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

Tiny
ONEILLS735
  • MEMBER
  • 2008 PONTIAC G5
  • 4 CYL
  • 2WD
  • MANUAL
  • 130,000 MILES
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.
Thursday, January 18th, 2018 AT 12:36 PM

4 Replies

Tiny
STEVE W.
  • MECHANIC
  • 14,544 POSTS
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.
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Thursday, January 18th, 2018 AT 10:28 PM
Tiny
ONEILLS735
  • MEMBER
  • 3 POSTS
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.
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Friday, January 19th, 2018 AT 2:54 PM
Tiny
STEVE W.
  • MECHANIC
  • 14,544 POSTS
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.
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Friday, January 19th, 2018 AT 4:21 PM
Tiny
ONEILLS735
  • MEMBER
  • 3 POSTS
Thanks, will try this.
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Friday, January 19th, 2018 AT 4:28 PM

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