I've heard of where it is but I still cannot find it. I have an 86 S4 N/A. Anyone have a direct picture of where it is or at least what it looks like?

 

its a pale yellow colour located right under the intake runners. Look for your injectors, the look down the fuel rail they are attached to. Its kinda deep down and behind the thermostat housing
Why are you looking for this, is it bad?

 

at the end of the bottom fuel rail you will see something like this: