Contents: General information ⇊ Examination ⇊
General information
The system accumulates fuel vapors that accumulate while the vehicle is parked and ensures that they are discharged into the intake manifold for combustion during normal engine operation.
Any EVAP system necessarily includes a special adsorber filled with activated carbon, which actually collects fuel vapors. The method of removing vapors from the adsorber may vary depending on the design of a specific system. The following will allow the reader to understand in sufficient detail the operating principles of any type of EVAP system.
Note: The design described does not necessarily correspond exactly to the design of the system installed in a particular vehicle, but the operating principle is common to all models equipped with a fuel injection system. In case of any deviations, check the information provided on the VECI label.
The fuel filter cover is equipped with a two-way safety valve. In the event of a system failure, the valve ensures the removal of fuel vapors into the atmosphere.
Another two-way shut-off valve is installed on the fuel tank and provides regulation of the removal of fuel vapors into the carbon adsorber depending on the pressure/vacuum differences associated with temperature changes.
After passing through the two-way valve, fuel vapors enter the charcoal adsorber through the ventilation hoses, where they accumulate during the entire idle period.
When the engine is started, the purge solenoid valve of the adsorber remains closed until it warms up to a certain temperature, allowing the purge diaphragm valve to open due to the increase in the depth of vacuum in the intake manifold. From the adsorber, fuel vapors are blown out through the diaphragm valve into the intake manifold, from where they enter the combustion chambers, where they are burned out during normal engine operation.
Examination
Note: A complete inspection of the EVAP system is beyond the skill level of the average DIY mechanic. Fortunately, the EVAP system, like all other emission control systems, is covered by an additional 5-year/50,000-mile warranty. The most common EVAP system failures are due to a damaged canister or a failing vacuum hose.
Vacuum hoses
1. First of all, always check the condition of the vacuum hoses, damage or disconnection of which is most often the cause of system failure (please refer to the hose routing diagram on your vehicle's VECI label). Replace damaged hoses if necessary.
Purge control solenoid valve
1. The purge control solenoid valve removes accumulated vapors from the carbon canister by creating a vacuum in the intake manifold. The valve provides a vacuum when the engine is warm and remains closed when the engine is cold.
2. Make sure that the engine temperature does not exceed the temperature indicated on the solenoid valve:
- 73°C for 1994 and 1995 models.
- 68°C for models from 1996 onwards.
3. Locate the purge hose at the top of the carbon canister (see accompanying illustration). For assurance, refer to the hose routing diagram on the VECI label.
4. Having found the necessary hose, disconnect it from the adsorber and connect it to the vacuum gauge. Start the engine at idle speed - there should be no vacuum.
5. Wait until the engine warms up – now the vacuum should appear.
6. If the test results are negative, locate the purge control solenoid valve on the intake manifold and disconnect the wiring from it. Disconnect the vacuum hoses and connect a piece of rubber hose to one of the valve fittings.
7. Using jumper wires, connect the valve directly to the battery. Try to blow through the valve through the hose. When the battery is connected, the valve should be open and freely blown through; when disconnected, it should be closed (some models use the opposite valve operating scheme).
8. If the test results are negative, replace the solenoid valve.
Two-way valve
1. Remove the fuel filter cover.
2. Disconnect the fuel vapor line from the gas tank and connect the tee to a vacuum pump and vacuum gauge (see accompanying illustration).
3. Slowly and continuously increasing the vacuum depth, monitor the vacuum gauge readings. The vacuum should instantly stabilize at 5.1÷15.2 mm Hg. If the valve opens before the required vacuum depth is reached, replace it.
4. Switch the vacuum pump to the pressure fitting. Increase the pressure in the line and read the vacuum gauge. The pressure should stabilize in the range of 10.2÷35.6 mm Hg. (the valve is open).
5. If the valve opens before the required vacuum depth is reached, replace it.
Carbon adsorber ventilation shut-off valve
1. To determine if your vehicle is equipped with a canister vent shutoff valve, look at the top of the canister. If there is a 2-pin electrical connector on the cover, then the valve is present.
2. Follow the hose from the valve down to the three-way valve, assembly from the canister. Disconnect the hose from the three-way valve and connect it to the vacuum pump.
3. Remove the shut-off valve from the adsorber without disconnecting the electrical wiring and vacuum pump from it.
4. Turn on the ignition (without starting the engine) and create a vacuum in the hose. If the valve holds a vacuum, disconnect the electrical connector. If the valve still remains closed, replace it.