Erratic idle

1990 GMC SIERRA
194,000 MILES • 5.7L • V8 • 4WD • AUTOMATIC
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BAGGYPANTS
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  • 1 POST
Idle oscillates from slow to fast and if the throttle is opened a bit it speeds for about 30 seconds, then backfires through the TBI and shuts down.
Oct 23, 2019 at 7:02 PM
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SCGRANTURISMO
  • ✓ASE Certified Mechanic
  • 4,897 POSTS
  • ASE Certified Mechanic
Hello,

An idle that is high or one that is "searching" is usually do to a vacuum leak. A vacuum leak or intake leak is defined as any unmetered air entering the air intake system from the Mass Airflow [MAF] sensor to the engine's combustion chambers. So if you could please do a visual inspection of the air intake tract, vacuum hoses, and the EVAP system looking for misaligned components, broken or cut hoses, or loose connections. Here is a link below with a guide on how to do this:

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

Please go through this guide and get back to us with what you are to find out and we can go from there.

Thanks,
Alex
2CarPros
Oct 24, 2019 at 12:14 AM
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