Im having issues with a 98 3.8l Windstar im working on ( Friends car ).

It sounded like there was a bad vacuum leak coming from the Intake Manifold or Throttle Body area. The car is stalling out and backfiring when you give it above 20-25% throttle, but if you keep it under that it will rev up and run just fine.

It had codes 0171 and 0174 plus one other (Cant remember) when I first scanned it, so i figured it was the Plenum or Intake Manifold gaskets. I got a Intake Manifold Gasket Set, and Isolator Bolts, replaced them, and smoke tested it and noticed smoke coming out of the plenum to lower gasket area. Pulled it back apart, used some Permatex Ultra Black to seal it better, Smoked it again and had no issue. Went to start the car and it is still having the same issue. Any time you go above 20-25% throttle the car dies out, and you can hear it back firing. I checked the MAF and it seems clean, but im not certain as I dont have any cleaner on hand.

Im unsure if it could be the Fuel Filter or Pressure regulator, Maf or TPS? Those seem like the next best places to start.

Any ideas?