Exhaust Vibration, Resonance or Moan - keywords 4.8L 5.3L 6.0L A/C acceleration air conditioning fluctuation idle intermittent L59 LM7 LR4 LQ4 LQ9 moan noise resonance rough vibration

This PI has been revised to include the 2006 Model Year. Please discard any copies of PIP3121D.

The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.

Some 1999-2006 C/K sport utility vehicles and trucks with GEN III V8 Engines and a single pipe exhaust system may exhibit an exhaust vibration that is engine load sensitive. Typically, this will happen just off of idle to 2000 RPM on light acceleration.

Important: The following does not apply to 2003-2006 Full Size C-K Utility Vehicles with an IDLE ONLY vibration. Reference PIT3203E for IDLE ONLY vibrations on 2003-2006 Full Size C-K Utility Vehicles.

Important: The following does not apply to dual pipe exhaust systems like the ones used on some 3/4 Ton and 1 Ton trucks or like the ones used on the SS Chevrolet Silverado or the GMC Denali Pick Up Truck.

1. Determine if this is an exhaust vibration by using an EVA tool to gather Hertz readings from various locations (seat track, frame, radiator support, etc.) during the condition.
2. Multiply the Hertz readings by 60 and the divide the sum of that by the exact RPM that the readings were taken at. The sum of this will determine what order vibration it is. If this is exhaust related, a fourth order vibration will be calculated.

Here is an example of a 4th order vibration at 900 RPM:

58 Hz multiplied by 60 = 3480

3480 divided by 900 RPM = 3.87

3.87 Rounded Up = 4

4 = 4th Order Engine Vibration

If this is a 4th order vibration, fabricate 2 dampers using the following procedure:

1. Obtain 4 dampeners, Part Number 10199232.
2. Line up 2 dampeners so the dampener pipes are opposite of each other.
3. Line up one mounting hole from each dampener with the holes of an appropriate size exhaust clamp.
4. With all of the holes lined up, weld the flat portion of the dampeners together and to the flat part of the exhaust clamp.
5. Attach the dampeners to the exhaust system using exhaust clamps and the following locations: On trucks with an 8 ft. bed, install one modified damper in front of the muffler and one just behind the muffler. On trucks with a six foot bed, install one modified damper in front of the muffler and one over the top of the rear axle. On Utility Vehicles, place one in front of the muffler and one anywhere between the muffler and rear axle bend. Fine tuning will most likely be necessary, especially on Utility Vehicles.

Note: The locations above are just starting points. If the vibration is not improved, move the dampeners around while using an EVA tool to fine tune the exhaust system by noting the G readings on the exhaust system.

Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.