Audi Repair: 2001 TT Quattro 1.8, crankcase ventilation, mass airflow sensor

Question
Hello, I have a 2001 TT Quattro 1.8T on 69K mileage.
I got the check engine light on last week. I went to an audi dealer and they found that broken hoses for crankcase ventilation system. They replaced hoses for crankcase system and vacuum lines.
However, the check engine light came back on next day. I went to the dealer again and they checked the on-board diagnostics and found fault for system (too lean). And, they found valve cover not sealing. They replaced valve cover and gasket.
HOWEVER, I got the check engine light on again within 6 hours after I picked up my car. And, my engine sounds shaking (stalling sound) at 2500~3000 rpm. I pointed out it but the dealer could not find any loosen parts.
I have to take my car to dealer tomorrow and they will charge me one more time for another problem. So far, I paid over $1200.00. Is there any way to find what the real problem is?

Answer
Your problems sounds like the dealer.  They seem to be obsessed with crankcase ventilation.  What did they do to cure the lean system code?  It sounds like you have a problem with the mixture, and this can be caused by a throttel position sensor or a Mass Airflow Sensor.  I would go back to the dealer, take the receipts of what you have already paid and ask to speak to the service manager.  Then explain to him or her that you have paid over $1200 to them to fix things that were obviously not the cause of the check engine light.  Explain that you would like to see your old valve cover, that you did not have any oil leaks, and that you want them to fix the actual problem, not something else.  Ask that anything they want to replace can be verified as the cause of the check engine light.  Alternatively you can get the codes read out for free at an advanced autoparts store, and you can let me know the codes.  Let me know how you get on,  Jan