The MIL illuminated soon after I got it home.
Torque Pro shows Code P0446
--P0446 is triggered when the ECM has detected a short in the circuitry to activate the vent control valve or the EVAP system failed to maintain the vacuum during the active EVAP leakdown test.
No shorts in the wiring folks.
Most of the threads I've seen on EVAP codes leap to the conclusion that the filler cap is bad and if that's not bad then "replace the purge valve" then ...
I don't just throw parts at problems praying I guessed right. I troubleshoot. Hail Mary Shotgun repairs are way too expensive.
I plugged in my Knock Off Tech 2 and ran some Evap tests.
-Both Purge and Vent solenoids made substantial clicks when I commanded them on and off. This put them on the back burner.
-When I closed the vent and opened the purge valve 20% with the engine running I heard a sucking noise from the filler door and the tank pressure sensor noted ZERO change... Dead Steady at 6.6in Hg. <---That dead steady pressure reading, way above .2" Hg, should've been a clue that it wasn't just the dead gas cap seal...
Okey Dokey. Off to O'Riley for a Stant filler cap to replace the knackered cap on the fill pipe.
New filler cap installed. Cleared the DTC codes with Torque. The MIL re-illuminated on the way home. WTF is going on here Did I buy a lemon?
Nope just an Onion... OK... Peeling the Onion...
-Tech 2 still shows 6.6in Hg no matter what the state of the EVAP valves. No sucking noise from the filler neck anymore so the cap really was worn out. I can now make the engine stumble from vacuum being pulled on the tank with the purge open and vent closed... The pressure sensor is still reading rock steady at 6.6in Hg. <--- Hey big dummy ya got a clue yet?
-Change the EVAP tank pressure sensor with a brand new GM sensor from the dealer in Wells River... Don't trust China made cheapo parts to last in-service and this sensor is in a really quite unpleasant location so I paid for an OEM part.
The gas cap was bad but this was a major chunk of the problem...
Tip for anyone doing this sensor. As you can see it's just like a MAP sensor. The tank pressure sensor is press fit through the top of the sender but there was no need to drop the tank or lift the bed... at least on my 2005 extended cab 2500 HD. I could access and change this sensor by blind feel reaching up between the frame and the LH bed side.
I made sure there was no crud on top to drop in the tank and kill the fuel pump and spread through the fuel system... GM bean counters decided it was a good idea to de-content these trucks by removing the inline fuel filter. A backpack blower with an elbow would've made quick work of any loose dirt and I did use one out of paranoia even tho I didn't feel any dirt or sand.
The new sensor shows negative tank pressure values, indicating vacuum, when the purge valve is open and the vent valve is closed. No sucking from the filler and the PCM now aborts the test when the vacuum reaches a low enough point. It shows 0.00in Hg with no commands to the Evap solenoid valves. No more MIL? It won't run an Evap test with the engine temps up but I'd say it's fixed.
It finally got around to running and passing the EVAP readiness test after 5 weeks. All the other emissions monitors ran and passed within 3 days and there were no pending codes it just didn't feel like doing it.