[Biturbos4] All causes to turbo failures known?

[Keman]

>> 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!

What kind of failure?

>> 1)    Wear

With infrequent oilchanges and poor quality oil, a turbos bearings can
certainly wear out. They don't have to, Gerret for example will tell you
that their turbos will outlast any engine you attach them to so long as
nothing breaks that controls the turbo.

>> 2)    Boost leak causes over-spinning of the turbo -> rpm to high ->
>> mechanical breakdown

IMO there's your #1 cause. I highly recommend a good boost gauge .. I've had
one since 10k miles, even though my S4 still isn't chipped.

>> 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

The O2 sensors detect this though and that's how you know the MAF is gone,
there are codes for reaching the adaptive limit rich set and a quick glance
at the MAF under heavy throttle shows much less grams/second measured than
what is really coming in.

>> 4)    Dump valve or bypass valve

When the bypass valves fail, they can cause an invisible boost leak since it
can dump air back in front of the turbo, but still behind the MAF sensor.
Aftermarket valves are a good idea.

The wastegate diaphram can rupture, overspinning the turbo. Rare, but
nothing is impossible. I've seen it once.

>> 5)    Insufficient oil supply for the turbo - oil lines clogged or
>> leaking

Crappy oil and infrequent changes, sure.

>> 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
>> sensor?

All biturbo S4's have two EGT sensors, one for each side just inches after
the turbo outlet.

>> Is a K04-type turbo charger a solution to cope with the mechanical
>> and thermal breakdown, independent whether the car is tuned or not?

I honestly don't like the K04's. They're just a K03 housing with a different
turbine wheel. I've compared them side by side now. There are some
ballbearing alternative turbos going around now that fit our cars which are
a much more appropriate design and are more efficient. If you're going
through the hassle of replacing the turbos, you might as well go with
something else.

>> To you think the air-mass gauge is a problematic sensor? Do you have
>> a remedy?

It's problematic more often than not, but you'll get a check engine light
before it becomes a serious issue.

- Keman

>> Thanks,
>> Joe