I found a shop that wanted to look at my truck for free. I'm having the issue to where at idle it will backfire through intake when trying to accelerate at completely random times, not every time I stop. The shop checked compression, plugs, did a leak test with smoke machine. Checked all o2 sensors. Only time it ever acted better was when they removed the o2 sensor before the cat. No codes showing. They are thinking it is a bad cat but I keep thinking plenum gasket. There is no puddle of oil in intake and it does not burn oil. 247,000 miles on engine. 2000 v6. Any help would be nice.

 

Put a vacuum gauge on it and see what the readings are at idle, at speed, and when accelerating/decelerating.

While possible you don't have the common signs of a bad plenum gasket.

Removing the O2 sensor reduces backpressure.