5kCSTQ boost Q

[quk@sievers.com (Phil Payne)]

In message <199608250656.XAA19166@netcom2.netcom.com> orin@netcom.com (Orin Eman) writes:

> I noticed this today on the freeway - accelerating in 5th gear, the boost
> would build up to 8psi then drop down to 5psi then back to 8 and so on,
> cycling once every couple of seconds.

> Is this normal?  If it is the effect of the wastegate solenoid, I would have
> expected it to cycle faster and over a smaller range than this.

I postulated a day or two ago that this cycle might prove to be a means
of spotting or even preventing turbo failure.

Let us suppose that your turbo is becoming less effective at providing
boost - either because of vane damage or because the high-pressure air
system (hoses, intercooler, inlet manifold) has developed a leak.

When boost is required, the turbo has to be spun up.  This takes time -
"turbo lag".  If it is less effective at generating boost than it should
be, it will have to spin faster in order to provide the boost required
to lift the wastegate.  It takes longer to reach higher speeds, so one
symptom that all is not well is that turbo lag increases.

The overboost control valve opens the wastegate under computer control -
this causes a loss of boost until the point where the computer decides
to shut it again.  The turbo is repeatedly slowed and accelerated
through the same band, but if it is less effective than it should be,
this band will be further up the rpm range than normal and it will be
wider - leading to a longer overboost control cycle.

Eventually, of course, boost level will fall to the point at which the
computer no longer has to intervene to open the wastegate.

I'm beginning to believe that the length of the overboost control cycle
(the duty cycle of the overboost control valve, if you like) is a more
important indicator of health in a servo-controlled turbo engine than
the boost level, because the system can control the latter to cover an
increasing level of sickness.  Just like doctors take pulses as well as
blood pressures.

It would be fairly easy to design a circuit to monitor the duty cycle,
and even easier to devise an alarm that would be triggered by
non-operation of the valve after n seconds of the "full load" switch
being on.  The cycle changes with air temperature and pressure, of
course.

My 1988 ur-quattro has a cycle length (guessed) of just over a second
uphill in third gear at sea level.  I would have expected yours to be
shorter, but it's a hard call.  Perhaps other owners of the same vehicle
could comment.

In any event - should you decide to check the turbo output for metal
fragments and the system for leaks, I'd be interested in what you find.
Especially if you find that a small air leak is causing the turbo to
spin faster than it should on its path to ultimate destruction.  Then we
would made really useful progress towards preventive instrumentation.

(Other Audi-heads - feel free to tell me I'm talking garbage.)

--
 Phil Payne
 phil@sievers.com
 Committee Member, UK Audi [ur-]quattro Owners Club