During the inspection of an exhaust system, close attention  should be given to all external surfaces of the exhaust system  for cracks, dents, or missing parts. This also applies to welds,  clamps, supports, support attachment lugs, bracing, slip joints,  stack flanges, gaskets, and flexible couplings. Each bend  should be examined, as well as areas adjacent to welds. Any  dented areas or low spots in the system should be inspected  for thinning and pitting due to internal erosion by combustion  products or accumulated moisture. An ice pick or similar  pointed instrument is useful in probing suspected areas. The system should be disassembled as necessary to inspect  internal baffles or diffusers. If a component of the exhaust  system is inaccessible for a thorough visual inspection or is  hidden by nonremovable parts, it should be removed and  checked for possible leaks. This can often be accomplished  best by plugging the openings of the component, applying  a suitable internal pressure (approximately 2 psi), and  submerging it in water. Any leaks cause bubbles that  can readily be detected. The procedures required for an  installation inspection are also performed during most regular  inspections. Daily inspection of the exhaust system usually  consists of checking the exposed exhaust system for cracks,  scaling, excessive leakage, and loose clamps. Approximately half of all muffler and heat exchanger failures  can be traced to cracks or ruptures in the heat exchanger  surfaces used for cabin and carburetor heat sources. Failures  in the heat exchanger surface (usually in the outer wall) allow  exhaust gases to escape directly into the cabin heat system.  These failures, in most cases, are caused by thermal and  vibration fatigue cracking in areas of stress concentration.  Failure of the spot-welds, which attach the heat transfer pins,  can result in exhaust gas leakage. In addition to a carbon  monoxide hazard, failure of heat exchanger surfaces can  permit exhaust gases to be drawn into the engine induction  system, causing engine overheating and power loss. Exhaust manifold and stack failures are usually fatigue  failures at welded or clamped points (e.g., stack-to-flange,  stack-to-manifold, and crossover pipe or muffler connections).  Although these failures are primarily fire hazards, they also  present carbon monoxide problems. Exhaust gases can  enter the cabin via defective or inadequate seals at firewall  openings, wing strut fittings, doors, and wing root openings. An exhaust leak is indicated by a flat gray or a sooty black  streak on the pipes in the area of the leak. An exhaust leak  is usually the result of poor alignment of two mated exhaust  system members. When a leaking exhaust connection is  discovered, the clamps should be loosened, and the leaking  units repositioned to ensure a gas-tight fit. After repositioning, the system nuts should be retightened  enough to eliminate any looseness without exceeding the  specified torque. If tightening to the specified torque does  not eliminate looseness, the bolts and nuts should be replaced  since they have probably stretched. After tightening to the  specified torque, all nuts should be safetied. With the cowling  removed, all necessary cleaning operations can be performed.  Some exhaust units are manufactured with a plain sandblast  finish. Others may have a ceramic-coated finish. Ceramic- coated stacks should be cleaned by degreasing only. They  should never be cleaned with sandblast or alkali cleaners.  Internal failures (baffles, diffusers, etc.) can cause partial or  complete engine power loss by restricting the flow of the  exhaust gases. If pieces of the internal baffling breaks loose  and partially or totally blocks the flow of exhaust gases,  3-25 Internal Muffler Failures Exhaust Manifold and Stack Failures Figure 3-42.  Primary inspection areas of three types of exhaust  systems. A B C Muffler and Heat Exchanger Failures