irregular idle

There is an idle air control valve that can fail or become "carboned up" and stick causing a lack of proper engine idle control. This type of valve is commonly used on a lot of Bosch fuel injection systems (Porsche, VW/Audi). Air entering the engine is measured and a signal as to its volume is sent to the computer in your car to determine how much fuel is needed to be injected into the engine of your car. Any "false air" or air the the air flow meter doesn't know about is not reported to the computer, the computer then calculates the fuel requirements for the engine errorrng on the too lean side and this affects the idle. Many automotive repair shops have a machine that generates an inert non flammable low pressure smoke that looks similar to cigar smoke. The intake manifold is sealed off, and smoke is put into the engines intake manifold. The smoke then fills the engines crankcase and if there are any vacuum leaks from gaskets, hoses, or housing smoke emits from the source of the leak. Some mechanics will use propane or aerosol carburetor cleaner, with the engine running they will spray the cleaner or propane around the intake manifold and vacuum hoses. A leak will cause a "lean mixture" condition, when the flammable fuel (carburetor cleaner or propane) comes in contact with the source of the vacuum leak the engine idle speed will raise as the engine has received "fuel". this method of finding a vacuum leak works but is not recommended as it can easily cause a fire.