EVAP System Operation

WARNING: This page is about a different car, the 2025 Jeep Grand Cherokee. However, it is still accessible from the selected car via links, so may be relevant.

REFUELING OPERATION: Fuel Tank pressure can build because the vehicle may operate for some time in electric powertrain mode without Fuel Tank vapors being purged to the engine and burned. The PHEV Fuel Door locks to prevent the vehicle from being fueled while the Fuel Tank is pressurized. The components used to control the opening of the Fuel Door are the Fuel Tank Isolation Valve (FTIV) , Fuel Tank Pressure Sensor , Fuel Door Release Switch and the Fuel Door Unlock Solenoid , which also includes an internal Fuel Door Position Switch . When the Fuel Door Release Switch is pressed the PCM will read the Fuel Tank pressure and open the Fuel Tank Isolation Valve (FTIV) to depressurize the Fuel Tank. Once pressure is relieved below a calibrated threshold, the Fuel Door is unlocked using the Fuel Door Unlock Solenoid and refueling can begin.

The Fuel Door Release Switch is hard-wired to the BCM and is the input that begins the refueling process. When the BCM receives a pressed signal from the switch it sends a wake up command to the Powertrain Control Module (PCM) along with a corresponding fuel refill bus message.
- The Fuel Door Release Switch signal is a 12.0 volt supply signal. The switch has two internal resistors and operates similar to a multiplex switch. The PCM determines any voltage reading above 3.0 volts (but less than the open circuit threshold) to be not pressed and any voltage reading under 2.0 volts (but not less than the circuit low threshold), to be pressed . A voltage reading between 2.0 volts and 3.0 volts is considered to be an irrational signal.
The PCM will check the Fuel Tank pressure and open the Fuel Tank Isolation Valve (FTIV) to depressurize the Fuel Tank. Once pressure is relieved below a calibrated threshold, the Fuel Door is unlocked using the Fuel Door Unlock Solenoid and refueling can begin.
- If there are no Fuel Door Unlock Solenoid faults present, the BCM supplies 12.0 volts to the Fuel Door Unlock Solenoid and the PCM controls the operation of the solenoid through a Low Side Driver .
- The PCM also monitors the Fuel Door position to determine if it is opened or closed. This is done by monitoring the status signal input from the Fuel Door Position Switch which is integrated with the Fuel Door Unlock Solenoid.

During refueling the normally open change-over valve inside the ELCM is open to atmosphere preventing excessive pressure build-up in the fuel tank and early fuel shut-off.

EVAPORATIVE NON-INTEGRATED LEAK MONITOR (ENILM) OPERATION: The Evaporative Leak Check Module (ELCM) is an electronic vacuum based flow meter that performs the required on board EVAP system leak testing. The Evaporative Non-Integrated Leak Monitor (ENILM) diagnostics are performed in the engine off (after run) state. The diagnostic will run after the ignition is off for 6 hours. The reason for the wait time is that there are cases where the Fuel Tank pressure increases greatly after ignition off (due to high temperatures) and is too large for the ELCM Vacuum Pump to create vacuum. This could cause a false failure during small leak testing. After the calibrated wait time the Powertrain Control Module (PCM) will initiate the Evaporative System Small Leak diagnostic. The Evaporative Leak Check Module (ELCM) and Fuel Tank Isolation Valve (FTIV) are key components used in diagnosing the system. The ENILM system used on the PHEV vehicles is divided into two parts at the FTIV (review the illustration below), the Fuel Tank side and Fresh Air side . The two sides of the system are diagnosed for leaks independent of each other by the PCM. However, if the Fresh Air side of the system is determined to be leaking, the FTIV is opened and the entire system is checked for a leak. Because of this a leak on the Fresh Air side will typically set a leak fault against both the Fresh Air and Fuel Tank sides, whereas a leak on the Fuel Tank side should only set a leak fault against the Fuel Tank side. One exception to this could be a small leak internally through the FTIV. This condition could cause the Fresh Air side to fail leak testing and set a fault but pass leak test of the entire system and therefore not set a Fuel Tank side leak. In most cases the FTIV Stuck Open DTC should also be set but may not if the leak is small.

Courtesy of CHRYSLER GROUP, LLC

PURGE FLOW OPERATION: When the engine is running, purge flow takes place as the vapors are drawn from the Charcoal Canister into the intake manifold through the Purge Solenoid. The Turbocharged engine requires the ability to draw fuel vapors from the Charcoal Canister during both Naturally Aspirated operation and during boost conditions. Typically engine vacuum is used to draw the fuel vapors from the canister. This is easily accomplished on a naturally aspirated engine that has sufficient vacuum in the Intake Manifold during most engine operating conditions. It is more difficult when a Turbocharged engine is in boost mode creating positive pressure in the Intake Manifold. This is accomplished using a different Purge Solenoid than is used on non-Turbo engines along with some additional hardware. The function of the purge system in each mode of operation is as follows:

Naturally Aspirated: When the Purge Solenoid is energized the vacuum from the Intake Manifold pulls check valve 1 open. The vacuum created inside the Purge Solenoid pulls check valve 2 closed, sealing it off to the Ejector Tee. Fuel vapors are drawn from the canister, through the Purge Solenoid, and into the Intake Manifold.
Boost: When the Purge Solenoid is energized the pressure in the Intake Manifold closes check valve 1 and opens check valve 3 allowing airflow to the Ejector Tee. The airflow through the Ejector Tee creates a venturi effect creating vacuum in the line between the Purge Solenoid and Ejector Tee, pulling check valve 2 open. Fuel vapors are drawn from the canister, through the Purge Solenoid, and into the Air Cleaner box where they travel with the intake air to the Intake Manifold.

For information on how the Powertrain Control Module monitors and performs diagnostics on the system. Refer to: EVAPORATIVE EMISSIONS - DIAGNOSIS AND TESTING .