Awful knock at high RPM

Tiny
MARY RICHARDS
  • MEMBER
  • 2006 PONTIAC G6
  • 3.4L
  • V6
  • 2WD
  • 125,000 MILES
This car runs good except a knock that is worst at high rpm's. My friend thought it was a cracked flex-plate because it is not a rod knock but the plate looks fine. He says it is coming from the bottom of the engine. Any suggestions?
Monday, June 10th, 2019 AT 11:29 PM

7 Replies

Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
Good morning,

Can you upload a video of the noise so I can hear it?

It is either a rod bearing or main bearing from your description.

https://www.2carpros.com/articles/engine-noises

A flex plate would sound like a valve tap.

Roy
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Tuesday, June 11th, 2019 AT 4:22 AM
Tiny
PATENTED_REPAIR_PRO
  • MECHANIC
  • 1,853 POSTS
Main bearings don't usually knock but rod bearings do. Drop the oil pan and plasti-gage all the rod bearings.
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Tuesday, June 11th, 2019 AT 5:00 AM
Tiny
MARY RICHARDS
  • MEMBER
  • 6 POSTS
What does plasti-gage mean?
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Tuesday, June 11th, 2019 AT 5:22 AM
Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
That is when the clearance is checked between the bearing and the crankshaft.

Roy
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Tuesday, June 11th, 2019 AT 5:28 AM
Tiny
MARY RICHARDS
  • MEMBER
  • 6 POSTS
Thank you.
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Tuesday, June 11th, 2019 AT 5:58 AM
Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
You are welcome.

We are always glad to help.

Roy
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+1
Tuesday, June 11th, 2019 AT 6:04 AM
Tiny
PATENTED_REPAIR_PRO
  • MECHANIC
  • 1,853 POSTS
To use plasti-gage, clean the motor oil off a rod bearing in the connecting rod end cap, lay across the bearing 1 string of the plasti-gage, then install the bearing and cap, torque it down to spec, then remove the bearing cap and measure how much the plasti-gage string got smashed and that tells you the amount of oil clearance and if it's too much, it means the bearing is worn.
Here is another source for how to use it: https://www.plastigaugeusa.com/how.html
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Wednesday, June 12th, 2019 AT 4:12 AM

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