I have a 2000 a4 quattro 1.8t with a vaccum leak somewhere...i know this because i can hear the obvious hissing but i just cant locate the leak. Whats the best way to locate where it coming from. Ive read that blowing smoke into one of the lines will identify the location. Has this worked for anyone and if so whats the best location or line to use for this?

 

You can spray carb cleaner around the engine bay/vacuum lines and listen for the engine to bog down. Try to spray it in more of a "mist" than a direct "jet". Quick, short bursts from a distance would be best IMO.

 

Blowing smoke, spraying a fine mist (water, carb cleaner, etc.)

i.e. anything that hangs in the air long enough to see if it's being pulled by some suction.

 

You can also find a vacuum diagram to help you look for potential problem areas while troubleshooting.