Start Idle RPM problem
 

my 318i E30 M40 '91 having problem with the start in the morning,the engine dice and run as usual with start in 1500 rpm as manual says, but after reach 1500 rpm immediate drop to 400 rpm than again goes up to 1500 than drop to 800rpm within approx 4 second from dice the engine.
If I try to restart the engine after reach working temperature it work normal...dice,run to 1500rpm than drop to 800rpm instantly
any one has a advice..what went wrong here

Its very rare in every pattern.......

Yap....for more clue...if I switch the AC on and release the accelerator pedal from higher RPM for example 2000rpm ,the rpm drop to 500 and get back to 800rpm in second...:confused:

Check all your vacuum hoses and check the coolant temp sensor.