Objectionable Exhaust Resonance - Boom Noise Heard in Cab at Idle - keywords droan extended LL8 moan muffler pipe resonance rough thump vibrate vibration

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

Customers may comment of an objectionable exhaust resonance "boom" noise heard in the vehicle at idle with the engine temperature at the normal operating range. This will only occur on vehicles equipped with the LL8 4.2L engine. This objectionable level of resonance noise or vibration may be due to the exhaust alignment. Although this concern is considered characteristic of the vehicle, improvements have been made following the procedure below.

Align the exhaust per the procedures below.

1. Drive the vehicle to get the exhaust up to normal operating temperature.
2. Carefully loosen the 3 nuts at the exhaust manifold to down pipe flange. Leave the nuts on the studs to hold the pipe and flange from completely falling away from the exhaust manifold.

Caution: The manifold, the bolts, the pipe, the catalytic converter, and the flange will be hot!




3. Carefully loosen the 2 nuts at the resonator flange. Leave the nuts on the studs to hold the pipes loosely together at the flange.

Caution: The bolts, the pipes, the resonator, and the flange will be hot!




4. Start the engine. Apply the brake pedal firmly and shift from reverse to drive several times to settle the engine and transmission in their mounts with the exhaust loose.
5. Place transmission back in park and shut off the engine.
6. Tighten the 3 bolts at the exhaust manifold flange.

Important: Tighten each bolt a little at a time until just snug being careful to apply even pressure at each bolt during tightening. Then go back and tighten each bolt a little more one at a time until all 3 are equally tight.

Caution: The manifold, the bolts, the pipe, the catalytic converter, and the flange will be hot!

7. Tighten the 2 bolts at the resonator flange.

Important: Tighten each bolt a little at a time until just snug being careful to apply even pressure at each bolt during tightening. Then go back and tighten each bolt a little more one at a time until both are equally tight.

Caution: The bolts, the pipes, the resonator, and the flange will be hot!

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.