This concerns a 1998 Bonneville with 86000 actual miles on car. Engine blew a head gasket and caoused a hydraulic Block on the cylinders. It was decided that We should replace the engine with a OEM GM complete motor including a new intake manifold. Car runs and sounds like a dream and idles smoothly. We now have about 200 miles on the engine and are getting an Engine check light (solid) and after checking, we find a P0171 Code (engine lean bank 1) Mechanic replaced both of the Oxygen sensors, before and after the catalytic converter. Ran car for 20 miles and the light came back on. Funny thing is, that the light comes on at the same place each time. Kind of going down a canyon, backing off the accelerator at times and braking to go around curves. Looking for suggestions.

. Check for vacuum leaks (hoses, manifold, PCV Valve). See if spraying WD-40 or water helps you locate the leak.

. Clean the MAF Sensor with CRC MAF Sensor Cleaner.