[Ol'Jim]

Guys, thanks for your input.
I passed it along to him. Sorry for lack of more detail on vehicle when I first posted. He told me last night it is a '98 Chevy 3/4T Ext Cab, 350. He said about the same time it started running bad, it also started smelling "gassy". I asked him if he meant as in an overly rich mixture, or as in raw fuel vapor. He said he couldn't tell for sure, but it is not leaking fuel, and he thinks it is in the exhaust. So this might make sense then, based on what j_CAt posted about DTC P0441. Thinking if there is a vac leak, that would result in lean exhaust, which the O2 sensor would report to the PCM. PCM then would attempt to order up more fuel from injection, which would then result in rich-smelling exhaust?
I told him to check over the vapor canister, the entire length of the vapor lines, including line connections to canister, the purge valve, and the fuel tank, as well as the seal of the gas cap.
If no cracks or apparent leaks, would you suspect the purge valve/solenoid? Can it be tested with a vacuum source? (He is near my home in TX, and I have a hand-operated vac pump there).
Also told him about potential distributor issue with bearings. He said his truck is right at 170K miles, so that sounds like a real possibility. I suggested he put an ohm meter on his plug wires. He has replaced plugs and dist cap.

As always, I really appreciate you guys and A/F for the help.