Checking For Vacuum Leaks

Some of the most elusive and hence most frustrating, engine problems are vacuum leaks. Anytime the engine won't idle right or a piece of emissions equipment won't function properly, a quick inspection of the vacuum system is in order. Start by looking for obvious offenders, such as torn hoses, broken connectors, or T-fittings, or just plain old dried, cracked, or worn-out rubber vacuum lines.
More obscure vacuum leaks can easily be found with the aid of a propane torch. WARNING, WARNING, WARNING.. Do not ignite the propane torch when you are using it to track down vacuum leaks!!! You are not using the heat of the propane torch in this procedure: rather you are using a small amount of the flammable gas to detect vacuum leaks. First, start the engine, and open the valve on the propane bottle just enough to begin the flow of gas. While the engine is running, pass the UNLIT torch over all vacuum lines, past the intake manifold gaskets, etc...should you pass the nozzle of the UNLIT torch near a vacuum leak, the idle speed of the engine will increase momentarily because the propane will be sucked in through the vacuum leak and burned as fuel inside the engine. Listen carefully for a rise in engine rpm's.

Courtesy of  manninej