This article will review a technical help request from a shop for a 2009 Audi A4 running rough and setting fuel trim fault codes.
The following fault codes were stored:
- P0171 System Too lean
- P12A2 Fuel Rail Pressure Sensor Too high
- P0507 Idle Air Control System RPM Higher Than Expected
- P2279 Intake Air System Leak
- P0300 Random Multiple Cylinder Misfire
- P0301 Misfire Cylinder 1
- P0302 Misfire Cylinder 2
- P0303 Misfire Cylinder 3
- P0304 Misfire Cylinder 4.
Before giving Autologic a call for help, the vehicle was in fact running lean, confirmed by fuel trim. The shop determined the crankcase breather was faulty, and once replaced they disassembled it and the diaphragm was ruptured.
VW FSI Crankcase Breather Design and Function
On Audi and VW 2.0 FSI engines, the engine crankcase is maintained in a constant vacuum while the engine is running. The engine is equipped with a crankcase breather valve installed at the valve cover with a direct connection to the intake manifold.
Crankcase blow-by gases are separated in two stages. Stage 1 – The primary oil separator in the oil filter housing removes the majority of engine oil from the gas. Stage 2 – The separator on the valve cover removes the remaining engine oil from the gas.
On turbocharged engines a more sophisticated pressure control system is used. A two-stage pressure control valve is located on the valve cover. When intake manifold vacuum is present blow-by gas flows into the intake manifold. When turbo-boost is present, the one-way valve in the breather valve assembly closes and blow-by gas flows into the valve cover.
The engine control system can detect improper installation of the pressure control valve and unmetered air leaks are detected using the oxygen sensors.
Repaired But Not Fixed (yet)
After reassembly the engine still ran rough and set lean faults. We asked the shop to perform a fuel trim test once again, and the engine was still lean.
Scan data showed Short term maxed at 25% and long term at 0%. When engine RPM was raised (off idle) short term fuel trim dropped to a normal range. This indicated a vacuum leak.
For a complete background and diagnostic information on fuel trim, see this Autologic technical article. Fuel Trim & Fuel Control - Diagnosing Faults Part 1
Once the lean condition was confirmed a crankcase pressure measurement was taken, it proved to be lower than expected.
Audi and VW 2.0 FSI Engines usually create about 4-6 Inches of Water.
This specification was developed after testing multiple good known engines, VAG does not supply a crankcase vacuum specification. Keep in mind, a reading higher than noted can also be a faulty breather causing too much vacuum in the crankcase. Symptoms would be similar.
Audi VW (VAG) Test for Crankcase Breather faults
VAG suggests testing the valve by removing the breather pipe between the intake manifold and the breather valve. Then use a plug to seal the intake port. If the engine runs normally at idle, the breather valve (located at the valve cover) is likely at fault. If the issue does not go away, the fault may be at the oil separator at the oil filter housing.
The valve side of the breather pipe can be left open to atmosphere for testing purposes.
Smoke Testing the System
A smoke test of the crankcase located a leak at the front timing cover and the rear main seal.
Notes on Seal Failure
When crankcase breather valves rupture, the crankcase will be put in high vacuum condition, which can lead to engine seal failure. Audi / VW did release a TSB for front main (crankshaft) seal failure due to high crankcase vacuum. Check with Autologic or with your latest repair information subscription to see if this TSB applies to the vehicle you are working on.
Repair Confirmation Test
Once the engine seals were repaired a new crankcase pressure measurement was recorded and determined to be within specification. The engine once again was running well with the check engine light OFF.
Repair also confirm with scan tool data. With the engine idling after repair, short term fuel trim returned to a normal range, as shown below.
Need help diagnosing a drivability fault on an Audi or VW? Get in touch with our technical support specialists.