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Unfortunately there is no one fits all solution to this problem. I have had this happen at least 3 different times on my car an each time the solution has been different. First time it was a bad crank sensor. Then it was a loose spark plug ground bolt to the valve cover that had worn threads and was popping out. A year after my crank sensor replacement I had to change it out again so I went and did it myself. This required removing the 3 stage manifold which has about 2,500 wires running to it which I had mixed up and forgot to attach a vacuum hose to the bottom of the manifold during reinstallation which caused the problem again. Your best bet is to scan for codes if you're having this problem with a bmw code reader, not some generic OBD2 reader. They won't tell you much of anything unless your O2 sensors are bad. After you scan the codes return here (or the mechanical maintenance section) and post your findings. If you don't scan for codes and the fault is not obvious (broken hoses etc.) then you're mostly just guessing and that gets expensive on these cars.
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