Re: idle problems

[Eddi Jew <jew@rysa.dtc.hp.com>]

 > I am still having idle problem on my '91 Audi 90. So far I have cleaned   
 > ISV, replaced crankcase breather hose, checked idle switch.
 > When the car is stationary it idles well, when I pull out oil dipstick it   
 > recovers in a second. When I am in traffic not going more that 40km/h or   
 > so and when I stop it still idles well, i.e drops down to 800RPM and   
 > never below. But as soon as I go faster (approx more that 50km/h) and   
 > then stop idle falls down to ~600RPM and then it slowly moves up to   
 > 800RPM or it occasionaly stalls. Even if I try to assist it by depressing   
 > gas pedal slightly does not help, it seems to wants to get up to 800RMP   
 > by itself.

I had a similar problem on my '86 VW GTI. The specifics may not apply 
(I forgot anyway), but perhaps the root cause is the same- the FI idle 
delivery is not "centered". I would think this would be done in one 
of the maintenance cycles, so if your car gets its regular "tune-ups", 
this may not be your problem.

Even if the ISV is working fine, it may not be operating around its
"home" position at static idle. It's job is to provide an amount of _air_
for the idle- the higher the duty cycle of the control signal, the more
air is let in. "percent duty cycle" is the pulse's ratio of on/off time.

If the _fuel_ delivery at idle is also not "centered", the ISV (and any
modulator for fuel control) could be trying to compensate by opening up 
too much or too little. That's bad because these control devices have 
a limited range-of-motion so it may (literally) be in a bad position 
to dynamically maintain correct air/fuel during (and just after) those 
throttle-downs. 

So for the ISV to maintain mixture (to keep idle), the fuel delivery 
needs to be adjusted/calibrated. My VW uses a "frequency valve" in the
FI unit to modulate the fuel delivery. This also has a control signal
specified as "percent duty cycle". The frequency valve is effected by
adjusting (offsetting) the static resting position of the FI "flap" 
(the round disc that is used to measure the air flow thru the FI unit). 
The adjustment screw (allen head) is "tamper resistant"- a lead plug 
is pressed over the screw to prevent you from playing with it. After it's
taken out, some have replaced it with a rubber plug (with a wire loop
connected to it).

Note, YMMV. FI units vary with model year. On your Audi, you may need to
monitor the duty-cycle of the ISV, or of any FI frequency valve, or some 
other output signal from the computer. Plus, there's procedures to prep the
control system before calibrating- open/plug vacuum lines, disconnect wires,
etc. Special tools or a "tap-in" wiring harness could make the job easier.
A duty-cycle meter is necessary. The Bentley manual is a good reference, 
so is the proper (Bosch?) FI handbook for your model Audi.

-- Eddi