To begin to diagnose the idle problem, we will need to first need to look at the vacuum hoses.In this case, I have decided to replace all the hoses just for extra insurance.Just remove the hose clamps on either end, and pull the old hoses off.Now cut the new hose to the same length as the old ones, push them on and tighten the hose clamps.
In some cases, even a leaking valve cover gasket or oil filler cap can cause this leak, so make sure your valve cover is properly sealed.Vacuum hose is cheap, so you might as well replace all the hoses.Now look at the passenger side of the engine at the top of the valve cover.First off, most modern BMW's from 1986 and on use the Bosch Motronic fuel injection system. If you look at the throttle body or carburetor of any car, it is essentially a valve or a series of valves.
The Motronic system is probably one of the most reliable fuel systems on the market, and is still widely used in most European cars. A vacuum leak will usually cause the car to idle higher. When you begin to open the valve, air is able to flow into the engine, increasing the engine speed.
Now look above the valve cover on the intake manifold. On the left side, the hose is plugged up, on the right side; the hose connects to the fuel pressure regulator.