"Tighten Fuel Cap" Message
 

Good Morning
I have a 2005 Chevy Silverado Crew Cab, 5.3 L , 4x4 ., Single Tank,The Tighten Fuel Cap Message started showing , so I went to Autozone and bought a new Fuel Cap. Helped for a day or so then came back on, so went to Chevy bought a GM Cap, same Results, Mechanic friend says its a Problem in the "Evap System" and they do a "Smoke test" to see where the Leak is, Does this sound correct? Is this a common problem??
Thanks Ed aka Tennessee Z

UPDATE!!!!! It was the FUEL PUMP!! Yep! "The Smoke Test" revealed a LARGE Plume and that the Vent Tube on top of Fuel Pump was Rusted out, Sooo, Had to Pull the "Box" ,Quite a job as the Bolts holding the Box to Frame were rusted also, truck came from West Virginia. Luckily som friends who own a shop did it with a Fork Lift and Correct tools, Further Update, Fuel Pump was Installed Fuel Gauge was Stuck all way on Full, Had to Pull the Box AGAIN and ReClock the Fuel Pump, Man! Pump was a "Delphi" too! Go Figure, Luckily the old one hadnt been thrown away so Comparison to Determine what was wrong was fairly easy.

good find.
the fuel cap message sets when the system detects a large evap leak. As you have found the leak can be anywhere.

Some vehicles will set the message if the leak test fails after a change in fuel level is detected...the vehicle knows a fueling event took place and sees an evap test that previously passed, now fails. it reasons you left the cap off or loose after refueling.

You are Welcome! I tell ya , This truck has had a Harder Life than I Imagined when I bought it, The Radio had been Changed, & All the Speakers rattle, I Bought a XM reciever because of Radios Poor Performance, I am Thinking if I can Figure out what GM radio will fit in stock Location, that also has XM capabilities? I will change it and the Speakers, Could sure use some Advise on that!!, Of Course Having fixed all the previous Mechanical Issues, NOW, The Power Steering system is leaking somewhere! Siggggh, :-( :-( Maybe the Radio and Speakers will be later. :-(