Audi A4 turbo idle fault

After a couple of previous visits to us, to investigate an intermittent poor idle and engine management light on, I was determined to get to the bottom of the poor engine idle speed. A diagnostic scan revealed three fault codes, that gave me vital information as to why the idle running was so poor;

  1. Turbo pressure loss between turbo and inlet manifold;  the system was recognising a fault with the intake system.
  2. Fuel trim too lean; the emissions into the exhaust was measured by the oxygen sensors as having too much oxygen.
  3. Multiple cylinder misfire; this code revealed that the fault wasnt restricted to one cylinder and so had to be something that effected all cylinders.

From the information already gathered I decided to investigate a possible air-leak on the inlet system. I ran the car on our emissions tester so i could see the make-up of the emission gases, from this i can see how bad the fault is. The readings showed me that the engine was running very weak due to an excessively high level of oxygen in the exhaust. We checked the entire intake system, from the air mass meter fitted to the air filter box, all the turbo hoses and control pipe-work and valves were tested, as was the turbo intercooler, hidden behind the front bumper, so we could guarentee the integrity of the complete intake system…….but still no fault!!

Perplexed, I kept thinking and rung a mechanic friend, who used to work at an Audi dealership, to run through a few theories. After a discussion, it was suggested to check the crankcase breather valve that was hidden under the inlet manifold. I searched and located the crankcase breather valve, which had indeed failed, the plastic disc in its centre had fallen sideways allowing excess and unmetered air into inlet manifold, see picture below;

audipcvplastic disc, cente of valve had slipped

Without the knowledge on this system i could have wasted lots more time looking for the fault, no matter how experienced you are as an independent technician, sometimes you need to research the system to familiarise yourself with how they operate.

pcvlocationarrow shows where valve is hidden under inlet manifold



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