The evaporative leak detection diagnostic strategy is based on applying a vacuum to the EVAP system and monitoring the vacuum decay rate. The VCM monitors the vacuum level via the fuel tank pressure sensor input. At an appropriate time, the EVAP purge solenoid and the EVAP vent solenoid turn ON, allowing the engine vacuum to draw a small vacuum on the entire evaporative emission system. If a sufficient vacuum level cannot be achieved, a large leak or an EVAP system malfunction is indicated. The following conditions can cause this problem:
A disconnected or malfunctioning fuel tank pressure sensor.
A missing, malfunctioning, improperly installed, or loose fuel cap.
A disconnected, damaged, pinched, or blocked EVAP purge line.
A disconnected or damaged EVAP vent hose.
A disconnected, damaged, pinched, or blocked fuel tank vapor line.
A disconnected or malfunctioning EVAP purge solenoid.
A disconnected or malfunctioning EVAP vent solenoid.
An open ignition feed circuit to the EVAP vent or purge solenoid.
A damaged EVAP canister.
A leaking fuel sender assembly O-ring.
A leaking fuel tank or fuel filler neck.
Any of the above conditions can set a DTC P0440.
Your may have to drop the tank and check the vent valve, see if it's broke off at the vacuum line fitting since you mentioned that gas las leaking from the tank.
When checking the vent/purge solenoid, depends on whether you have a 2dr or a 4dr. I can't remember the exact location of the 2dr vent/purge solenoid, but on a 4dr, just drop the spare tire if you have it underneath and it'll be in the middle of the cross member directly behind the rear axle. It'll have one large vacuum line and an electrical connector.
You can also see the gas filler tube to the rear of the gas tank, check for any damages where the gas may be coming out of.
Good luck and hope this saves you some money, I believe most dealer will charge close to $400.00 to drop the gas tank and do this job.
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