Wrangler 'slow emissions leak'

Hello! I’m new here and really hope you guys may be able to answer a question for me.

Car: 2005 Jeep Wrangler 36,000 miles

What is happening: Check engine light comes on when fuel tank reaches around 3/4

Sometime this winter I replaced the original locking gas cap to a Mopar non-locking gas cap. Not long after I started getting the check engine light to come on. I have been to NAPA and Autozone to get it read and it says ‘slow emissions leak’, which typically means there is a problem with the gas cap. They have told me to tighten the cap and it will go away in a couple days. It will not turn off by itself, so I have disconnected the battery for a couple mins to reset it. The light will stay off until the next time my fuel level gets to about 3/4 and then comes back on. Is there something else that could be triggering this light? Or, why would an original Mopar part cause my computer to read an error?

Thanks for the help. Let me know if I left something out.

The gas cap is one item that seals the gas tank. The ECM monitors the pressure and vacuum in the tank to see if they are acting correctly. If there is exit of vapor or entry of air the computer sets the code. So any connection, hose, or device that is not holding pressure is suspect. You will have to go deeper. When you remove the gas cap after the tank has dropped 1/4 do you get a rush of air in or out of the neck as the cap is removed? Have you observed pressure buildup on a hot day?

The fact that the code sets when the tank gets below 3/4 may indicate that the gasous leaking spot may be covered when the fuel is higher. It also may be a red herring because the complete test cycle may take that long to complete the requisite number of times.

Post back with what you find.

Researcher’s reply is accurate but I will add that the ECM won’t run its “evaporative leak” test unless the tank is well over 1/2 full.

both replies above,are way misrepresented,and WRONG.well 90% wrong.

read the criteria before you blast back.

thank you and ,we hope you love the Concord as much as we do,and enjoy your day at Hershey park.