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Rough idle no check engine light

Tiny
TIMOTHY SCHAEFFER
  • MEMBER
  • 2003 CHEVROLET TRAILBLAZER
  • 4.2L
  • 6 CYL
  • 2WD
  • AUTOMATIC
  • 195,000 MILES
Jiggled key while driving through a parking lot cause seat-belt light wouldn't go off, accidentally turn off car. Hurried up and came to stop put in park restarted the car to a rough idle and lack of power. Once moving will pick up speed but at a stop in park or drive real rough.
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Tuesday, June 4th, 2019 AT 9:36 PM

1 Reply

Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
Hi Timothy,

The scenario doesn't add up to cause a rough idle. Just turning it off while in gear would not cause a rough idle. The most common cause of a rough idle with no check engine light is a vacuum leak. Basically, unmetered air is coming into the engine causing it to run lean. The PCM has not had enough time to turn the light on.

Here is a guide that takes you through how to test for vacuum leaks. If you can, just look over the engine and see if you find a line that has come off. This would make more sense for it being so sudden rather than a line cracking or an intake leak.

Let me know what you find and we can go from there. If you find no leaks, I would put a vacuum gauge on it and ensure you have a steady needle while it is running rough. If no leak, the needle will be still.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge
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Wednesday, June 5th, 2019 AT 4:22 PM

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