P0171-2001 Audi A4 System Too Lean

Question

I have a 2001 Audi A4 and I have recently replaced the mass air flow sensor, camshaft position sensor, air filter, timing belt and water pump, catalytic converter and spark plugs, but I keep getting this P0171 code that comes back. Now I know I need to get an oil change and also am going to check and see if there are an bad ignition coils but as Ive been reading alot of people say it may be a vacuum leak and ai was just wondering could his be the cause? Just trying to nip it in the bud as the P0171 code has been the peskiest ine to get rid of. When driving the car it accelarates way better, but it still runs kinda rough at idle in neutral. Does anyone know what this may be?

Answer 1

P0171 Fuel trim lean

Yes it is vacuum leaks that are mostly the cause of a lean fuel system.One of the clue you give is , "it rough at idle" When it comes to a turbo engine a vacuum leak doe not create a noticable lean condition.  Unless it with the PCV system. and this includes the oil dipstick .and the crankcase ventilation  hoses. The PCV and its hoses  rupturing would cause a lean condition  and is mostly the cause for a lean condition especially with the rough idle.