Our AAN engines are controlled by a computer (ECU or "ECM"). One of the things the ECU does is control the idle speed by allowing air to bypass the closed throttle valve (throttle body) when sitting idle. This is especially necessary when running things like the air conditioner compressor that puts a variable (on/off) load on the engine. This is accomplished via an electronic stepper motor and a variable valve opening in the N71. This is what VAG says about the N71 ISV:





This is what a real one looks like:


Sometimes, if there is a lot of oil vapours from the crankcase ventilation system or the turbo seals (as evidenced by oil in the cross-over tube or the Michelin Man hose), the ISV will become dirty and fail to function properly, i.e. idle will hunt around a lot. To clean, remove from the car (two clamps and one electrical connector) and clean with isopropyl alcohol or carb cleaner. DO NOT MOVE the "shutter" by hand, instead, apply 12V across the pins to activate the stepper motor. If cleaning doesn't restore proper function, then the solution is a new one or a very good used one.

The N71 ISV part number is 034133455B as shown below (or Bosch 0280140512 as shown above), is very common across the Audi line from 1987 to 1997: