two SCARY sudden acceleration incidents 92 Audi 100S

The cruise control system in your vehicle is completely separate from
any sources of engine vacuum and is controlled by a separate control
unit. The entire system is completely isolated from any other and
only uses inputs from other systems such as speed, etc.

All quite irrelevant since you have effectively removed the cruise
control system from the scenario by completely disabling it.

Leaving very little to possibly suspect. There is no other mechanical
control of throttle.

The components of the throttle body system a the throttle body, a
potentiometer that senses throttle position and movement, the idle
stabilizer valve which meters air in order to keep a steady idle, an
EGR valve for emmissions control, and a temperature sensor. None of
the latter items mechanically manipulates the throttle body.

Without mechanical actuation of the throttle body and in order to
achieve the power output you describe, the engine will require
accurately metered air and fuel. I have no idea if the ISV or EGR,
either together or separately, could introduce a volume of air
equivalent to a fully opened throttle plate.

Nevertheless, air and fuel and accurate management of both must be
present to achieve anything remotely resembling full engine power.

Do you recall noticing the position of the throttle pedal? Was it on
the floor?
What was the nature of your most recent throttle application?

The engine vacuum system on this model seems rather tortuous. There
is a "damper", the function of which I have no idea, a solenoid valve
of some sort, a "vacuum unit for intake manifold" which I believe is
merely an tap into engine vacuum, and some check valves here and
there. Then there's an entire subsystem built around a "suction
pump" (p/n 078 133 753) which also taps into the intake manifold. By
it's name it does not seem to be something that would introduce air
into the system, but in your case, something is clearly malfunctioning
so who's to say.

Finally, have you run the diagnostic output from the ECU to see if
there are any error codes stored? This may give you some add'l
insight.

I hope some of the above helps get closer to the cause. Give the age
of the vehicle, I'm very doubtful Audi or the NHTSA will do anything
whatsoever.

Ed

Ed,

Thanks for your thoughtful reply. Pedal was not on the floor when
uncommanded acceleration occurred, I recall tapping it on the first
incident thinking it might be hung up and found that it had a normal
amount of travel remaining.

On the first incident I had steady throttle, going uphill at 55 mph
with cc on but not engaged. On second incident I was in 5 mph bumper
to bumper traffic so I wasn't doing any significant throttle
excursions. I drive very smoothly and slowly in traffic, not jerky.

No floor mat issues. Accelerator pedal was unobstructed. When I
restarted the car in neutral, it went wide open, near redline, limited
only by the rev limiter (5500 RPM?). On the next restart idle was
normal. There is some link to a vacuum issue, I am pretty sure. Right
before the second incident the brakes took extra effort as if vacuum
boost was decreased or gone.

Have not run diagnostics. I am sure dealer will. I don't recall any
panel lights illuminating.

I hope NHTSA and Audi will take it seriously. An old defective car can
kill someone as easily as a new one. Audi seemed paranoid that I was
trying to get a legal case going, but I am not at all interested in
suing. I just want to keep someone from getting killed.

I was the biggest skeptic in the world about uncommanded acceleration
until it happened to me twice in one day. I am lucky that the second
incident happened with the cruise control fuse pulled. Otherwise I
would have incorrectly assumed that with the fuse pulled it could
never happen again and been driving blissfully unaware of the lurking
danger.

What puzzles me greatly is how the problem is intermittent.

Mark

There was a recall of some 92 100 cars to replace a rotary vacuum pump
with premature vane wear with some sort of "vacuum unit", perhaps a
tap into the vacuum line with a check valve.

On Jun 19, 10:30 am, wrote:
> The cruise control system in your vehicle is completely separate from
> any sources of engine vacuum and is controlled by a separate control
> unit. The entire system is completely isolated from any other and
> only uses inputs from other systems such as speed, etc.
>
> All quite irrelevant since you have effectively removed the cruise
> control system from the scenario by completely disabling it.
>
> Leaving very little to possibly suspect. There is no other mechanical
> control of throttle.
>
> The components of the throttle body system a the throttle body, a
> potentiometer that senses throttle position and movement, the idle
> stabilizer valve which meters air in order to keep a steady idle, an
> EGR valve for emmissions control, and a temperature sensor. None of
> the latter items mechanically manipulates the throttle body.
>
> Without mechanical actuation of the throttle body and in order to
> achieve the power output you describe, the engine will require
> accurately metered air and fuel. I have no idea if the ISV or EGR,
> either together or separately, could introduce a volume of air
> equivalent to a fully opened throttle plate.
>
> Nevertheless, air and fuel and accurate management of both must be
> present to achieve anything remotely resembling full engine power.
>
> Do you recall noticing the position of the throttle pedal? Was it on
> the floor?
> What was the nature of your most recent throttle application?
>
> The engine vacuum system on this model seems rather tortuous. There
> is a "damper", the function of which I have no idea, a solenoid valve
> of some sort, a "vacuum unit for intake manifold" which I believe is
> merely an tap into engine vacuum, and some check valves here and
> there. Then there's an entire subsystem built around a "suction
> pump" (p/n 078 133 753) which also taps into the intake manifold. By
> it's name it does not seem to be something that would introduce air
> into the system, but in your case, something is clearly malfunctioning
> so who's to say.
>
> Finally, have you run the diagnostic output from the ECU to see if
> there are any error codes stored? This may give you some add'l
> insight.
>
> I hope some of the above helps get closer to the cause. Give the age
> of the vehicle, I'm very doubtful Audi or the NHTSA will do anything
> whatsoever.
>
> Ed

First, I found a possible culprit. The neck on a 3 port plastic valve
in the vacuum line that runs on the top of the engine, driver side,
and feeds the brake booster was broken on the suction side (the side
that leads to the intake). The hose was sucked against the broken part
of valve when the engine was running so the vacuum loss was not
complete. What was disturbing is that the neck, which should have
remained firmly fit in the hose end, was missing and nowhere to be
found. Could this have disintegrated into small particles and have
been sucked into the throttle body? Could a foreign object here cause
increasing (not just stuck) fuel delivery? I don't know, but it is a
clue.

Audi America's response was waaaay beyond disappointing. They sent me
to a dealer who wrote up a $2000 repair estimate including $750 for a
new throttle pedal assembly, new floor mats, hundreds for
unspecified "decarboning" and did nothing to find the root cause.
Although this was CLEARLY an uncommanded acceleration issue, NOT A
STUCK throttle, they treated it as a stuck throttle issue and would
not even open up the cover to the throttle body on their dime. I
called Audi America HQ and spoke to Michael Harris. I told him I was
not making a legal case, not seeking money, just trying to prevent
fatalities. I told him that Audi could not possibly find the root
cause if they replaced every bad part before finding what caused the
problem. He was polite but wouldn't change anything. Oh well, I tried.
Who knows how many other 92 100S owners have had this happen. Three
have reported identical incidents to NSHTA, but I will bet there have
been others who didn't even know how to file a report. If this happens
to an old or non alert driver it cold result in multiple fatalities. I
am going to replace the broken valve, replace the old vacuum hoses in
that same line and see what happens. I am almost sure that the valve
lies at the heart of the matter since it causes partial/intermittent
loss of brake boost and that immediately preceded the acceleration.
The plastic was very brittle and the neck particles probably did get
get sucked towards the intake/throttle body, perhaps even entered it.
I don't know enough about the Audi fuel injection system to know if a
foreign object in this area could cause an increase in fuel delivery.
Does anyone else know?