Exhaust Systems
by Jim Kerr

Exhaust systems are usually forgotten about until a major problem occurs. Fortunately, the systems tend to very durable considering the conditions they operate under. A good exhaust system not only carries harmful exhaust gases away from the passenger compartment, but also converts most of the harmful gases into acceptable by-products. A simple understanding of exhaust system operation can help a driver recognize many problems before they require more expensive repair.

The exhaust system starts with the exhaust manifold on the engine. Hot exhaust gases flow from the manifold through the front or header pipe into a catalytic converter. The converter changes unburned hydrocarbons (fuel) and carbon monoxide into water vapour and less harmful carbon dioxide.

Inside the stainless steel case of the converter, there is a ceramic honeycomb coated with a thin layer of platinum. When the exhaust gases pass through the hot converter, they are changed. Many performance cars use dual converters and dual exhaust systems to allow more exhaust gases out of the engine.

From the converter, the exhaust flows through another pipe to the muffler. On most cars, the exhaust now flows out the tail pipe, but on some models, the exhaust passes through a secondary muffler called a resonator. Finally, clamps and rubber mounts called hangers support the exhaust system under the vehicle.

Inspecting the exhaust system is simple. The first step is to put on some work clothes because reaching under the car is a dirty business. The second step is to apply the parking brake and block the wheels so the car will not roll over you! Most exhaust parts are visible once you lay down beside the car. Inspect all the hangers to see if the exhaust system is properly supported. A broken hanger causes the exhaust pipes to flex and break.

Next, inspect the muffler and resonator. Poke the parts with a screwdriver to see if they are solid. Small rust holes usually start to form at the bottom and ends of the mufflers close to where the exhaust pipes enter. Many mufflers have a small hole at the lowest end to drain any water out of the muffler. The water is a by-product of the combustion process and is normal, but it is also the reason the exhaust system rusts from the inside first.

Any exhaust parts with rust holes in them should be replaced. Odourless and poisonous carbon monoxide gas can leak from the rust holes into the passenger compartment. Tap on the muffler a, resonator, and converter gently with a rubber hammer or block of wood. Any rattles heard from inside these parts indicate an internal failure hat could lower vehicle performance by blocking the exhaust flow.

The last parts to check are the exhaust pipes. Any pipes that bend easily when squeezed with a large pair of pliers require replacement. Many newer cars have exhaust pipes made of stainless steel. Although more expensive then regular steel pipes, the stainless pipes last much longer.

Noise and smell are other indications of exhaust system problems. A hole in the exhaust system not only makes the car sound louder, but can also cause the oxygen sensor in the fuel injection system to falsely sense a lean fuel mixture. The computer then injects extra fuel into the engine to compensate and your fuel economy drops. Sometimes the extra fuel passes through the engine into the exhaust system and combines with the oxygen let in by the exhaust leak to cause a loud backfire. This can quickly destroy a good muffler!

Some vehicle exhaust gases smell a little like rotten eggs. This is caused by the catalytic converter as it changes the exhaust gases. Sulphur in the fuel combines with oxygen to create the smell. Unfortunately, the odour is the small price we have to pay for cleaner exhaust, but if the smell is strong, the vehicle should be checked. An engine that is using too much fuel or is running rough causes the converter to work extra hard as it tries to convert the unburned fuel into other gases. The converter gets extremely hot under these conditions; exceeding 650 degrees C. and this soon causes converter failure. Drive the vehicle slowly and as little as possible until the rough running is repaired.

Finally, here is a tip for diagnosing a plugged exhaust system. Connect a vacuum gauge to an intake manifold vacuum port. Run the engine at 1000 rpm with the transmission in park and check the vacuum reading. Then increase the engine speed to 2500 rpm. Again, check the vacuum reading. If the reading drops at higher rpm, then the exhaust system may be plugged.

Jim Kerr is a master automotive mechanic and teaches automotive technology. He has been writing automotive articles for fifteen years for newspapers and magazines in Canada and the United States, and is a member of the Automotive Journalist's Association of Canada (AJAC).

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