Bee-when warm, any car is in closed loop mode so the maf is measuring air passing maf and sending an anticipated fuel injector spray duration period to the ECU. Then, the pre-cat O2 sensor has the final word and determines how well the ECU did based upon Maf signal. The leaner air at higher altitudes will tell the O2 that car is running rich cause there isn't enough air to support your adaptation values for normal operation. ECU will start to change the adaptation values to take fuel injector pulse duration down to make the O2 sensor happier with less gas to match the lessor air. The car does not know how long you are going to be in the higher altitude, so it may not make a full adaptation for the height above sea level in one trip. If you moved to higher altitude permanently, it would change your whole table and then your car would run lean at sea level until adaptation values got used to being at sea level and getting more air and lengthen the fuel injector pulse time. Make sense?
Last edited by 540iman; 12-06-2012 at 12:47 PM.