Recently replaced my o6 cummins with a rebuild. The new engine seems solid enough but lacks any low end torque. Mileage is down considerably - original engine was quick off the start and before I lost a cylinder would average 24-26mpg. After losing 1 cylinder it still gave 19-21mpg. New engine is a real dog. Mpg is no better than 15 at 55,000 miles. Could use some help here.