Car idles hard

2006 PONTIAC GTO
135,000 MILES • 0.6L • V8 • 2WD • AUTOMATIC
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TERRIRESSE MALONE
  • MEMBER
  • 1 POST
When driving and maintaining a speed car revs up and rpm go to five or six and stays there. Black smoke comes out of mufflers. Idles is hard and rough. Has been put on diagnostic machine. Nothing shows up. Been to three mechanics and no one can figure it out. Just had spark plugs changed.
Jul 21, 2017 at 1:19 PM
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STEVE W.
  • ✓Experienced Mechanic
  • 15,367 POSTS
Black smoke is excess fuel being burnt, rough idle suggests you have a misfire issue which would give you excess fuel.
I would start with a fuel pressure test, a bad pressure regulator could cause a rich condition, poor idle and black smoke. Next you will want to look at the fuel trims when the smoke is actually happening. High trims could suggest a sensor input issue like a bad coolant temperature sender. Basically the ECM thinks the engine is running cold and supplies more fuel. Extra fuel give you the higher rpm and smoke as well.

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

To look at the fuel trims and sensor data you need a scan tool that can show live data.
Jul 21, 2017 at 4:57 PM
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