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Truck vibrates at certain RPM

Tiny
SHADETREE30
  • MEMBER
  • 1976 CHEVROLET CHEYENNE
  • 5.7L
  • V8
  • 4WD
  • AUTOMATIC
  • 299,841 MILES
This old truck of mine has a bad vibration at certain rpm in park, gets worse when truck is in gear(like stopped at a red light). Shakes when the truck accelerates, but stops when reaches over certain rpm. It has a stock 350 5.7 4bbl rodchester quadrajet, with a thm350 transmission "203b" series transfer case and 1ton axels. Vibration stops when returns to idle in park or when accelerated past a certain rpm.
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Wednesday, July 6th, 2016 AT 12:06 PM

4 Replies

Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
If it is truly shaking and not misfiring then check for a weight fallen off torque converter or a bad harmonic balancer. Draw a chalk line across the rubber part then start engine and rev it to about 2500 rpm quickly see if chalk line has moved if it has then balancer is broken. Also check for broken motor mounts and trans mount. If not that then it is either in engine or transmission. Also disconnect belt and start engine if vibration goes away it is something in accessory drive.
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Wednesday, July 6th, 2016 AT 12:23 PM
Tiny
SHADETREE30
  • MEMBER
  • 3 POSTS
Balancer is good, engine is internally balanced. Mounts are all new. Has no accessories other than alternator, water pump, and power steering pump. When engine is at idle (750-850 rpm's) engine runs smooth run it up to about 900-1800 rpm's it shakes the whole truck, go past 1800 to 2000 rpm's and higher it stops. Gets worse when truck is under load in drive or reverse. Until past 2000 rpm's then it stops.
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Wednesday, July 6th, 2016 AT 12:33 PM
Tiny
SHADETREE30
  • MEMBER
  • 3 POSTS
Found the problem engine had wrong heads on it from previous owner. Changed the heads runs great now
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Friday, July 8th, 2016 AT 3:05 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 47,490 POSTS
Great news, thanks for using 2Carpros, we are here to help.

Best, Ken
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Saturday, July 9th, 2016 AT 1:35 PM

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