m) and the engine operated for at least 6 minutes since the start of drive cycle.
- Connect a suitable Data Scan Tool (DST) to the 16-pin Data Link Connector (DLC) and check for faults.
This canister is used as a storage facility for fuel vapors that have escaped from components such as the fuel tank. This canister prevents these vapors from entering the atmosphere.
Generally, the only testing done to the canister is a visual inspection. Look the canister over and replace it with a new one if there is any evidence of cracks or other damage.Evaporative Emissions Purge Control Solenoid Valve
The evaporative emissions purge control solenoid valve is located at between the canister and the intake manifold.
- Disconnect the vacuum hose from the valve.
- Connect a vacuum pump to the vacuum hose.
- Turn the ignition switch ON .
- Apply vacuum to the hose.
- If the valve holds vacuum, the valve is OK.
- Unplug the purge valve electrical connector. The valve located under the air intake hose or intake manifold. It has 1 electrical connector and 2 vacuum lines, 1 leading to manifold vacuum and the other to the carbon canister.
- The coil resistance should be 25-65 ohms.
- Attach a hand vacuum pump and gauge to the intake manifold side of the valve. Supply 12 volts to the valve to hold the valve closed. Apply 15-21 in. Hg. (500-700 mbar) of vacuum to the valve.
- Wait 20 seconds and check the gauge to determine if a vacuum has been lost. If a drop of more than 1.5 in. Hg. (50 mbar) occurs, the valve must be replaced.
- If the valve is good, but the system is still suspect, reinstall the valve, then start and warm the engine to normal operating temperature.
- When the engine has reached normal operating temperature and the system should be normally operating, carefully unplug the electrical connector from the valve. Connect a voltmeter to the connector terminals; the meter should cycle between 0-12 volts as the control unit supplies and denies ground.
- If there is no voltage, check the wiring between the valve, DME relay and the control unit for continuity. If the wiring is good, the control unit is faulty.
- If the voltage is ON continuously, turn the ignition switch OFF and check for a short in the wiring or short to ground.
Inspect all system hoses and tubes for signs of damage or cracks. Any damage or leakage must be repaired.