Gas cap light/check engine 2009 MKX

[BGrahovac]

I have had an issue with the gas cap light and occasional check engine light on my 2009 MKX. Its driving me nuts.
I replaced the gas cap and all wad good for a week. Then the engine light came on with aP0455 code for gross eval leak. Double checked the gas cap, cleared the light. Again all was good for a few days. Got fuel and the gas cap light came back on later that day. Again double check fuel cap and the light went off about a week later. A day after that the engine light came back on with the same P0455 code. Cleared light and all was good for a few days. The gas cap light came back on and is still currently on.
I checked the purge valve with the line disconnected and it unplugged. It has no vacuum so I believe that means its good.
I do have a smoke machine and was curious what ports I should use to test.
Has anyone else run into this? If so what was your fix for it?
Any help would be appreciated. I can't stand trouble lights being lit up on the dash.

 

[driller]

P0455 can be caused by several items(cut & paste):

• Loose or faulty gas cap
• Damaged gas tank fuel filler neck
• Structurally compromised EVAP canister
• Cracks, holes, or splits in EVAP hoses
• Faulty EVAP purge valve
• Defective vent valve solenoid
• Cracked fuel tank
• Faulty system pressure sensor

In my experience, the vent valve or pressure sensor are typical suspects but I have seen cracked/split hoses or mysteriously disconnected lines. I generally refrain from throwing parts at a problem but usually the vent valve is cheap and easy to get to.

Actual troubleshooting the EVAP system is usually most efficiently done following the factory PC/ED (Powertrain Control / Emissions Diagnostics) manual. It's not rocket science but EVAP systems tend to be very model specific.