Fluctuating RPMs after starting?

2006 BUICK LUCERNE
120,000 MILES • 3.8L • 6 CYL • 2WD • AUTOMATIC
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DSCHWINDT
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I drove my car yesterday and it operated normally. This morning, when I started my car, the RPMs fluctuated from 300 to 3,000. I waited a couple minutes to see if it would stop and it did not. I put the car in reverse, but the fluctuations remained. For some context, in the past I have noticed the RPMs fluctuating a bit when driving on the highway, but only by a few hundred RPMs. This is the first time the fluctuation has been so drastic. What might be causing this?
Mar 3, 2025 at 5:50 AM
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STRAILER
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Mostly this can be caused by 2 reasons, the engine has a vacuum leak, or the throttle bore needs to be serviced. Also, do you have a check engine light on? If so, we should run the codes to see if any are stored, this guide can help:

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

If no codes are found, I would use this guide while the engine is cool:

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

If no leaks are found let's service, the throttle bore. This guide will show you how with instructions for your car in the images below:

https://www.2carpros.com/articles/throttle-actuator-service

Check out the images (below). Please let us know what happens.
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Mar 3, 2025 at 1:21 PM
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DSCHWINDT
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I do not have a check engine light.

Thank you for the ideas, I plan to try them.

I have an update for you though. I just tried starting my car again and it is idling normally at 1,000 RPMs, so it seems the issue may be intermittent. I’m not sure if that helps solve the problem, but just letting you know.
Mar 3, 2025 at 2:47 PM
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STRAILER
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Yep, both a vacuum leak and a dirty throttle body will cause this issue.
Mar 4, 2025 at 10:27 AM
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DSCHWINDT
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I'm sorry for the late reply. I always forget to respond once the problem is solved. I wasn't able to clean the throttle body as thoroughly as I'd like without taking apart more than I was willing to. So, I cleaned the throttle body as well as I could while installed. I haven't had trouble since! Thank you!
Jul 3, 2025 at 8:54 PM
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STRAILER
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Glad you could get it fixed, thanks for letting us know.
Jul 4, 2025 at 9:15 AM