Feature May 1, 2009 by
Mike Farrell, Curriculum Developer CARS
Not All EVAP Systems Are The Same
Evaporative emission systems (EVAP) that collect fuel vapours present in the fuel tank have been mandated for over thirty years now. While all EVAP systems collect the fuel vapours, it is up to each m...
Evaporative emission systems (EVAP) that collect fuel vapours present in the fuel tank have been mandated for over thirty years now. While all EVAP systems collect the fuel vapours, it is up to each manufacturer to develop control systems. The modern systems are designed to be self diagnosing and will report a leak as small as 0.020-inch by using a fuel tank pressure sensor. While all systems perform the same function, the design and controls of some systems are radically different.
The self diagnosing system functions on the popular four-cylinder 1ZZ-FE 1.8 litre engine, found in both domestic and import vehicles, are slightly different to other manufacturer’s systems. Three PCM controlled solenoids are used to operate the system; most other systems employ two solenoids.
The Vent Solenoid is a normally open valve that is commanded closed to seal the EVAP system and stop air flow into the charcoal canister. The Purge Solenoid is normally closed but is opened by the PCM to allow manifold vacuum in the EVAP system — thus drawing fuel vapours from the EVAP system. The new player in this system is the Pressure Switching Solenoid. This EVAP system uses a dual chamber canister. One chamber is connected to the fuel tank to gather and store vapours, while the other half is connected to the air cleaner to supply air to the system. The PCM controls the Switching Solenoid which is normally closed. When the PCM commands the Switching Solenoid to open, the fuel vapours in the tank will be transferred to the canister.
There are two additional spring loaded check valves installed in the canister that are used to control air flow — there is an EVAP valve mounted on the vapour portion of the canister and an atmospheric valve is mounted on the air portion.
When the vehicle is parked and allowed to cool down, the valves will assume their normal positions. The Purge and Switching Valves will be closed and the Vent Valve will be open. As vapours are formed in the tank, they are routed to and stored in the canister. The atmospheric check valve controls the pressure in the system — it is set to 5.5-inch of H2O. Any pressure above this setting is allowed to vent into the air through the drain tube. Most other systems use the vent valve to control system pressure.
The system does not allow the Purge Valve to open until the engine coolant temperature reaches 74C, at which time the purge valve opens and allows manifold vacuum into the vapour chamber of the canister. With the Vent Valve normally open, air is drawn through the canister where it is mixed with the vapour and then drawn into the engine and burnt. The switching valve remains closed at this time.
The latent heat from the exhaust system slightly warms the fuel tank which increases fuel evaporation resulting in an increase in fuel tank pressure. During normal EVAP system operation, this is an important item to remember. The EVAP system pressure may actually increase with the purge valve open and not drop as expected. This can lead to a system misdiagnosis.
Once the vehicle has reached operating temperature and the correct diagnostic conditions are met, the PCM will begin to run a series of EVAP system tests. The system will be leak tested and the solenoid operation will be confirmed.
The EVAP system will be tested for a large leak by closing the Vent Valve, opening the Switching Valve and keeping the Purge Valve open. This will cause a vacuum to develop in the system which is monitored by the tank pressure sensor. If the vacuum does not reach a set level within a specified time, codes P0440, P0441 or P0446 will set.
After the system passes the large leak test, the system tests for vacuum decay by closing the Purge Valve. The PCM monitors for a large decay in the vacuum and, if found, will cause P0440 to set.
After the system passes the vacuum test, it will perform the small leak test. The pressure sensor will continue to monitor the vacuum decay in the system. The pressure sensor can detect a leak as small as 0.020-inch and, if the test fails, a P0442 will set.
The Vent Solenoid is tested by commanding it open to allow air into the system. The pressure sensor should see a rapid decrease in vacuum (increase in pressure) indicating correct operation.
The last item tested is the Switching Solenoid. The Switching Solenoid is commanded closed and the vacuum decay should not be as rapid because the pressure sensor will be isolated from the rest of the EVAP system.
After all of the tests are completed and have passed, the PASSED flag will be set in the PCM I/M 240 flag. Any system failure results in a diagnostic trouble code.
Don’t assume that all EVAP systems are the same. Always consult the current service information for the vehicle you are working on.
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