My 1986 328 had a 50k service earlier this spring. Shortly after that, it started behaving strangely. First, the starting wasn't as strong/quick as usual. It would take some more cranking at feel like it needs some throttle to get going. Then, there's no high idle at all, it would just idle at around 500/550 or so. Eventually it will get up to about 800 which is also lower than normal. Acceleration from stop is sluggish, at higher RPM above 3-3.5k it feels more normal but probably still less than it should. In the past, I had the front ignition go out due to corroded main lead and got both coils and leads replaced. That's the only repair i've ever needed in 6 years other than going in for the 50k service. I tried to feel for header heat suspecting maybe a bank was out and thought the front bank might have been too cool. So, disconnected front bank ignition to see if it made impact and car wouldn't start, then started when hooked back up. Next I thought it could be frequency valve and couldn't quite tell if it was buzzing so disconnected that and it ran terribly and cleaned up when reconnected. Ideas of what to look into next?
Fuel and spark are basic checks. Disconnect one spark plug at a time...and see if it makes a difference. Fuel is harder to check. Warm-up plug could be fouled -- that would explain cold start problem; again disconnect and try? Have you, or someone removed/cleaned the injectors?
My guess is from just what you are describing that you have a vacuum leak... and or a cracked wire to one of of the ignition modules... or a loose connection. my gut says vacuum leak with one of the hoses.. not connected or with a hole... or large crack. the idle is a function of the Aux Air valve, under the plenum, and then sending signals to the cold start injector etc...
