I have a 2007 Toyota Matrix, and sometimes(not always) it will rev up to about 1700-2000 RPM while idling. It normally idles at about 900 RPM once it warms up. I just changed the oil & air filters, so I don't think they're part of the problem. It has an automatic transmission, 4 cylinder engine, about 55,000 miles. Any ideas?
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I had a similar problem with a 2001 Hyundai Tiburon.
I would start the car and it would rev up on the initial start, lower to ~800 rpm then begin a process of revving up to ~2000 rpm and back down to ~800 rpm about every 5-8 seconds.
800-900 rpm was the normal idle range for that car.
The problem wouldn't occur every time, but often enough that it was aggravating.
My problem ended up being the Mass Airflow Sensor. It was either sending bad readings or no readings at all so the ECU did not know how much air the engine was taking in causing the rev cycling.