I'd think if the belt was off by a tooth that the truck would show signs of not running just right.
Thanks for the compliment Ian :act030:
I've run into the the 'timing out by a tooth' on several cars, and the results aren't always the same. A slightly rough idle, intermittent misfire codes, sometimes a hard start... it only takes a minute to set the crank on TDC and to pop the plugs out of the cam covers to check.
I totally agree that the 'filling it up sets the light' seems to be evap related and you are on the right track given the info. But evap codes are sensitive! To have anything fail on the emissions side and not set a code would be a rare one for sure. IF it is related to filling only, the most likely culprit is a bad purge solenoid/valve. As you fill the tank, vapours could be filling the intake side and causing a rich mixture on start up. This would set misfire and/or O2 sensor codes. But when the evap goes into self diag, the purge sol. should set a dtc of it's own.
So I'm not convinced the fill up is the only sign. There might be other factors that we haven't picked up on yet.
Just for giggles, if the OP is convinced this is fill up related, try unplugging the purge valve before filling. Does it change anything?
For my own curiosity (after you've check the timing marks and confirmed when all this started), how about mimicking a fuel stop but not adding fuel? Pull over, wait 3 minutes and then restart. If no change, same scenario, pull the fuel cap off, put is back on and try again... I'm not holding my breath it will act up, but might help with the diag.