This charcoal-filled plastic shoe box bolted to the bottom of my 2001 Tracker is the EVAP canister.


There are three air/vapor hoses connected to the canister.


One of the EVAP canister hoses collects gasoline vapors from the fuel tank. One hose is connected to the purge valve (bolted to the intake manifold). The last hose
is connected to the vent valve (sometimes called the air vent). Note that without a command from the PCM the purge valve is closed and the vent valve is open.


During normal driving the PCM opens the purge valve and the stored gasoline vapors are
pulled into the engine through the intake manifold.


Watch the eleven minute video above to see a Tracker purge valve trouble code caused by a faulty PCM.


The PCM routinely tests integrity of the EVAP system. The test works like this:

Step 1: the PCM closes the vent valve
Step 2: the PCM opens the purge valve drawing a vacuum on the canister.
Step 3: the PCM closes the purge valve and starts a timer.
Step 4: the PCM monitors the fuel tank pressure sensor.
Step 5: if the system maintains 3.3kPA of vacuum for 15 seconds there are no leaks.
Step 6: if the vacuum falls below 3.3 kPa, the PCM sets DTC P0440 (small leak).
Step 7: if the vacuum can't reach 3.5 kPa, the PCM sets DTC P0455 (large leak).

In 2001 a "small leak" was defined as that equal to a round hole with a diameter of 1 mm.
In 2002 a "small leak" was defined as that equal to a round hole with a diameter of 0.5 mm.


EVAP leaks are best detected by using a low-pressure smoke machine. The pressure in a standard smoke machine will damage the purge and vent valves.


This is the air filter for the vent valve.