01 Ram 1500, 5.9 gas. Replaced the plenum plate about 5,000 miles ago. Drove my truck from Alaska to New York, and didnt have a single problem. Had it up here for 2months, now its started to back fire and miss under throttle. Doesnt do it all the time. Idles fine, runs fine other than the missing, and back firing. Im thinking it might either be the CAT or maybe just bad fuel. Its been doing for the last 2 tanks. Just filled up with premium and it still does it. The cap, rotor, spark plugs and wires were done about a year ago. Never had a single problem and it started all of a sudden.....any ideas?

 

Possibly cam or crank sensor giving you fits. Is the service engine light on?

 

maybe an O2 sensor?
got any codes? poor mpg? smell rich?
maybe low fuel pressure? - check with a gauge.
pull plugs and check condition.
maybe check compression while out.

where's it backfiring? in the exhaust, or back into the plenum?