1997 Ford Mustang

Ok so you have no cats on the car? If this is true then that is why it can't trim the fuel correctly. See this is an OB2 system with 4 cats. The front O2's monitor whats going into the cats and the the rear O2's monitor whats coming out. Without the cats the rear O2's can't get to temp. The factory O2's are not a wide band O2 so it must be within set peramiters or you get bad fuel trimming. I am shocked that the light isn't on have you checked for any codes? See I had my engine light come on after I put in my Flowmaster cat back system as it cooled the rear O2s. I have many more mods that I have done to my Stang and no problems but I didn't remove the cats. Have you tried cleaning the mass air flow sensor with mass air flow sensor cleaner (DO NOT USE ANYTHING ELSE) this might help it is worth trying. Let me know what you find and if needed we can run a couple of test to see what we can find.

Ok if the O2 will operate in a wide enough band then the H pipe will not cause problems but with the rear O2's running cooler it will use more fuel make sense? The H pipe comes with the bungs because the car can't run correctly without the all the O2's in place however the H pipe is not smog legal so if you are in a smog state you will need to replace the cats before the next snog. The mass air flow cleaner is available at most any parts store don't use anything else on it. Now a cold air intake may help too but costs about 180 dollars but it will inprove intake flow and work great with the H pipe which increased exhaust flow. The smoke you meantioned what color is it? I have done many mods on my Stang so I am very familier with them so if you have questions ask away.