Lack of power, poor fuel economy, transmission shifts are hard and late. Reduced top speed. Intake manifold vacuum which drops excessively with RPMs. High exhaust back pressure (greater than 1.5 psi at idle and 3 psi at 2000 rpms on most vehicles).
Excessive Back Pressure / Restriction
Passageways within the muffler, resonator or converter have become blocked or restricted. This could be caused by rusted partitions within the muffler breaking away from their original positions and blocking exhaust flow. It can also be caused by chunks of converter substrate material caught in the muffler or from excessive carbon build up in the converter. Another possible source of restriction is crushed exhaust pipes or internal air gap pipe failure.
Inspect the exhaust system for crushed, bent or otherwise restricted pipes. Replace or repair as required. If pipes look good, temporarily remove the O2 sensor ahead of the converter. If symptoms are still present inspect for internal air gap pipe restriction ahead of the converter. If symptoms are no longer present, reinstall the front O2 sensor, remove the O2 sensor behind the converter and retest the vehicle. If symptoms are still present the converter is causing the restriction. Check for rich condition, excessive oil consumption, misfires or other root cause of failure. If symptoms are no longer present with the rear O2 sensor removed, the restriction is in the muffler or resonator. Inspect and replace resonator or muffler assembly as required.