My 1994 Chevy C1500 with the 4.3 6 cyl doesn't want to accelerate from a dead stop.

This is a common 4.3 problem usually EGR (exhaust gas re circulation valve) partial function it also effects higher speeds 50-60 sluggish vehicle feel not effected by being hot or cold. My shot in dark without vehicle diagnosis.

Also check and replace air filter if necessary.

Thank you! I'm trying everything suggested.

Check the air hose that is connected to the transmission module located on the passenger side of the transmission. Most people don't think to look there. The hose is exposed to many elements therefore it could have a small crack that would cause the engine to sputter at lo idle. It takes all of about fifteen minutes to change. Do not use the old clamps get new ones. I had to replace mine twice first time it was split. Two weeks after I installed new one it looked like someone took a razor to it. So when you buy the new hose inspect right there at the parts counter to make sure there are no splits in the new hose. Good luck yet I am sure this will fix your sputtering and backfire on a start up. Also it will eliminated hard starting.