Engine will rev up and down up and down

Hi,

The most common cause of this type of issue is a bad idle air control valve (IAC). The IAC is what is responsible for maintaining the idle speed. Here are a couple links for you to look through. The first explains how in general to service it and the second is replacement.

https://www.2carpros.com/articles/idle-air-control-valve-service

https://www.2carpros.com/articles/how-to-replace-an-idle-speed-control-motor-iac

This type of issue can also be related to an engine vacuum leak. However, based on your description, I feel it is the IAC.

Here is a link explaining how to check for a vacuum leak:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

The IAC is located on the throttle body. See picture 1

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Let me know if this helps or if you have other questions.

Take care,
Joe