Inspection of the exhaust system is recommended whenever a vehicle is in for service and there is a ready opportunity for a quick look (i.e., the vehicle is up on a hoist). The basic inspection is to see if there is deterioration of any exhaust system components. Deterioration may not always be obvious, such as through the appearance of holes and flaking rust, so close inspection is necessary.
The most general way of determining deterioration of the basic exhaust parts (pipes, muffler[s], resonator[s], brackets, clamps and hangers), is to visually look at them, and also to tap them with a small metal ball peen hammer or similar tool. The sound of solid metal should be heard in the pipes, and the muffler(s) and resonator(s) should have a solid sound, without rattles, or excessively flaking metal.
Finding a system that is nearing failure gives the customer a "heads up" that may influence their decision as to where the eventual exhaust system repairs will be made.
If the customer complains about vehicle performance that includes a "no start" condition, poor fuel economy, sluggish acceleration, off-idle hesitation or power loss at around 50 miles per hour, there could be an exhaust restriction. Quite often an engine will start and idle in a normal manner when the exhaust is restricted. Once the throttle is opened, the excess backpressure caused by the restriction prevents normal engine operation. The converter may be the culprit, with a melted and/or clogged substrate. Before replacing the converter, further tests are required. Note that misfires can cause a converter to overheat and meltdown. An overheated converter is usually caused by an ignition or compression problem, requiring diagnostic testing. Engine problems must be corrected before the converter is replaced. It is also important to remember that simple failure of an emissions test is not grounds to replace the converter without establishing "legitimate need".
Inspect the listed components below for deteriorated or broken welds, cracked joints and corrosion damage. Be aware that damage or deterioration in these areas could allow exhaust fumes to enter the passenger compartment of the vehicle.
- Exhaust pipes
- Exhaust gaskets and seals
- Catalytic converters
- Tail pipes
Inspect the exhaust system for any of the following concerns that could cause poor sound quality or vibration:
- Leaking joints, seals, or components
- Binding of components
- Contact with vehicle undercarriage
- Excessive vibration or movement of the exhaust system
- Torqued or twisted exhaust system hangers or brackets
Exhaust system contact with the undercarriage of the vehicle can be a major concern to driver and passenger comfort. To help resolve this concern, check the system for the following conditions:
- Loose or damaged pipes
- Loose or damaged clamps and brackets
- Loose or damaged heat shields
- Loose or damaged exhaust seals/connections
- Loose or damaged flange bolts
Be aware that alignment of components is critical to proper exhaust system operation. There should be no excess stress or pull on mounting hardware when the exhaust system and vehicle are in their at-rest position.
Inspect the catalytic converter for any concerns. Lightly tap the converter with a rubber mallet or thump it with a properly gloved hand. Use Caution: Remember the unit may be hot! Use a heat resistant glove for protection. On a properly functioning monolithic or honeycomb converter, the sound should be a solid thump. If you hear rattling noises or banging around, it possibly means that the substrate has broken-up. The unit will have to be removed and visually inspected to confirm this problem. Check that the noise is not the result of loose shields or other loose components in the exhaust system. If you confirm that the substrate has broken-up, check pipes and muffler for loose pieces which may have become lodged in these components.
Inspection of the exhaust components on modern vehicles is much more technical than in the past. As an example, proper air fuel ratio or "fuel control" is required for modern converters to operate efficiently. The engine fuel management system relies on accurate MAF, oxygen, and air fuel sensor readings. Very small leaks in the intake, or the exhaust system, can dramatically impact the accuracy of these readings. Key points of interest should include:
- Inspect for small "pinhole" leaks in welds around the O2 sensor ports.
- Inspect all connections, and exhaust components, upstream of the converter for small leaks.
- Inspect all connections within 3' downstream of the converter outlet for small leaks.
- Inspect intake boot connections between the MAF sensor and the intake manifold.
- Inspect the intake manifold for proper sealing.
Most technicians rely on a smoke machine to help locate these small leaks, but there are a number of other methods which also work well. Keep in mind that the exhaust system should be inspected for leaks when it is cold and again after it has been warmed to normal operating temperature.
Once all intake and exhaust system leaks are resolved, proper MAF, O2, & AFR sensor operation should be verified. It should be noted that both pre and post O2 sensor operation are important to converter efficiency. Most technicians find that graphing readings from these sensors are helpful in identifying issues. Since converters are designed to store oxygen, post converter O2 sensor readings of 450mv or higher will typically indicate high converter efficiency.
When working to resolve converter efficiency concerns, technicians need to be aware that a perfectly balanced air/fuel mixture (technically referred to as "Lambda of 1") is one of the main requirements for high converter efficiency. Using a recently calibrated 4, or 5 gas analyzer is one of the fastest and most effective ways to calculate Lambda.
If a converter has failed, the technician should identify and resolve the root cause of the failure prior to replacing the converter. The main causes of converter failures are:
- Overheated, melted, or broken substrate (typically due to engine misfire)
- Coated/oil fouled substrate (Carbon build-up, excessive oil consumption, internal coolant leaks, improper fuels or additives, & use of non-converter safe sealants)
- Structural damage (Impact damage, corrosion, stripped threads)
- Thermal shock - (cold quenching of a hot converter)
TEMPERATURE DIFFERENTIAL TEST
The conversion process produces heat. So, exhaust gases entering the converter should be cooler than the gases exiting the converter.
The temperature differential test measures the surface temperature at the inlet and outlet bushings of the converter. If conversion is taking place, then the outlet bushing reading should be higher than the inlet bushing reading. However, different pipe wall thicknesses and corrosion, along with different heat transfer rates, may cause inaccurate results. So, the U.S. EPA recommends that this test be used only to prove that a catalyst is good. Make sure that the engine is fully warmed up and running. Make sure the heater is OFF. Using a pyrometer or infrared thermometer read and note the temperature of the pipe just ahead of the converter inlet at the weld ring. The weld ring is the point where the inlet pipe is welded to the converter body. Read and note the temperature at the weld ring of the outlet pipe. Calculate the difference between your readings. If the outlet reading is higher than the inlet reading, you can be assured that at least some conversion is taking place. On well-tuned, newer vehicles, the catalyst can be fully functional at only a 20° F difference.
GAS ANALYZER TEST
This test measures the levels of HC, CO, CO2, NOx, and O2 gases coming out of the tail pipe. Using this data, you can isolate problems under the hood and under the vehicle.
Excessively lean, or rich mixtures are beyond the capability of the catalyst to convert. Even if the converter is working to specifications, measurable levels of pollution will be noted at the tail pipe under these conditions. The validity of this test to isolate a converter problem, depends on a properly tuned and operating engine. Fortunately, a gas analyzer can first be used to locate engine problems.
Following manufacturer's instructions connect analyzer to exhaust pipe, then read and note the levels of Oxygen, Carbon Dioxide, Hydrocarbons, NOx and Carbon Monoxide produced by the vehicle. Compare your data with the Primary Causes of High Gas Levels, listed below. The chart will help isolate engine emissions control problems. There may be more than one engine related problem, in addition to a failed converter.
TESTING FOR AN EXHAUST SYSTEM RESTRICTION
These tests are used to determine if blockages exist in the exhaust system, creating excessive backpressure. The following test can help determine the location of an exhaust restriction.
ENGINE VACUUM TEST
The intent of the vacuum test is to determine if there is a blockage or restriction in the entire exhaust system. It may or may not indicate converter problems. This test can be performed using any suitable engine vacuum source. After the vacuum gauge is connected to a vacuum source, readings are noted at idle and then again at 2,500 RPMs. As engine speeds vary, readings should initially drop slightly, then rise to within 2 to 3 inches of the vacuum level established at idle. A large drop of 8-10 inches of vacuum typically indicates an exhaust restriction. Erratic swings of the vacuum readings may indicate periodic blockages caused by loose components temporarily blocking the exhaust system. Remember that vacuum levels are also affected by factors other than exhaust system restriction, including valve and ignition timing.
If a vehicle fails the engine backpressure vacuum test, you can pinpoint the component in the exhaust system causing the concern by measuring backpressure at different points in the exhaust system. These measurements can typically be made through the O2 sensor ports. Start by inspecting the system for crushed, bent, or otherwise restricted exhaust pipes. Replace or repair as required. If the system passes the visual inspection, remove the most rearward O2 sensor. This sensor is typically located directly after the converter. The use of heat or penetrating oil and a back and forth motion may help if the sensor threads are rusty. Most O2 sensors use an 18mm-threaded port, so one adapter fits many applications. Install adapter in the O2 port and tighten to manufacturer specifications. Then connect the gauge hose to the adapter. Begin by reading, and taking note of, pressure at idle and at 2500RPM. On most vehicles, at idle back pressure should not be higher than 1 psi. At 2500 RPMs the reading should not be higher than 3 psi. If the readings are high at this point, the blockage probably exists downstream of the test point, which typically means it is in the muffler or resonator. If the back pressure is okay at this location, move to the O2 sensor just ahead of the converter and retest. If the back pressure is high at this point, the converter is causing the issue. If the O2 sensor ahead of the converter tests okay, then the restriction is most likely in the y-pipe assembly, or there could be an internal restriction in the air gap pipe located between the exhaust manifold and the converter.
- Verify the application in the catalog. Do not overlook any footnotes that will influence installation.
- Intermingling of competitor's parts could cause a system to not fit the application properly. The reason for this is that many OE exhaust systems leave the factory as a single welded unit - there are no breaks in the system. For aftermarket purposes, this type of exhaust system is sometimes impractical. To make installation easier for the installer, aftermarket manufacturers produce replacement parts in smaller pieces that are easier to handle and install. Not all exhaust system manufacturers break the OE system at the same points. This difference in connection points sometimes creates complications when mixing aftermarket components from a variety of manufacturers.
- Always check parts for damage before installing them. Sometimes parts are damaged during shipment. Minor handling damage to pipe ends or muffler bushings can be easily repaired.
- If a new part is being used with existing parts in the system, make sure the old parts are in good condition. A broken pipe end could cause the system to be too short.
- If the muffler and pipe were originally welded assemblies, and the catalog does not recommend replacing both parts together, be certain the cut-off was made within one-half (1/2) inch of the weld.
- Make certain that the vehicle has not been modified in any manner that would void the catalog listing. A frequent example of this are engine swaps, header installations, etc.
- Check to see if the part is designed in such a manner that the inlet end can be confused with the outlet, and the part could be installed backwards. Most Walker mufflers are marked to designate the outlet end and it is easy to see if the muffler is positioned correctly.
- Is the muffler rotation correct? Many mufflers can be installed 180 degrees rotationally incorrect, causing fit problems with the rest of the system.
- Check the silhouette of the vehicle. Does the exhaust system hang too low? The top of the exhaust pipe should not be visible when viewing the vehicle from the side.
CAUTION: MINIMUM CLEARANCE FOR ANY PART OF THE MUFFLER TO THE UNDERBODY OF THE VEHICLE SHOULD BE AT LEAST ONE (1) INCH.
- When a heavy-duty clamp is required, the clamp should be torqued down to approximately 40 foot-pounds to ensure a tight seal. If the seal is not airtight, loosen the clamp, rotate it 180 degrees and then tighten it again.
Any and all of these conditions can lead to complaints concerning excessive exhaust system noise.
- Ensure application matches the catalog listing - year, make, model, engine - and that all pertinent footnotes have been considered. Do not accept "broad" application information in the catalog listing header. When receiving a noise complaint, obtain the exact model, make, year and engine information.
- Note catalog listing or bulletins to see if any of the parts have been superseded. In the catalog, these are noted with an "or" listing. The alternative part may have better acoustical performance. Also, ensure that part numbers have not been transposed.
- If possible, a record of exhaust system complaints and solutions should be maintained to determine if the problem is really with the part, or if the specific vehicle or installation procedure could be at fault.
- Using caution (parts will get hot), move the exhaust system from side to side, and up and down, to determine if the system is hung securely and not contacting the undercarriage of the vehicle.
- Start the engine in an open area and allow it to warm up. Once the engine reaches operating temperature, listen to the exhaust tone at different rpms and loads to determine when the undesirable tone is most noticeable.
- If the noise is a popping sound at the tail pipe end, check under the hood and determine if the engine is equipped with an Air Injection Reaction (A.I.R.) pump. If an A.I.R. pump is present, disconnect the belt from the pump. If the noise ceases, the valve in the pump is faulty and should be replaced. If the vehicle is not equipped with an A.I.R. pump, the problem could be in the engine.
Should further investigation be required, place the vehicle on a lift, raise the vehicle, and inspect the exhaust installation. Check for the following:
- Contact with undercarriage
- Broken hangers and insulators
- Loose/leaking joints
- Muffler and resonator installed backwards
- Missing converter
- Resonator substitute pipe in use
- Worn out heat riser
- Engine with faulty timing
- Use of an "economy" or performance muffler
- Alterations to the original installation