Angie's LIST Guide to
Auto exhaust systems

Your vehicle's exhaust system primarily routes exhaust gases away from the engine, but it performs a number a number of other important functions as well.
 

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How exhaust systems work

Internal combustion, by which fuel-burning engines derive power, can be a messy process. Fuel from the vehicle’s gas tank is drawn into the combustion chamber or cylinder within the cylinder head, where it is mixed with air drawn in from outside the vehicle through the air filter. From there a spark plug produces a spark from the vehicle’s electrical system, namely the distributor cap and wires, which ignites the fuel-air mixture, producing a small explosion.

This explosion drives the piston – which is seated flush in the cylinder – up, which translates the force of the explosion into the crankshaft. The crankshaft translates this power into a circular motion, eventually reaching the transmission, which transfers this power to the wheels. Finally, the remaining fuel-air mixture, both burned and unburned, exits through the exhaust port, and the cycle begins anew. This happens thousands of times a minute when a vehicle’s engine is running.

But what happens once the exhaust gases exit the engine’s combustion chambers? First, it travels out of the engine via the exhaust manifolds, also known as exhaust headers or simply headers. Since most vehicles have multiple cylinders, each combusting their own fuel-air mixture and thereby producing their own exhaust, the exhaust manifold collects the spent fuel from each cylinder. The exhaust is then sent further down the exhaust system via a single tube known as the tailpipe.

How an automobile exhaust system works

Reducing polluting emissions

Exhaust gases undergo an important change before they exit the vehicle’s exhaust system. One of the most important in the process is the catalytic converter, which reduces the amount of toxic pollutants emitted into the air. In the U.S., automobiles are the primary source of air pollution. That’s one reason the EPA began regulating vehicle emissions and fuel standards under the Clean Air Act starting in 1970. Following the act’s passage, vehicle manufacturers began adding catalytic converters to the exhaust systems of their vehicles.

The catalytic converter doesn't use moving parts to scrub exhaust gases. Instead, it utilizes chemical reactions to almost totally eliminate toxic pollutants such as nitrogen oxide, carbon monoxide and hydrocarbon in a vehicle’s exhaust. Within the catalytic converter a series of planes covered in catalytic metals such as platinum, rhodium or palladium chemically converts the pollutants into less harmful nitrogen, carbon dioxide and water, thereby reducing the amount of polluting emissions.

Reducing noise pollution

After leaving the catalytic converter, the exhaust gases travel further down the exhaust system tailpipe to the muffler. Much like its name suggests, the primary purpose of the muffler is to reduce the noise produced by the combustion process. Have you ever been to a stock car race or drag strip and noticed how loud it is? The main reason you’ll need ear protection anytime you’re in the vicinity of race cars is their lack of muffler.

dual exhaust

Although a the muffler is the most easily visible component, it's only one portion of a vehicle's exhaust system.(Photo by Fred Patton)

A muffler turns down the volume on the vehicle by passing exhaust gases through a series of tubes and filters, which progressively absorb, diminish or delete the sound waves produced by the engine. Because the absorption of sound waves also produces back pressure, a muffler that operates properly is an integral component in a vehicle’s engine operating efficiently.

Once the exhaust gases have passed through the muffler and out via the remaining tailpipe through an exhaust tip or resonator tip, the exhaust system process is complete.

Signs your exhaust needs repair

The cost to repair the exhaust systems or its components can vary widely by vehicle type, make and model, as well as which component needs repair and the extent of the damage. Replacing a catalytic converter, for example, can be significantly expensive, while re-hanging a loose exhaust pipe can be relatively inexpensive.

Whatever the repair, continuing to drive a car with a damaged exhaust will almost always cost more in the end than repairing the problem at the first sign of trouble. And it's important to remember that the muffler and exhaust system do more than just quiet the car, they also prevent carbon monoxide fumes from building up in the cabin, reduce polluting emissions and ensure the vehicle runs at peak efficiency.

Here are some indicators that your car may need muffler repairs or exhaust system maintenance:

1. Increased volume when the engine is running
When your car's exhaust note suddenly sounds more like a lawn mower or motorcycle, it's a good sign that your car needs muffler repairs. For other drivers, the sound of needed exhaust system repairs may sound like buzzing, a low rumbling or a slightly louder exhaust note.

muffler

Because mufflers and tailpipes are constantly exposed to the elements, they may wear more quickly than other parts. (Photo by Fred Patton)

Don't just turn up the radio to drown out the noise - get the car to a muffler repair shop. Delaying needed repairs can eventually cost more in the form of more extensive damage, such as having to replace an otherwise usable component such as exhaust pipe, catalytic converter or muffler because it fell from your car.

2. Rattling at stop lights or stop signs
You might notice you get drowsy when driving, you might see a dashboard warning light, or you might notice your car occasionally hesitates or chugs when you try to start it. If you notice any of these issues, or if your car rattles when not moving, schedule an appointment for the muffler shop right away.

The catalytic converter is one of the highest priced exhaust system parts. As far as safety goes, it's one of the most important parts to keep up. If a catalytic converter has a tiny hole, you might not notice it right away. The signs start out small.

3. Engine vibrations
If you notice a new vibration or loss of power when operating your car, have the exhaust system checked by a professional. You might notice the new vibration when you touch the steering wheel, foot pedals or car seat.

You might or might not hear a rumbling sound, depending on the size of the damaged area. An exhaust leak makes your car's engine work harder, and that extra effort sometimes causes vibrations so intense that the driver feels them before hearing the leak.

4. Decreased fuel efficiency
If you notice you have to fill up your gas tank more often than normal, have your exhaust system checked out by a muffler repair shop. Similar to the vibrations, increased fuel use is a sign something is wrong with your car. When the exhaust leaks, your engine works harder. When your engine works harder, it requires more fuel.

Avoiding catalytic converter theft

With the rise of precious metal prices in the last few years, a component of every vehicle’s exhaust system has become the target of thieves: the catalytic converter. Because catalytic converters contain precious metals like rhodium, platinum and palladium, scrap metal thieves find them particularly lucrative targets. Within just a few minutes, a thief can slip underneath a vehicle and remove the catalytic converter using a powered saw or a wrench.

These metals can command per ounce prices of hundreds or thousands of dollars in metals markets, but metal thieves typically receive $100 to $200 per catalytic converter. Replacing a catalytic converter can cost a victimized car owner more than $1,000.

To avoid becoming the victim of a costly catalytic converter theft, check out these ways to protect your vehicle.

1. Know if your vehicle is a more lucrative target
Any vehicle manufactured in the United States after the mid-1970s features a catalytic converter but some vehicles are more prone to catalytic converter theft than others. If you drive a truck or an SUV, those vehicles’ higher ground clearance offer thieves more access to their prize. Vehicles that feature multiple catalytic converters – such as newer vehicles that meet stricter emissions standards –can also pose a more lucrative opportunity for a thief.

2. Reduce your risk
When possible, avoid parking in unsecured parking lots for long periods of time. Look for lots that are in busier areas and that feature ample lighting and on-site security. When parking, look for the most well lit parking areas, such as the near the entrance. Installing a security system that’s sensitive enough to detect a person in close proximity is another possible solution (although likely annoying).

3. Beef up security
Several products are available to help prevent catalytic converter theft. The products usually consist of wire cables or cages that surround the catalytic converter and are bolted or welded to the frame. Although costly at $100 or more, they’re far less expensive then replacing a catalytic converter.

If your catalytic converter attaches to the exhaust system by bolts, you can also ask a local welding or auto service shop to weld the bolts, preventing them from being removed by a thief. This would not prevent a thief using a powered saw, however.

4. Make your mark
Although it may do little to prevent theft, having your car’s vehicle identification number (VIN) or license plate number engraved on the catalytic converter itself may deter thieves from selling it a scrap yard.  

Comments

Excellent explanation of location within the vehicle, function, importance of protecting from theft, and cost.
I suspect my catalytic converter has been stolen. The noise in my care seems as though it is under the driver seat.

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