Q: Q: RPM fluctuating

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What could any possible problems be ?

My car has 150000 miles.
My car has a manual transmission.

There are a few things that may cause this such as a vacuum leak, a dirty or faulty mass air flow sensor or a dirty or failing idle control valve. A vacuum leak is a very common issue related to this and will cause the motor to run lean while starving for fuel. As a result of the high air volume entering the motor, this will cause the motor to race as you describe. The same concept is true with the mass air flow sensor as well in that if it produces a faulty reading to the computer (i.e. too much fuel is present), the computer may reduce the amount of fuel being supplied to the motor. I would suggest having a professional from YourMechanic come to your location to assist with diagnosing your vehicle as vacuum leaks can be very difficult to trace.

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