Whistle at idle, slightly rough idle and a lean code
- Make and model
- Volkswagen Passat B7
- Year
- 2012
- Engine
- 1.8 TSI (CDAB)
- Fuel
- Petrol
- Mileage
- 136,000 km
Customer complaint
A whistling or howling noise from the engine at idle, a slightly rough idle and the engine warning light on. The oil filler cap is hard to lift with the engine running.
Fault codes
| Code | Description | Status |
|---|---|---|
| P0171 | System Too Lean (Bank 1) | Confirmed |
Measurements and live data
| Parameter / measurement | Measured | Expected |
|---|---|---|
| Total fuel trim, idle | +21 % | within ±10 % |
| Total fuel trim, 2,500 rpm | +6 % | within ±10 % |
Tests performed
- Fuel trims compared at idle and 2,500 rpmLean mainly at idle: points to unmetered air
- Smoke test of the intake and crankcaseSmoke from the vent of the PCV valve on the oil separator
- PCV valve diaphragm checkedDiaphragm split
Solution
Root cause
Split diaphragm in the crankcase breather (PCV) valve, letting in unmetered air and losing crankcase pressure control.
Repair
Replaced the oil separator with its integrated PCV valve (latest version), cleared adaptations and the code. Fuel trim at idle back to about +2 % and the whistle gone.
Parts
Oil separator with PCV valve
Labour time
1 h
Lesson learned
Lean at idle but normal at 2,500 rpm is the classic unmetered-air pattern; on this engine look at the PCV valve first.
Training case: the usual fix for this fault, not a confirmed repair on a specific car.
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