I recently had a Hyster with a Vortec that started doing the same kind of stuff you were saying. I'm not sure if your truck carries the same vacume governor set up like my hyster. But when i went through this problem theres a electronic controller that regulates the vacume going to the governor. When i checked the voltage it was spiking to 60Volts at times. When i traced the wiring someone had mixed a diagnostic wire with one of the coil wires so it was back feeding to the vacume electronic controller. after fixing the wiring the issue went away.

The backfiring happens when quickly accelerating or getting out of the throttle and this blows the air cleaner cover off. It only backfires like this a couple of times a day if at all. The lift only has 1700 hours on it and it's the only one of it's kind in our fleet. The factory first stated that it was an ignition problem but after I changed all that now they say it's our propane but I'm not having any issues with any of the other LP lifts. I was talking to a friend who is still doing field service and he said he's having the same problem with a vortec powered Daewoo. Coincidence?

sounds like early valve timing which can be caused by seats receding into head which acts exactly like over adjusted valves. most of the time this backfiring occurs when steering (which lugs the engine) and attempting to accelerate. occaisionally lowering the steering pressure will help. if the valves are opening early it should show up on a vacuum guage. (a really good diagnostic tool that everyone seems to have forgotten about). make certain even with new parts that moisture is not getting into the cap through tiny cracks. i have had this happen in the past. mist the cap with water and see if the problem gets much worse. can be a hard problem to find.....hope you get it knocked out soon