Q: car starts shaking when it's idiling,sometimes stalling.service engine light on on 2003 BMW 525i

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when the car is idiling,it will start to shake pretty hard until i start to drive then it will start slow and kind of putt along. I did notice at certain times the RPM will go between 4& 5 before the gears change. It recently started to stall and run pretty rough.I had it towed to a repair shop but i'm scared they are going to try to sell me things i don't need, so i would like an idea of what it might be before i have any work done.
(1) Answer
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It could be many things, unfortunately and without experiencing the problem first hand or being able to do some basic diagnostic tests its hard to say. First place to start is to see why the check engine light is on, retrieve the codes and see what they indicate as being the problem. Then check the basics, is the ignition system contributing to a misfire (spark plugs and ignition system in good order?), is there a vacuum leak from the intake manifold or air induction boot?, or is there an engine mechanical problem, (maybe a tight valve or reason for low compression in a cylinder, perhaps a bad fuel injector. Too any possibilities to be able to guess at without doing basic tests.