Rick's Free Auto Repair Advice

P0171 VW

Diagnose and fix a P0171 VW trouble code

What does a P0171 VW code mean?

A P0171 code indicates a lean condition that can’t be corrected by the computer. This is a common code on a VW and can be accompanied by misfire codes like: P0300, P0301, P0302, P0304, P0305, P0306. If you get a P0171 code along with misfire codes, diagnose and fix the p0171 code FIRST! Do not start with the misfire codes because a lean air/fuel mixture can CAUSE those misfires.

Additional enhanced codes may also set, like:

P1297 Fuel trim bank 1, system too lean at idle speed
P2187 Fuel trim bank 1, system too lean at idle speed
P2279 Intake air system leak

What causes a P0171 VW code to set?

A lean code on any vehicle can be caused by a vacuum leak or some source of unmetered air. For example, if the air duct running between the MAF sensor and the throttle body develops a leak, that would let in unmetered air that the computer cannot correct, resulting in a P0171 VW code. So start by checking the air duct for tears. If that checks out, move on to the very troublesome VW PCV system

VW PCV crankcase breather is a problem

Every engine creates blowby gasses that

P0171 VW

VW Pressure Control Valve

seep past the piston rings and flow into the crankcase. The pressure control valve (PCV) system is designed to suck those blowby gasses out of the crankcase and route them back into the intake manifold to be burned. Since the engine uses normal manifold vacuum to remove blowby, and since the crankcase can’t run on negative air pressure, the PCV system must allow outside air in to the crankcase to restore normal air pressure and crankcase breathing.

Most car makers use an open system to route filtered air into the crankcase. VW uses a different system. Depending on the engine, VW uses two crankcase breather valves to allow fresh air into the engine. One valve is located on the valve cover, the other on the oil filter housing. The valves contain a diaphragm that is known to fail. If one of the crankcase breather valve fails and the engine crankcase can’t breathe, it’ll create a negative air pressure situation in the crankcase.

Test the crankcase breather valves

While the engine is running, pull the dipstick and check for a vacuum condition on the dipstick tube, or remove the oil fill cap. Neither should have a vacuum condition. Next, clamp off the vacuum tube to the valve cover breather valve to stop the flow of vacuum. If crankcase vacuum disappears, the valve cover breather valve is the faulty component and must be replaced. If blocking the vacuum tube from the valve cover breather doesn’t change vacuum, try the valve on the oil filter housing breather. Remove the tube at the housing and block off the vacuum from the intake manifold. If that fixes the crankcase vacuum problem, replace the oil filter housing.

©, 2017 Rick Muscoplat

Posted on by Rick Muscoplat

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