I've got a 1991 BMW M50 2.5L engine in my car and I have some ebay stainless steel headers. It uses clamps to attach to the exhaust rather than flanges and I can hear quite clearly that it leaks in that area. The O2 sensor is placed in the exhaust after the headers rather than in the headers as it will not fit there. Would the exhaust leak at the clamps cause a sufficient spike in oxygen in the exhaust that the O2 sensor would be giving false readings causing a rich run and mileage 16-20mpg when it should be 25-30?
To run rich the leak would have to suck in outside air and mix it with the exhaust so the sensor would think it is lean and richen the mixture. But the exhaust leak should leak from the inside out, not the outside in so the change would be zero.
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