Four-cylinder throttle return springs can be improperly installed, exposing the hook of the spring to the heater hose. If the hook catches against the hose, it can hold the throttle open. The heater hose bends beneath the throttle linkage, passing very closely to it. There is a potentially dangerous incorrect re-installation position, where the hook can hang up on the hose.
The correct installation, has the hook facing away from the hose. Even with the spring on correctly, care must be taken to insure the heater hose is properly installed, since inadequate clearance between the hose and the rest of the linkage may also cause the throttle to stick open
On 2002tii engines, there is a spring in the throttle richness control that can break if it has been removed or bent, not then but after. This spring has been known to break of its own accord and there's not too much you can do about that. Depending upon throttle position at the time of the break, anything from full throttle to lean running and backfiring can occur. Only BMW offers a spring which is the correct length. shape and tension. A ballpoint pen spring can be used as a quick fix to get you home again.
Six cylinder engines have a potentially dangerous condition -- the dashpots can adjust themselves so as to hold the throttle open for a couple of seconds, resulting in rapid acceleration. The driver, of course, is subjected to undue trauma. Check the locknut on the dashpot output shaft frequently to insure proper actuation length and tightness.
(Reprinted from the BMW Automobile Club of America's Puget Sound Chapter newsletter Aus Freude Am Fahren. )