I have a 88 740 turbo manual with 225000 miles. I am having the same symptoms as N7SC. The idle is very erratic going from near stall to 1500 or 2000 rpm eventually evening out at 800rpm a little warm. Going into a stop in nuetral the idle will always drop to near stall and then jump high. There is also the sucking or hissing noise in the engine compartment. So far I have changed all vacume lines, cleaned Idle Air Valve, tested and cleaned MAF, replaced the engine temperature sensor, cleaned the throttle body, and installed new o rings on the injectors. Any advice would be appreciated. Ive been working on this problem for a while now.