1998 Nissan Altima, Trouble code P0171, Ongoing problem

There should not be any sludge in the engine, I or my son changes the oil once every 3 months and there is usually not 3,000 miles on it by that time, this engine was replaced about three years age because the timing chain broke and trashed the original engine, the replacement engine was used, about 68,000 miles on it, when installed, the oil was changed @ 1,000 miles then at a maxium of 3,000 there after, can't understand if the engine is sludged up, I'll take the PCV valve out again tonight to see if it'e clogged up, if it is, I guess there's a problem.

look vor vacuum leaks or intake or exhaust leaks. p0171 lean means theres too much oxygen and most likely is pulling air in at the intake manifold gasket or if you're smelling it inside car could be exhaust leak which may be sucking air in there causing the oxygen sensor to read too much oxygen and setting codes. so inspect intake and exhust for leaks.