Hi I had the same problem with the engine light coming on when going uphill, code P0171. I tried several things, cleaned the MAF, changed the air filter and spark plug but the problem was still there. I took it to a garage and they changed the oxygen sensor, checked for vacuum leaks, fuel pressure, bad earth to the MAF, adjusted the hot idle but the problem was still there. In the end I clean the MAF again and noted that there was a bit of play in the air filter housing. On one of the front bolts holding the air filter housing the rubber washer had been compressed and was not holding the corner of the filter housing firm. I put an extra rubber washer under the bolt to remove the play and the problem seems to be fixed. I have tested it for about 2 week with no sight of the light, prior to this the light came on daily. Hope this is of some help.