5kCSTQ boost Q

[scott.john.mockry@bangate1.tek.com]

With all this talk about the boost control solenoid on the 5000CS TQ cars I thought I
would pipe in with a few tidbits.

Regarding the problem with the boost cycling between 8psi and 5 psi you  may
want to verify that the WOT switch is not intermittent. I have had problems
with cracked solder joints inside the idle/full throttle switch on both my 86 5000CST
and on my 89 200TQ. This could cause some strange behavior under boost.

I remember reading some time ago in the  "1986 New model information" from Audi,
that the MC engine turbo system ECU  will  also shut down the boost control  system 
(ie duty cycle controlled solenoid)  if the specified boost read from the ECU boost map 
is not reached  within a few seconds.  I need to review my documentation to get the 
exact time. I wonder if your ECU was switching the solenoid off and on at the time 
you noticed the problem. Just a thought..

As Scott (PDQSHIP) mentioned using a WG spring that is too stiff will cause the ECU
to rapidly reduce the duty cycle to 0% (negative duty cycle) going to the WG solenoid
 because the required boost value ( from the ECU map) is reached just from the WG spring
pressure without the need to  add boost pressure on top of the WG spring/diaphram.

I  measured the boost value on my 89 200TQ with the stock ECU and WG spring using a 
pressure transducer and an Oscilloscope to graph out what the boost profile looks like.
Under full throttle runs in 3rd gear from 35mph at around 800ft elevation (70 degrees F),
 with the  boost control solenoid system disconnected and the upper WG open to the 
atmosphere, the turbo output is  (1.3bar) about 0.1 bar less turbo pressure 
 than the specified 1.42 bar. The WG solenoid system provides more 
pressure on top of the WG at  higher altitudes to ensure the absolute 
boost remains at 1.42 bar. 

Recent tests done with my modified ECU with higher overboost cutout values (1.9bar)
and a stiffer WG spring (WG solenoid duty cycle fades to 0% quickly under full throttle
3rd gear tests). This 0% duty cycle means that the WG upper chamber is connected
to aproximately atmospheric pressure (actually a little lower than atmospheric
because the connection is on the suction side of the turbo). This has the effect of actually
lower the available boost by 0.1 bar (I get 1.7bar instead of 1.8bar) when the boost 
control solenoid is left connected. Because the ECU boost map profile has not "yet" been 
reprogrammed for the higher boost levels it trys to lower the boost back down to 
the 1.42 bar level.  

At higher altitudes using this approach the absolute boost available is lower 
(around 1.5 bar at 5000ft) which is ok in some repects because it limits my K24 turbo
speed which Scott (PDQSHIP) has indicated would likely over-rev if the 1.8 bar would
continued to be produced at higher altitudes.

The newer 91 200TQ and the S4's use a barometric sensor to provide altitude information
to the ECU to reduce the K24 boost levels at higher altitudes to prevent over-reving
the turbo.

Regarding leaks in the turbo hoses/intercooler/vacuum lines etc. When you start running
higher than the 1.42 bar pressure, any weak points in the turbo plumbing will become
apparent rather quickly. The intercooler to intake hose  will split
on the bottom and the intercooler end caps can be blown apart, many have indicated
that the intercooler should be held together with some large screw type
hose clamps. When my Intercooler to intake hose leaked previously it was bad enough 
under boost to cause a loud "hissing"  sound.


Last week, I developed a small vacuum leak  after a hard run and had rough
running along with my frequency valve (O2 mixture control) duty cycle going up to
around 85% indicating a lean condition. After checking all the obvious crankcase hoses ,
idle stabilizer hoses, and vacuum lines coming from the intake manifold, I removed
all the turbo hoses coming and going from the intercooler checked them for tears and 
after re-installing them and tightening the hose clamps, the vacuum leak was gone.

While looking for a vacuum leak you can take the crankcase breather system out of the
equation by removing the large 1" hose from the intake to the crankcase and plugging
the end.  While looking at the crankcase breather hoses, don't forget to check the oil 
filler cap gasket, dipstick tube seal, the Valve cover gasket and any of
the cam or crank seals for leaks. 

Good luck
Scott M.