I finally cured the rough idle of my E28 535i this weekend, but I'm not exactly sure how. I replaced the coolant temperature sensor, which supplies information on engine temperature to the idle control unit, because the old one didn't meet the resistance specs in Bentley's. However, while doing so, I accidentally knocked loose a couple of thin vacuum hoses that connect on top of the thermostat. The ends of the hoses were pretty well stretched so that they didn't fit tightly over the connectors. I cut off about a quarter inch from each, and each reattached firmly. Fired her up and the engine now smoothly idles around 700 rpm, even when cold! (And it is getting cold here in DC now, brrrrh!) Just a suggestion for those of you out there searching to eliminate a rough idle, like I was for the past year. BTW, before the engine idled low, often dipping below 500 rpm, and sometimes stalling, especially before it was warmed up. YMMV.

Rebel A. Cole
'86 535i (187,000 miles, *smooooth* idle!)