Drives OK but no power when under load

here is an article I wrote to help folks and shops deal with the P0171 and P0174 codes. To this day, they really throw off shops. I do Emissions work in N Ca and have to deal with these codes every day, so I have spent many, many hours dealing with their causes. http://repairpal.com/OBD-II-Code-P0171-and-P0174 take this to a good shop http://repairpal.com/gmc-repair-in-rising-sun-maryland