engine vibration at 1100 rpm

in my 2002 chevrolet silverado with a 4.3 l v-6, which i just bought used with 46,000 miles, a strange problem, occurs whether in 1st, 2nd 3rd or 4th gear,
while driving, at around rpm 1100 very noticable engine vibration will occur. if i keep at that speed (rpm stays the same), vibration will be constant. if i increase or decrease speed (rpm), vibration will disappear.
mechanic friend says all chevy 4.3 's have similar problems/qirks
hoping someone will access to TSB's can look it up.
many thanks
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received email from someone, as below. i am posting it incase anyone else has similar problem. looks to be a common issue. =========================================================it is the v6 and the exhaust heres the bulletin ,steve
Exhaust Moan/Vibration (Install Exhaust System Flex Pipe Kit) #02-06-05-003C - (02/05/2004) Exhaust Moan/Vibration (Install Exhaust System Flex Pipe Kit) 1999-2004 Chevrolet Silverado 1500 Series Extended Cab or Regular Cab/Long Box Pickups (New Style)
1999-2004 GMC Sierra 1500 Series Extended Cab or Regular Cab/Long Box Pickups (New Style)
with 4.3L Engine (VINs W, X-- RPOs L35, LU3) and Automatic Transmission (RPO M30)
This bulletin is being revised to add the 2004 Model year. Please discard Corporate Bulletin Number 02-06-05-003B (Section 06 -- Engine/Propulsion System).
Condition Some customers may comment on a moan or vibration at low vehicle speeds between 900-1200 RPM under load.
Cause Exhaust system resonance that is transmitted through the powertrain mounts into the body may cause this condition.
Correction An exhaust system flex pipe kit is available to correct this condition. If diagnosis leads to this condition, install the applicable exhaust system flex pipe kit listed below using the following procedure. Refer to the Parts Information for kit applications and usage.
Important This fix and the kit part numbers are specific to vehicle frame type. A Level 3 frame was introduced interim 2000 model year (approximately January 2000). With a Level 3 frame, the transmission crossmember was moved forward of its previous location. Pre-Level 3 frames will require the installation of additional components to correct the condition.
1999-2000 4WD Models Built Prior to 1/20/00 and All 2000-2004 2WD and 4WD Models Built After 1/00 These models only require the installation of a flex coupling. The flex coupling has adapter sleeves for the installation of U-bolt clamps. This procedure uses the U-bolt clamps, but the preferred installation method is to weld the flex coupler to the exhaust pipe.
1.. Raise the vehicle. 2.. Locate the first available straight section of pipe rearward of the mid-joint flange and forward of the muffler assembly. 3.. Measure approximately 228 mm (9 in) on the straight pipe and mark. 4.. Using a suitable cutting tool, cut out the marked section of the pipe and remove. 5.. Remove any burrs, if necessary, before installing the flex coupler pipe.
6.. Install the flex coupling (1). Expand the sleeves of the flex coupler, if necessary, to properly fit over each pipe end. The centerline of the installed flex coupling should run parallel to the straight section of pipe (no bends in the coupling) for optimum performance. 7.. Install the U-bolt clamps (2) on each sleeve end of the flex coupling. Tighten Tighten the U-bolt clamps to 30 Nm (22 lb ft).
8.. Lower the vehicle. 1999-2000 2WD Models Built Prior to 1/00 These models will require the installation of a flex coupler, two Y-pipe support brace rods w/integral U-bolt clamp assemblies on the LH and RH sides of the transmission rear mount and new transmission mount studs and nuts. The Y-pipe support braces are used in combination with the flex coupler to reduce exhaust moan.
1.. Install the flex coupler using steps 1-7 of the above procedure. 2.. Support the transmission with a suitable adjustable jack stand. 3.. Remove the two bolts and nuts from the transmission rear mount. 4.. Position the transmission rear mount and install two new studs to the mount. 5.. Install the new studs supplied with the kit to the transmission mount.
6.. Loosely position the Y-pipe support braces w/integral U-bolt clamps (LH and RH) to the underside of the transmission rear mount and the crossover pipe as shown. The top ends of the braces attach to the underside of the transmission rear mount. The bottom ends of the braces attach to the crossover pipe using the integral U-bolt clamps. The braces support the LH leg of the Y-pipe where it crosses over the vehicle to meet the Y-pipe collector. 7.. Install the two nuts to the transmission rear mount and the two nuts to the U-bolt clamps. Tighten a.. Tighten the new transmission mount nuts to 25 Nm (18 lb ft). b.. Tighten the two U-bolt clamps to 30 Nm (22 lb ft). 8.. Lower the vehicle. Parts Information Part Number Description Usage
15076982 Exhaust System Parts Package 1999-2000 2WD Models Built Prior to 1/00
15076990 Exhaust System Parts Package a.. 1999-2000 4WD Models Built Prior to 1/20/00 b.. All 2000-2004 2WD and 4WD Models Built After 1/00
a.. Kit P/N 15076982 includes a flex coupler, two support brace rods w/integral U-bolt pipe clamps, two U-bolt pipe clamps (for use with flex coupler) and two new transmission mount studs w/fasteners. b.. Kit P/N 15076990 includes a flex coupler and two U-bolt pipe clamps.
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