2000 Impala 3.4L : Stumble when you press the gas pedal when it reach at certain rpm either at park or drive. Starts right up and idle wonderfully and runs great when it catch on at higher rpm (above 45 mph). This been going for almost 2 years. There's NO MIL light or code.

Things that have been done and checked two days ago:
Changed fuel pump, fuel filter, fuel pressure regulator, ignition control pack and module, TPS, IAC, coolant sensor, EGR, MAP and cleaned MAF.


Changed six months ago: spark plug, ignition wire and camshaft sensor.

No kink or pinch return line, Fuel pressure at 53 psi. Compression ok on all cylinders. No vacuum leaks, checked 10,000 times. No other issues or problems.

Absolutely no changes after all the parts where replaced.

Now, I need all your brain power please to figure it out. All recommendations and suggestion are greatly appreacited because I'm out of ideas.

r/Gerry

 

I know you've checked for vacuum leaks 10000 times, now how did you check them? Also, with it running take the plug wires and gently shake them around, does the rpm change?

 

I've checked the leaks by unconventional way: placed dry ice in an empty gallon of milk with little water and attached a clear plastic hose on it that I pointed to all vacuum fittings and lines checking the smoke if it is being sucked up.

Been there done that: shaked not only the plug wires but all the electrical connections and lines. No changes. I even went further by removing and reinstalling the spark plug and wires.

I forgot to mention that if I maintain the throttle at the stumble point, it will not die but idle rough and after a while it backfires.

Thanks for your input. I hope one of you guys can figure it out.