I have a 1999 4.3l Chevrolet Silverado 1500 with 156,279miles 2wd that has had a misfiring problem and has been checked several times by several people and they have found nothing. Spark Plugs and wires, fuel filter, and cap and rotor have been replaced and still nothing. I got the AC Compressor fixed recently and engine now misfires more than before and loses power. Could it be possible it is my fuel pump causing it; The fuel sending unit has gone bad on it like 2years ago.
Most common reasons for misfires is bad plugs or vacuum leaks. When they did the plugs I wonder if they gapped them correctly.
I just scanned it with an OBDII Scanner and it gave me 3 codes P0300, P0446, and P0442
The P0446 refers to a voltage drop in the electrical system causing the evap vent valve to malfunction. This may be a clue that the problems are electrical. When you turn the AC on it puts a heavy load on your electrical system with the AC compressor clutch and typically the radiator and condenser fans will also run with the compressor. That's quite a few amps.
https://repairpal.com/obd-ii-code-p0446
this engine is notorious for eating intake gaskets. Spray some starter fluid around it to see if it failed
I just scanned it with an OBDII Scanner and it gave me 3 codes P0300, P0446, and P0442
Our FAQ has info on what to do with trouble codes.