The old harness and computor, MEFI-3 only read air coming in and the new harness and computor MEFI-4 reads air coming and going out, it has a 02 sensor. The computors have different part numbers and the harness are not the same. It sounds to me like the first series was a design problem and they tried to correct it with the new one. The moter was expensive and the original computor didnt work and the upgaded kit helped it idle some what better but it still serches,and standing behind the car running gags you. I have checked for vaccume leaks and are none.