Vacuum Applications

Almost all modern cars use engine vacuum to assist with power braking and to create vacuum in the engine crankcase. Other vacuum systems include windshield wipers, climate control and emissions controls.

Air/Fuel Ratio Effects

Gasoline engines require 14.1 parts of air to one part fuel. A leaking vacuum tube can allow more air into the engine than it is tuned for, leading to misfires, rough idling and loss of power and fuel economy.

Leaking Couplers

Most often, vacuum leaks occur where the rubber vacuum lines connect to either the engine or accessories.

Dry-Rotted Hoses

Though the usage of modern synthetic hoses prolongs this effect, all flexible lines will eventually begin to harden. Under sustained engine movement, these brittle hoses will begin to crack and leak.

Diagnosis

Computer controlled cars will often throw a check engine light when a vacuum leak is present. To track down a leak, lightly spray the vacuum line connections one by one with carburetor cleaner, brake cleaner or (preferably) ether starting fluid. If you hear the engine increase in RPM, then you've found your leak.

Source:

AA1car.com

E30 Forum

ASA Shop

Resource:

Spray Testing a Vacuum Leak (Video)