engine speed and AT

[John Cody Forbes]

Kent McLean wrote:
> Snoopy wrote:
>> Hi. Does my car have any kind of engine speed increase/regulation,
>> when the car is not moving, transmission is in "D", and brakes are
>> applied?

> Not that I'm aware of. The engine does have various sensors, to adjust
> the idle. But "normally" the idle should be steady.

I would disagree. Every Audi that I know of since the early '80's has an 
idle air controller valve (IAC/idle solenoid/ or a few other names for the 
same device). If the IAC is working properly it should be able to compensate 
for idle fluctuations, although it is possible and somewhat likely that it 
isn't reacting fast enough when the battery voltage is low. At idle with the 
brake lights on with a typical old type 44 alternator is likely to drop the 
voltage below 12v, decreasing IAC roesponse (among other things). A common 
issue with these idle valves is slow reactions due to getting stuck closed 
by grime. Try revving the engine a bit in park and suddenly letting it drop 
back to idle. If it stumbles and the idle drops low before jumping up and 
eventually settling to a normal smooth idle I would say your IAC needs to be 
cleaned. On manual tranny cars this is usually more noticeable because it 
would happen virtually every time you pushed the clutch, but I can see on an 
auto trans car it being sort of hidden until just the right situation.

I'm also going to venture to say that I'm guessing this car may have a bad 
motor mount. These engines are pretty smooth runners and even with bad 
mounts they don't make a whole lot of vibrations at normal idle, but they 
will start rattling things if the idle drops. Motor mounts in good shape on 
these cars in my experiance keep the car rattle free down to 500rpm-ish.

-Cody Forbes