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Q: when in park or stopped at a red light my car seems like its about to shut off

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when i'm in park or stopped at a red light my car all of a sudden drops RPMs to about 500 about to shut off and then goes back to 1000-1500 RPM.it also starts to shake but then when i start to drive it seems to be fine

My car has 101000 miles.
My car has an automatic transmission.

A: Low or surging idle speed can be caused by ...

Low or surging idle speed can be caused by a number of different things. A vacuum leak can cause this. If a vacuum line is broken or detached, or the intake manifold gasket is cracked, the engine will draw in too much air, effecting the air/fuel mixture and ultimately the idle speed and performance. Also, a faulty idle speed or idle air control motor can be at fault. Sometimes the idle air control motor can be "lazy" - does not respond as quickly as it should - and not set a fault that would cause the check engine light to come on. That can happen due to age/mileage, or the throttle body can have carbon build-up that slows it’s reaction time. The mass air flow sensor can also be at fault. If it does not measure incoming air correctly, it can cause stalling, poor idle, lack of power and poor fuel economy.

Have a certified technician look into the poor idle to isolate the exact cause.

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