Q: Car revving without gas

Do these "excursions" in engine speed occur only when the car is in gear (driving on the road)? If you have uncommanded acceleration in park, or failure of engine speed to return smoothly to idle after speeding up the engine in park, you may have a throttle body or throttle sensor fault. Even if the problem only occurs in drive, in an area where it is safe to do so try moving the gear selector to neutral during one of these incidents and see if the engine speed drops to neutral, as it should, once you remove your foot from the accelerator. If it does drop to neutral, versus the irregular operation you experience if the car is left in gear, there may be an engine/transmission control issue. Quite a few engine faults can cause inconsistent idle speed and, to answer your question, yes, vacuum leaks are one such cause but some tests have to be performed to determine whether this is an engine issue alone or whether there is a problem, too, with the transmission. To request an engine/transmission diagnostic, please use YourMechanic’s service request page. If you have further questions, do not hesitate to re-contact YourMechanic because we want you to make the most of your repair dollars and help you to get the best possible results.