tune issues
A mail order tune will not get the fuel exactly right simply through math. That is the reason why cars have o2 sensors. Different temperatures/humidity/etc depending on your location as well as the type and condition of various components in the car will cause the calculated fuel to be off. In most cases, it should only be off by a little. But that won't always be the case.
With a street tune, the goal is to gather data from each trim cell and then adjust the tune (MAF or VE depending on which you have) to bring the calculated a/f to within as accurate a range as possible. This will result in better open loop operation (when the car is cold) and will ensure the o2 sensors have plenty of margin for error during closed loop.




