All 318i's have this problem eventually, so used is not advised.
Maybe you have a wiring problem. The ICU is a small black box,
literally above the glovebox. See
http://www.realoem.com/bmw/select.do
Now that I think about, disconnecting is a little involved. You must
block all flow thru the silver cylinder located engine top center. It
has a two-pin connector on the back. Remove the back hose, tape a penny
over the opening, and reassemble, leaving the connector off.
Having blocked this air valve, you must now adjust the throttle cable
to keep the throttle from fully closing. Turn the nut on the bracket
thru which the cable passes until the engines idles at about 750 RPM
when fully warm.
>From this point on you must depress the gas pedal to start it and to
keep it from stalling during warmup.
>> Stay informed about: 1985 318i Will Not Idle Properly (HELP!)