Check engine light on rpm's are going crazy and idle is rough

2008 PONTIAC G5
130,000 MILES • 4 CYL • 2WD • MANUAL
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ONEILLS735
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Check engine light came on and brought it into mechanic who scanned and said the rpm's were going crazy. idle was rough and sounded like a fan was jumping. Checked the timing chain which was good, replaced the two cores but still idle is rough. Runs great on highway no problem.
Jan 18, 2018 at 12:36 PM
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STEVE W.
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What was the code or codes that came up from the engine light? high rpm and rough running that seems to get better at higher rpm sounds like a vacuum leak or lean fuel condition. I would look for a bad hose or leak around the intake system. Also, check that the throttle body is clean and the IAC system is working as it could stick open and let in extra air which will also cause a higher rpm.
Jan 18, 2018 at 10:28 PM
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ONEILLS735
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The codes were saying timing issue, timing belt crankshaft and sensors checked all good. when trying to read the tool the rpm's were jumping all over could not even read rpm.
Jan 19, 2018 at 2:54 PM
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STEVE W.
  • ✓ASE Certified Mechanic
  • 15,200 POSTS
  • ASE Certified Mechanic
Okay, the rpm signal comes from the ignition control module. Check the wiring to it for corrosion or a bad ground. That could cause jumpy rpm. I would still look over the throttle body as well, if the IAC has an issue it could be causing the rpm's to fluctuate as well.
Jan 19, 2018 at 4:21 PM
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ONEILLS735
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Thanks, will try this.
Jan 19, 2018 at 4:28 PM
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