Q: Rough idling, no power on acceleration, revs by itself, shuts off

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I have a 94 BMW 525i with 260k miles. In the past 2 months, my car has shut off by itself while at a red light twice. Nothing out of the ordinary before or after it shut off. A few days ago, while idled in the parking lot, started rough idling, rpm shot up to over 1000, the check engine light came on, and then the car shut off. It started right back up afterwards. Then on the way home from work, I noticed that the car sounded differently than normal. There was no power when I tried to accelerate, whenever I slowed down or came to stop, The check engine light would come on and the would rough idle and want to die out. However, when I give it gas to start going, the check engine light would go off, but still no power.

So far, I've changed out the throttle body sensor and there was no change. I also noticed that when I start the car, the inside lights and dash lights would dim and light back up with the rhythm of the rough idling.

My car has 260000 miles.
My car has an automatic transmission.

The first thing I check for in any drivability problem is air leaks. Any air that gets into the engine without going through proper channels will lean out the system and cause these kinds of problems. Look under the hood for any hoses or rubber boots that are disconnected or in bad condition. Sometimes an engine will backfire once and blow a hose off of the intake and nothing runs right after that. As for your lights flickering; that may or may not be important. You will want to check the charging system carefully and if you have the instruments, check for any AC current in the system by switching the DVOM to AC. Anything above .5VAC will cause problems and means you need a new alternator. If you’d rather farm this work out to a professional, contact YourMechanic. They can send a technician to your home or office to check out your rough running problem and let you know what is the solution.

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