Fluctuating RPMs after starting?

DSCHWINDT

MEMBER
2006 BUICK LUCERNE
3.8L
6 CYL
2WD
AUTOMATIC
120,000 MILES

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?

Monday, March 3rd, 2025 AT 5:50 AM

3 Replies

KEN L

MASTER CERTIFIED MECHANIC
50,271 POSTS

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.

Images (Click to make bigger)

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Monday, March 3rd, 2025 AT 1:21 PM

DSCHWINDT

MEMBER
30 POSTS

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.

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Monday, March 3rd, 2025 AT 2:47 PM

KEN L