Truck vibrates at certain RPM

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.

Wednesday, July 6th, 2016 AT 12:06 PM

4 Replies

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

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

SHADETREE30