Poor cold running
I own a 1990 BMW 316i with the M40 engine. I have owned the car for the past 20 years and it has now covered 140,000 miles. No modifications have been carried out, the vehicle is in overall excellent condition and is now used only perhaps one day per week.
The following problem has crept in over the past year. When starting from cold, the engine fires up immediately, but splutters and idles badly at only 600/700rpm. If you open the throttle it is possible to slowly increase the revs and then suddenly the engine bursts into life and idles evenly. However, the cold idle speed goes up to 1800rpm, which is too high. Despite the high idle speed, when pulling away the engine tends to hesitate a little. After a few miles, the idle speed starts to reduce, gradually settling down to an even 750rpm, and the engine performs satisfactorily. Hot starting is never a problem.
Work carried out so far includes replacement of the idle control valve, replacement of the thermal transmitter, replacement of the inlet manifold gaskets and replacement of all hoses adjacent to the inlet manifold/throttle body. Genuine BMW parts were used. Colm O’brien
Given that you have already replaced the components I would have suggested, such as the coolant temperature sensor and idle control valve, I would next check the crankcase breather – although you have replaced the hoses, the small plastic breather valve may be faulty. This is easily tested by detaching the oil filler cap while the engine is running; once removed, if the engine revs die dramatically and you can detect a sucking of air into the cam cover, this will proves the valve has failed. If this test above does not cause the engine revs to die or produce a rushing of air, then I suspect the cold running problems may be caused by a residue within the fuel injection system.
In this case, I would try using a system additive to clear the residue before taking more extensive action. Something like Wynn's Xtreme Petrol System Clean should help to remove the deposits that may have built up over the past 25 years. Alternatively, the problem may be the result of a failing fuel pump that is struggling to deliver the required pressure immediately. Unless you have done so recently, I would also replace the fuel filter, although normally a problem with the filter or pump would cause symptoms at cruising speeds, which you do not seem to have.