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Remember back when you were just a kid and you told your mom you were going to hold your breath until you got your way? I'm sure that didn't work out too well for you, because she knew that whatever air you took in would eventually have to come back out. You really didn't have a choice in the matter.
Imagine, though, if you were a runner and you could breathe in fine but it was extremely hard to exhale. I don't think you would be a very efficient runner. The same thing holds true for our vehicles and the way they exhale — their exhaust system.
Exhaust Flow at a Glance
For an engine to be efficient, it must have the proper exhaust flow to get rid of all the products of the combustion process. The air that is taken in through the intake has to be expelled after being used and there's only one way out: the exhaust system.
The exhaust flow actually starts in the combustion chamber where the fuel/air mixture is burned. After that it becomes a byproduct of the combustion process and has to leave the engine via the exhaust valves in the cylinder head and then out through the exhaust manifold or header pipe, depending on engine design. While exhaust systems vary based on engine design, manufacturer, environmental standards or even size constraints, they all follow some basic design characteristics.
After leaving the engine, there's the required catalyst that can be a close-coupled converter or a catalytic converter in the exhaust pipe itself. A close-coupled converter actually can be a part of the exhaust manifold and is used to allow for rapid heat up, which aids in a quicker cleanup of the exhaust gasses.
Some systems may have an exhaust resonator just pass the converter to aid in the reduction of exhaust harmonics that might cause unwanted noises or vibrations. Exhaust pipes then route the flow underneath the vehicle and into the muffler. Here, of course, is where most of the exhaust noise is eliminated. This is done by the use of various chambers and dampening material used within the muffler itself.
Finally, we reach the termination point at the tailpipe or exhaust tip, depending on system design. By this time the exhaust has been cleaned up and quieted down to acceptable levels. Based on engine size and design, this is done through either a single or dual exhaust system.
The Pieces and Parts
Let's take a look at some of the parts that get the job done, starting at the exhaust manifold. Usually made of a heavy cast iron material, the exhaust manifold mounts to the engine at the cylinder head. It is subjected to some serious temperatures, and for this reason it's susceptible to cracking or warping.
Manifolds should be inspected regularly for these signs. If the exhaust manifold encompasses a close-coupled catalyst, then the catalytic converter is actually part of the manifold in some cases. The bolts and studs that are used for manifold and pipe retention are also subjected to extreme temperature inversions and environmental factors. This can make for some tricky R&R techniques when the bolts are aged and rusted.
This is a concern throughout the exhaust system and might require a good bit of penetrating oil, extraction tools, a torch and a lot of patience. Some vehicles have a flexible joint just after the manifold to allow for engine and exhaust system movement during operation. These then would connect to the flanges on the pipes. At this point there may be a thin lead covered gasket, a doughnut seal or compression flange seal, either of which could be a potential leak site.
If the system contains a standard catalytic converter, we would probably run into it next. As we know, a catalytic converter is an emissions control device used to reduce hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOx) pollutants from the exhaust gas. The converter is made up of a ceramic monolith substrate that is supported in insulation. It's housed in a metal shell and the substrate material will be coated with Palladium (Pd), Platinum (Pt) or Rhodium (Rh). These are the materials that actually do the work of helping to burn off the exhaust gasses.
Once you get past the converter, you run through the piping and into the muffler or resonator if equipped. The configuration varies by design. Resonators are used in systems designed to have mufflers with less back pressure and where design requires specific exhaust tuning. The muffler greatly reduces the noise from engine combustion and is comprised of tuning tubes that create different channels in the muffler for the exhaust to travel through.
At the end of the system, you'll have either a tailpipe or exhaust tip for a point of termination and to direct the flow from underneath the vehicle.
There are a few problems that are common with exhaust systems, and some of them are obvious. Of course noise is probably the first complaint you hear from customers and can usually be grouped into three categories: loud exhaust, external rattle or vibration and internal rattle.
A loud exhaust may or may not be a problem. The first thing you should do, if possible, is to compare the exhaust sound to that of a like vehicle and see if the concern is related to a design characteristic of the vehicle. If the noise is abnormal, then begin with a good visual inspection of the system. Look for signs of damage, rust through, loose connections, soot marks or anything abnormal such as ruptured muffler or converter housings.
Sometimes the integrity of exhaust piping can be checked with a small ball peen hammer. Simply tap on the pipe in different areas with the ball side and listen to the sound. A good section of pipe should have a nice ping or ring to it; whereas, a section that is deteriorated will have a dull thud sort of sound. External rattles or vibration can be linked to several different causes. Heat shields on the body, engine and exhaust system itself can be a prominent area for rattles. This is an area where an assistant comes in handy to get the noise occurring while you search for its source.
Some repairs can be as simple as bending a misplaced heat shield back into location. Remember that exhaust systems can set up resonations that can cause other components on the vehicle to rattle. A vibration could be caused by a resonation issue as well. Some companies manufacture dampeners that can be installed on the exhaust system components to aid in these issues.
An exhaust component contacting the body or chassis will almost certainly be a cause of noise concern for a customer. Inspect exhaust system mounts and hangers to see if they have been bent or misplaced during operation. Deterioration of rubber mounts and bushing can cause the same thing and will require replacement. Remember that any damaged component should be replaced as soon as possible.
Internal rattles can sometimes be a little tricky to find. Many times rattles in an exhaust system component carry through other components in the system and make it hard to locate the origin of the noise. If the location can't be determined with the aid of an assistant operating the vehicle while you perform your inspection, grab a rubber mallet.
Start with the more obvious components like the catalytic converter. A converter whose substrate has broken apart can be located by tapping on the converter with the mallet and listening for the loose material in the converter to rattle around. The same thing can be done with the muffler by tapping on it and listening for a loose or broken baffle or tube inside. Many exhaust pipes have a double wall design that can break apart and rattle internally. Once again, the mallet can help locate them.
Always check for leaks as you're inspecting other components. Nobody likes a leaky exhaust sound, but more importantly is the inherent safety issue of allowing exhaust gasses to reach the occupants.
Restrictions are a bigger problem than noises. Usually restriction problems will not only cause an audible difference in the exhaust but will be accompanied by a drivability concern. Customers may have a complaint of loss in power, higher than normal engine temperatures, a drop in fuel economy or peculiar odors. The most common cause would be the catalytic converter. If the substrate has deteriorated or come apart it can block the exhaust flow and cause excessive back pressure in the exhaust system.
This can damage exhaust system components, and it can be detrimental to the engine if not repaired. Excessive back pressure is fairly easy to check with the use of a back pressure gauge. You would simply remove the first oxygen sensor in the exhaust stream after the manifold and install the gauge in that location. Operate the vehicle at 2,000 to 2,500 rpm and observe the gauge. Most systems should only show around 2 to 3 psi if there are no restrictions. If the reading is higher, you should inspect the converter and other exhaust components for blockages.
If a catalytic converter has had a catastrophic failure, it is likely that the muffler or resonator could be contaminated with debris and require replacement as well. Either way, make sure the entire system is checked and is free of restrictions after such an event.
Look Before You Leak
Many exhaust system problems can be avoided with a little maintenance and inspection from time to time. It's very easy to inspect an exhaust system while a vehicle is on the lift, and it takes only a couple of minutes to give it a look over. This step can save time and money for your customer. Damage can be spotted quickly and can be eliminated before causing a problem.
Leaks sometimes can be as simple as tightening a clamp or bolt but could require a gasket or seal replacement. Other exhaust system problems might not be as easy to determine and can be related to another system or the result of a previous repair.
Some odors can indicate exhaust system problems and due to the serious nature of exhaust gasses, should never be taken lightly. Exhaust gasses are dangerous and can cause serious injury or death. You should remember though to make customers aware that fuels containing high sulfur contents can have a really bad odor. Advise a customer to try fuel from another reputable facility before addressing this issue.
Know that exhaust system repairs can entail the use of torches or welders for some repairs, and if you are not certified in the use of this equipment, it would be best to sublet these repairs to a facility that is specialized.
Exhaust system maintenance and repair can be done quite proficiently if you simply take a little time and have a little patience. Thoroughly inspect everything. Pre-failure inspections are a must. Finally, listen to your customer. No one knows their car better than they do. Evaluate their concerns and steer them in the right direction for the repairs they need. Exhaust system repairs don't have to be, well, exhausting.
Randy Wilson is an ASE Master Technician working for a Chevrolet and Cadillac dealership in Dothan, Ala. He is Advanced Engine Performance Specialist Certified with 25 years of experience. Wilson specializes in drivability, diagnostics and electrical systems.