With the advent of emission control legislation on the Federal level, as well as state emission inspection legislation, it has become increasingly important that idle adjustments do not violate the letter of the law. The only way to make sure that the idle mixture setting remains at a legal level is to have it checked with an exhaust gas analyzer of known accuracy. This is an extremely expensive electronic device which your BMW dealer or a reputable independent garage is required to have on hand to make these adjustments. Therefore, it is recommended that the mixture adjustment be referred to them.
The idle speed and mixture can be adjusted ONLY with the aid of a CO meter. If this tool is not available, do not attempt any of the following procedures. The idle mixture can be adjusted ONLY with the aid of a CO meter on most models; on the 318i, it can be adjusted ONLY with a BMW digital mixture adjustment unit 12 6 400. Idle speed is not adjustable on any model with the Motronic control unit except the M5 and M6. If idle speed is incorrect, either the idle valve or the idle control unit must be replaced.