So I know that i have a 5.0L and that mileage is gunna suck anyway. But i can smell my car running rich. But when i plug the O2 sensors in it runs like crap but not as rich. They're not in the exhaust so maybe that's the issue. But could a VERY small vacuum leak and NO/improper O2 sensor cause it to run rich and get like 8 miles to the gallon considering that it should get like 17?
Yes and yes... When the pcm sees a vacuum leak it will richen the fuel trim to compensate for it. As for the O2 sensors...They read oxygen in the exhaust stream. Since they are not in the system (just hanging out in the atmo) all they are seeing is lots of O2 soo...Same thing. Get a couple of O2 bungs and weld em in either right befor or after your header flanges on the collectors/pipes. Then go from there. not plugging in the O2/S at all will cause the car to run poorly since the PCM needs the input from em to tune the fuel enrichment accordingly...Good luck!!!
