[Biturbos4] Turbo reliability

[Ti Kan]

Below is a post from Keman from a while back on turbo failure that's
good reading for the new members of the list, and a good review for
the rest.

-Ti

> 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