VW Passat & Audi A4 1990-2000

Evaporative Emission Controls



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. The charcoal granules absorb fuel vapors entering the canister. 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.

The canister purge valve controls vacuum to the canister. The valve is operated by the Engine Control Module (ECM). The ECM 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:

Fuel Tank
Fuel Fill Cap
Evap Purge Canister
Fuel Tank Pressure sensor
Engine Control Module (ECM)
EVAP Purge Control Solenoid Valve

The evaporative control system does not require any service under normal conditions other than to check for leaks. Check the hoses visually for cracks, breaks, etc. Also check the seal on the gas tank filler cap. Replace the cap if the is split. If any hoses are in need of replacement, use only hoses marked EVAP, available from your local automotive supply store. If there is a strong smell of raw fuel from under the hood when the engine is not running, test the purge control valve.

The Evaporative Emission (EVAP) Controls are monitored by the Engine Control Module (ECM) and if found to be malfunctioning the ECM records the problem in the fault memory as a Diagnostic Trouble Code (DTC). If the problem persists or compromises the vehicle's emissions, the Check Engine Light/Malfunction Indicator Light (MIL) will be activated.

The system is considered to be malfunctioning when:

No purge airflow can be detected

Before the Evaporative System can be tested, no fault conditions must be met for the following:

Vehicle speed,
Engine coolant and/or intake air temperature
Engine idle speed
Oxygen sensors
Purge valve
Fuel tank pressure sensor
Evaporative emission canister shut off valve
Throttle position

  1. 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.
  3. Connect a suitable Data scan tool (DST) to the 16-pin Data Link Connector (DLC) and check for faults.

Click image to see an enlarged view

Fig. Evaporative canister frequency valve and control valve system used 2.0L 16V engines

Click image to see an enlarged view

Fig. The evaporative canister is not a regular service item, and can be tucked out of the way. This one is located in an under-body cavity below the trunk-1.8L Passat shown

Canister Purge Regulator Valve
  1. With the engine OFF, disconnect the hoses from both valves and connect a clean length of hose. It should be possible to blow through the frequency valve (normally open) but not the solenoid valve (normally closed).
  3. With the engine coolant less than 140°F (60°C), disconnect the purge hose from the canister to the frequency valve. With the engine at idle, there should be no vacuum.
  5. As the engine warms to operating temperature, the solenoid valve should be ON and open. The frequency valve should begin to cycle ON and OFF , open and closed. There will be strong vacuum for about 30 seconds, then little or no vacuum for about 60 seconds.
  7. If the system does not perform as described, pull back the connector boots so a voltmeter or test light can be connected with the wiring still connected to the valves. If there is no voltage, the wiring or the engine control unit may be faulty. If voltage appears at the intervals described, 1 or both of the valves is faulty. Using a suitable Digital Multi-Meter (DMM) check the resistance of the purge regulator valve as follows:
  9. Disconnect the EVAP canister purge regulator valve harness electrical connector.
  11. Measure the resistance at the electrical terminals of the EVAP canister purge regulator valve. The resistance should be 20-28 ohms.
  13. If out of specification, replace the EVAP canister purge regulator valve.

Click image to see an enlarged view

Fig. The EVAP purge regulator valve is typically located near the air filter housing-1.8L Passat model shown

  1. Disconnect the electrical connector.
  3. Loosen and remove the hose clamps and remove the valve from the EVAP hoses and remove the valve.
  5. Installation is in reverse order of removal.