[Biturbos4] All causes to turbo failures known?

Josef Hebenstreit josef.hebenstreit at hispeed.ch
Thu Apr 29 19:29:46 EDT 2004

Hi friends,
I have a S4 2,7 Biturbo/1999 and about 160.000 km. The chip-tuning
pushed the performance to 310hp instead of 265hp. Even though I put my
car to the service on a regular basis a turbo failure was inevitable! 
Conclusion: One has to learn the lessons.
Following arguments regarding the cause of a turbo failure may be valid:
1)    Wear
2)    Boost leak causes over-spinning of the turbo -> rpm to high ->
mechanical breakdown
3)    Defect or mal-functioning air-mass gauge causes ECU to read out
lower values for the air-mass flow -> ECU responses with lower fuel
injection -> lean fuel-air ratio -> high temperature on turbine of the
turbo -> thermal breakdown
4)    Dump valve or bypass valve
5)    Insufficient oil supply for the turbo - oil lines clogged or
Normally the third case shouldn't be possible due to
exhaust-gas-temperature sensor sitting between turbo and lambda sensor.
ECU should detect high temperature (not only peak but also average
value) and protect the turbo using the waste gate and/or switching over
to limp program.
What year of manufacture of the S4 has this exhaust-gas-temperature
Can you verify this arguments especially the 3rd one and how one can
cope with it?
Is a K04-type turbo charger a solution to cope with the mechanical and
thermal breakdown, independent whether the car is tuned or not?
To you think the air-mass gauge is a problematic sensor? Do you have a

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