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Evaporative System (EVAP) Monitor — I/M EVAP Monitor

Evaporative System (EVAP) Monitor

All OBD2 vehicles are equipped with a fuel Evaporative system (EVAP) that prevent fuel vapors from evaporating into the air. The EVAP system stores fuel vapor in a charcoal canister during fillup. After fillup, the ECM commands a purge cycle where the fuel vapors are burned in the engine and the system conducts and integrity test to ensure there are no fuel vapor leaks. Each manufacturer conducts the integrity test in their own way.

How the EVAP system works

After fillup, the system open a purge valve connected to the intake manifold. It then opens a vent valve connected to the charcoal canister. Engine vacuum sucks fuel vapors from the charcoal canister and burns the vapor in the engine. It expects to see an increase in engine RPM due to the introduction of this extra fuel. At the same time, fresh air enters the charcoal canister through the vent valve. When all the fuel vapor is removed from the canister, the ECM will see a drop in engine RPM due to the lack of fuel vapor from the purge valve. In then knows the canister is purged.

The EVAP Monitor checks for proper fuel vapor flow to the engine, and pressurizes the system to
test for leaks.

The computer runs this Monitor once per trip.
The EVAP Monitor is a “Two-Trip” Monitor. If a fault is found on the first trip, the computer temporarily saves the fault in its memory as a Pending Code. The computer does not command the MIL on at this time. If the fault is sensed again on the second trip, the PCM commands the MIL “On,” and saves the code in its long-term memory.

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