noch eine Moeglichkeit, defekte(r) Kurbelwellensensor, hier kopiert aus USA:
Got back from replacing Crank Position Sensors on a 1991 M70 engined 850i, and have a few interesting facts I wish to pass along to the rest of the forum.
The car in question had the following symptoms:
surging idle
rough running (like it was starving for gas)
lack of response to the throttle.
hard starts
These symptoms have been diagnosed on the forum as possible FPR trouble, fuel filters clogged, burned out fuel pump(s), bad injectors, cracked distributor(s), leaking intake manifold gaskets, low batteries, bad plugs, wires, bad impulse sensors, bad gas, driver error, etc, etc...
I told the owner to check the CPS sensors. He used an ohmeter and the values came back within specs according to the ETM. I told him again to visually inspect the wires - he did, but couldn't see anything out of the ordinary.
I told him he would need to replace both sensors (which I did today) and here is what I found:
Both of the sensors had the correct resistance across the terminals, but when I started to remove them it became real obvious that they were infact DEAD. Both wires are held to the upper timing cover by a metal clamp which at some point had a rubber lining. Over the years, that rubber has deteriorated, and the wires eventually came into contact with the insulation on the wire.
Now, the CPS are magnetic reed switches, and the wire actually has a core and a shielded jacket. As soon as that jacket is exposed, the CPS stops working correctly - even while still maintaining the correct resistance values.
New sensors installed (without said brackets or clamps), and the car fired right up and ran as smooth as silk.
Moral of the story: If you are experiencing any of the above mentioned symptoms, and have not found the cause, you might want to remove that top clamp and see if the wire is damaged. If your CPS wires are hard, instead of pliable, you had just better figure on replacing them as soon as you can, because they WILL leave you stranded on the side of the road.
http://i48.photobucket.com/albums/f2...fanito/M70.jpg
Those would be the ones. The wires with the "donut" on the ends are impulse sensors.
The clamps I was talking about are shown in the pic below by the yellow ellipse.
If you decide to remove the clips altogether, make sure you reinstall the bolt back into the timing cover.