Engine misfiring. I have a 1996 GMC Suburban with a 454 ci engine and 235,000 miles. The misfiring started a month ago, I have replaced the plugs (ACDelco Set at 0.035), the Spark Plug Wires (2yrs old/15,000 miles on them) look to be in goog shape My next move is to replace the rotor and cap. Question: Am I correct on the gap for the plugs? What is the firing Order? Where is number one on the distributor cap? Should I replace the wires again?

Lets start at the beginning. Did anyone check fuel pressure before and after replacing the pump ?
Has anyone checked the fuel pressure regulator and the injector spider ? The 4.3w engine uses a spider with one injector that supplies all 6 cylinders. And it runs at 65psi or more.

1. Air filter clogged
2. Fault with the electrical systems and or fuel.
3. Fuel not reaching the injectors.

These are some of the problems that would cause this.

And did this happen right after they replaced the injectors, or recently?

no this is one strong engine.

Replace the sparkplugs or try holding the gas pedal to the floor and try to start but keep pedal to floor until engine starts and revs, this may take a few trys, dont crank more than 10 to 15 seconds, if this doesnt work then replace the plugs,

could be anything from the weird to the sublime so i couldnt evan hope to tell you unless i had the vehicle in front of me sorry

can be lifter going bad,rocker arm loose (once oil warms and thins out) can also be a minor exhaust gasket leak

sound like an internal engine problem. either one or more of the pistons has "piston slap" and that knocking will diminish as the engine warms up or possibly a rod bearing is worn out. have a tech do a manual oil pressure test to make sure the oil pump and main bearings are ok. piston slap or knock that decreases when the engine warms up is not that serious, but a rod bearing knock or possible oil pump failure should be taken care of asap.

P0300 is a multiple cylinder misfire and is used by all OBD2 vehicles on the road in the USA that is 1996 and newer, P0301 is cylinder number 1..P0302 is # 2 and so on...that tune up may have been needed and parts and labor are expensive..you have a cylinder that is misfiring and the cause must be located..possible causes are intake manifold air leaks..secondary ignition problems..fuel injectors..plugged EGR system if it has one..failed head gasket..burned or cracked valves..I have seen some crazy stuff..you need a tech on that one not a mechanic.

Check fuel pressure, and regulator. A leaking regulator gives a warm engine more fuel [ it does not like it] a cold motor does. same as a flooded motor. Good luck

sounds like fuel pump pressure. I had the same thing happen to me in a Lincoln mark VII and the electrical connection was shorting out in the fuel pump. I changed the fuel pump..Returned under warranty) put the new one on with filter and had no problem with it.