I have an 2007 Workhose chassis with 8.1 engine.
It started back firing at 44,000 miles so I changed the standard wires to high temperature wires and at least 3 of old wires were burned. This improved the condition but did not cure it.
Next I installed new spark plugs again improved but did not cure. It back fires, normally, one pop on down shift and one on up shift 5th gear to 4th gear (6 speed tranny). On down shift into 3rd gear on steep hill it is constant popping and I drop cruise and go manual throttle to minimize problem.
Do not have any engine lights or codes. I can see the back fires on computer program but just as a blip with no info.
I do most of my own service but at total lose on this one.