Whether you’re a muffler/undercar specialist working on exhaust systems every day or a drivability tech working on them occasionally, you’ve inevitably banged your head on a few tough exhaust system problems and technology changes haven’t made the exhaust repair game any easier. This article will bring you up to date with technology, tools and techniques you need to get today’s exhaust systems running quiet, running clean and passing OBD II monitors/state emission tests.
If you think your job is hard, image the life an exhaust system component. Exhaust systems basically have three tough jobs to do.
Exhaust System Job 1 – Move air
It’s all about volumetric efficiency. Engine designers want to make the engine breath as efficiently as possible but also keep things quiet. Pressure waves from exhaust if allowed to expand, help to provide cylinder ‘scavenging’ in order to clear the combustion chamber of exhaust. The less residual exhaust in the cylinder, the more room there is for power contributing oxygen on the intake stroke that follows the exhaust stroke. Tuning exhaust header/manifold shapes and lengths is just as important as tuning the intake manifold runners. Crossovers have traditionally helped. Since each cylinder pushes a positive pressure wave of exhaust gas one exhaust stroke at a time, an exhaust pressure pulse from the left side of a V engine passes the crossover tube in the exhaust, creating a negative pressure (vacuum) wave. This vacuum pulse helps to pull out the exhaust pulse that just began its voyage out of the cylinder on the other bank of a V-configured engine. Adding to those exhaust ‘puffs’ of positive pressure are just as many ‘whiffs’ of negative pressure.
Exhaust System Job 2 – Quiet things down
Mufflers are obviously needed for the street but will limit the exhaust system’s No. 1 job — Volumetric Efficiency (VE). Reduction of the muffler’s back pressure (resistance to flow) most always means an increase in sound decibels. OEM mufflers typically use a multiple chamber approach with some insulation inside to help dampen the sounds that don’t get cancelled out in the chambers. Sound waves bouncing off the walls of the various chambers crash back into the incoming sound waves causing a cancelling effect. Aftermarket mufflers such as the Cherry Bomb muffler I installed on my first car (a 1961 Chevy) are still around in much the same form as they always have been; straight mufflers with a single internal pipe with small holes that allow exhaust gasses (and sound) to move outwardly into the surrounding insulation material lowering noise levels somewhat. Modern aftermarket exhaust system manufacturers have made great advances in these types of performance mufflers to afford better VE and still have decent noise levels. Variable displacement engines take on a whole new exhaust signature when a pair of cylinders from each bank (GM and Dodge) or an entire bank of 3 cylinders on a V6 (Honda) are shut off. For this reason, manufacturers like Tenneco (Walker Exhaust) have turned to variable tuning of the exhaust to offset the different/additional noises experience in these fuel sipping variable displacement modes. Corvette Stingrays beginning in 2014 have been equipped with this new technology. It works similar to variably tuned intake manifolds by controlling throttle butterflies near the muffler outlets in order to change the flow and sound note of the exhaust.
To help noise matters further, many newer vehicles have seen greater popularity with active sound management inside the cabin. Honda’s Variable Cylinder Management (VCM) uses an active noise cancellation via the OEM audio system for several years, an idea that has caught on with other OEM. Add a microphone inside the vehicle to monitor road noise frequencies, exhaust drone, etc. then create equal but opposite (delayed) sound waves to cancel out objectionable noises. I recently drove a new Chevy Equinox V6 equipped with an active noise cancellation system and there was a distinct improvement in cabin noise.
The catalytic converter has been around for over 40 years now. The definition of ‘catalyst’ is “causes or accelerates a chemical reaction but in the process is not effected by that reaction. Image courtesy: Walker Exhaust
Catalyst expectancy is similar to that of any other major component of the vehicle. It has been said that “most cats don’t die natural deaths – they are killed by what comes into them.” Engine oil, silicates from coolant and WAY too rich exhaust mixtures can make the best converters fail fast! Physical damage (broken substrate) can be detected by a modest wrap with a compound / rubber mallet but beware – smack on too hard and a good one is now broken! Photo: Dave Hobbs
Want more pressure in exhaust system leak testing? Some creative techs use a leaf blower sealed into the tailpipe outlet and go looking for the leaks! Photo courtesy: Rusty Savignac at Paxton Garage
Toyota Corolla had all the symptoms of a restricted catalytic converter. The pressure of the leaf blower pushing air stopped at the plugged up muffler. Photo courtesy: Rusty Savignac at Paxton Garage
Relax – nobody shaved their grandmother’s head! This is insulation that has came loose from the 92 Toyota Corolla’s plugged muffler. Photo courtesy: Rusty Savignac at Paxton Garage
Exhaust System Job 3 – Get the catalytic converter to light off
This requirement part is obvious – converters need to be 350 - 450 deg. F (on average) to light off (begin cleaning up exhaust) and will operate without damage somewhere in the 450-850 deg. F. range. They may see temps as high as 1200 deg. F. under adverse conditions. Sustained temps very much above that can lead to substrate failure. On the cold side of the discussion the age-old problem with converter light off is the time it takes. By the time you get that first couple of miles driven prior to light off you’ve already polluted more than your next 50 miles of driving! This is why the “pup” converters (downsized) have been split into 2 sections with the first being moved very close to the exhaust manifold outlet. Exhaust pipes and manifolds with an inner wall surrounded by an outer wall were not uncommon several decades ago. One failure sometimes experienced is due to thermal shock. Sudden cooling (driving through a flooded spot in the road, for example) can collapse the inner wall of the pipe. There is a simple test for this passed down to me from my dad who never played golf but always had a golf ball in his tool box. You roll the golf ball into the exhaust pipe inlet (removed from the manifold) and it should come out the other end (removed from the cat/muffler). If there was a double wall pipe with a collapsed inner wall the ball won’t pass through. Double wall exhaust pipes seemed to drop off the radar for a few decades until OBD II began to evolve and the need to get the cat lit off quick required some drastic measures. The extra wall provides insulation to keep hot exhaust gasses hot during warm up so they can pass that heat to the converter for quicker light off.
The modern three-way converter uses advanced catalyst chemistry to store and release O2 when coupled with a closed loop fuel system. This system utilizes one or more O2 sensors to oscillate the fuel mixture between lean and rich conditions. These oscillations are the reason you see a good O2 sensor switching from rich to lean in a very rhythmic fashion. The oscillating rich/lean fuel commands combined with the O2 storage and release on the catalyst surface, allows for the best oxidation/reduction of all three emissions IF the A/F ratio is right and there are no exhaust leaks. Remember those positive and negative pressure (puffs and whiffs) associated with multi cylinder engines explained in exhaust system job 1? Those “whiffs” pull in ambient O2 concentrated air that all but eliminates the chances for accurate fuel control and OBD II cat efficiency monitor accuracy.
Catalytic Converters – Exhaust system king of cost!
Monolithic three-way catalytic converters with a ceramic substrate have been the norm for many years. They’re comprised of an outer heat shield, stainless steel outer shell and a porous ceramic “honeycomb” substrate with a wash coat containing noble (expensive) metals such as platinum, rhodium and/or palladium. Surrounding the extremely dense ceramic substrate is an insulation mat that resembles a blanket. The mat protects the ceramic substrate (sometimes called the ‘brick’) from physical shocks, allows for some expansion and contraction and insulates the shell from some of the heat produced inside the converter. Gold (in very small doses) is being used in some newer converters which explains why some diesel oxidation converters on over the road trucks have as much as $1,000 of precious metals in them and explains why the crackhead across town used a Sawzall to cut the cat off your customer’s Chevy Malibu last night!
Three-way converters are named as such due to the 3 pollutants they address. The first pollutant is CO or Carbon Monoxide. CO is a byproduct of incomplete combustion. The second pollutant is HC or Hydrocarbons. HC is basically unburned fuel. CO and HC are formed at higher levels when the ideal stoichiometric mixtures (14.7:1 for spark ignited engines) run lower (richer). The third pollutant is Nitrogen Oxide or NOx. NOx is formed when Nitrogen combines with Oxygen due to high temperature combustion conditions. Those same conditions often result in spark knock/detonation. The easy way to remember when NOx is produced is to remember that when you hear “knocks” (ping) you are making NOx. NOx is formed at higher levels when the mixture goes higher (leaner) than stoich. NOx is often more likely to occur with overheating, lack of proper EGR flow/variable valve timing operation or insufficient fuel octane. All of these add air to the fire and your experience adding O2 to your cutting torch flame reminds you of what air does to the temperature of a fire! Of course dripping injectors, restricted air intake, skewed coolant sensor, (showing colder than it really is) and too much EGR can be the causes of air fuel mixtures lower than stoich (richer).
In order to achieve as close to 100 percent 3-way converter efficiency as possible the job of the engine management system (including the O2 sensors) is to maintain stoich under most all conditions. This will keep the converter from loading up with excessive HC that it can’t oxidize and prevent it from running so hot the ceramic substrate turns into Mt. Saint Helens.
Turbochargers boost VE for increased performance but can also be a source of exhaust leaks and even an exhaust restriction in rare instances. Make sure any exhaust diagnosis includes a thorough look at the turbo. Photo: Dave Hobbs
Using a smoke machine has become a very popular diagnostic tool for finding exhaust leaks. Slide the cone shaped adapter into the exhaust pipe outlet, hook up the smoke machine and look for smoke coming out along the length of the exhaust system. Photo Courtesy: Automotive Video Innovations
This C6 Corvette muffler cutaway shows the holes in the inlet’s baffle that allow sounds to move into the first chamber then on into the second chamber to be absorbed by the muffler acoustical ‘insulation. ’ Exhaust finally passes on out of the outlet as certain frequencies of sound waves cancel each other out. Photo courtesy: General Motors
This XR-1 performance muffler from Borla does not use chambers or baffles that restrict exhaust flow. Performance exhaust engineers analyze exhaust notes in order to tune the exhaust system to allow for the perfect mix of flow and sound to suit various customer tastes. Objectionable noises such as a drone within a particular RPM band can be tuned out. Photo courtesy: Borla Exhaust
Flex couplings like the shown here on this Prius are common and prone to leakage. Inspect, jiggle, inspect again. Don’t just listen for leaks. Make sure you use a smoke machine or effective alternative method to test for a leak when you experience a PO420 cat efficiency code (false air coming in via the leak can make a PCM think the cat is bad when really all you have is an O2 sensor breathing in fresh air instead of true exhaust samples. Photo: Dave Hobbs
GEN III Prius models (2010 and newer) use an exhaust heat recirculation system to aid in cold engine emissions. It takes the place of the insulated tank hot coolant storage systems. The line at the bottom is the cooler of the two lines while the line on top (partially obstructed in photo) is the hotter return line that has the warmed coolant that has travelled around the recirc unit’s outer perimeter. The actual main catalytic converter is in two stages. Since the exhaust heat recirc system is in the middle, it almost appears as if there are 3 catalytic converters! Photo: Dave Hobbs
Gen III Prius - Hot and cold running water to the cat?
That’s exactly what this exhaust heat recirculation system has on GEN III Prius models (2010 and newer) to aid in cold engine emissions. It takes the place of the insulated tank hot coolant storage systems used in Gen II models (2004-2009). One line at the bottom side of the exhaust heat recirculation unit is connected to a temperature actuated mechanical control valve. The valve forces exhaust to reroute within the outer shell of the recirc unit past the coolant passageways inside the unit. The other coolant line continues the circulation process of coolant back up to the engine. The unit is situated between the section of the converter closest to the exhaust manifold and the lower section downstream. Don’t confuse this unique emissions gadget with the early Gen I Prius (and earlier Lexus models) which used an electronically control vacuum solenoid to control a diaphragm that rotated a mechanical valve towards the front of those converters in order to route exhaust gasses on cold starts into an outer chamber of the converter containing hydrocarbon adsorbers. This also lowered cold start up emissions keeping those vehicles in the “AT PZEV” (Advanced Technology Partial Zero Emission Vehicle) club.
Diagnostics - Exhaust restrictions
Besides the collapsed inner wall of a dual wall exhaust pipe restricting the proper flow of air movement out the tailpipe there are a host of other causes and diagnostics techniques to keep in mind. The muffler’s internal structure can come apart or the insulation can dislodge and plug it up. Most mufflers are directional too, so don’t forget to check to see if the last tech working on the vehicle put a muffler on backward. When the converter substrate loads up with carbon, soot and other contaminates it can increase the exhaust system’s back pressure first to the point where power is reduced (runs well at idle low speeds but can’t get up to highway speed) and then progress to the point of restriction where the engine won’t start. If you can’t get exhaust out, you can’t intake air in.
1. Vacuum Gauge
The classic vacuum gauge test is still my first pick if I can get to a source of manifold vacuum. The simple process is to connect the gauge, record the vacuum at idle then rev the engine up to 2000 RPM and hold there for a few seconds. The initial throttle opening will naturally cause the vacuum to drop but as the engine gets up to the 2000 cruise RPM the gauge should recover and stabilize at a reading higher than the idle speed reading. A gauge that drops and stays down indicates a possible restricted exhaust and the cue to go on to step 2. Passing this test isn’t a guaranteed good health test of the converter and muffler but failing the test IS your justification to get some additional diagnostic time approved to perform an exhaust back pressure test.
2. Back Pressure Gauge
Most engines will allow you to remove the pre-cat O2 sensor in order to screw in a backpressure tester. Typically, 1.5 PSI or less means NO restrictions. 1-5 PSI to 2.75 PSI means MAYBE (consult OEM service info). 2.75 PSI or more means you have a restriction. Got excess back pressure? Reinstall the pre-cat O2 sensor and remove the post cat O2 sensor and install the back pressure gauge downstream from the cat. If backpressures are way down, the converter was the culprit. If the pressure gauge reads high, the restriction is downstream in the resonator/muffler.
3. Compound/Rubber Hammer
Giving the converters, resonators and mufflers a sharp “rap” with a compound hammer to locate a rattle is simple enough. If something is banging around in a converter when you smack it that something could be intermittently causing an exhaust restriction. There is one caution however. Do NOT bang too hard! There is no spec for how hard is too hard. How hard do you hit an electronic module to test for intermittent circuit board connections? According to Joe Bacarella at Tenneco/Walker Exhaust there have been numerous cases of techs hitting converters hard enough to make a perfectly good ceramic substrate a broken substrate. The converter should NOT rattle nor should it sound hollow. Hollow means the substrate is gone.
The Gen III Prius Exhaust Heat Recirculation system is shown in its entirety here. Exhaust travels through the reduction catlyist (not shown) first before going through the exhaust heat recirculation unit. Then the exhaust makes it way on downstream into the oxidation catalyst before heading to the resonator and finally the muffler. Drawing courtesy: Toyota
GEN III Prius model Prius exhaust heat recirculation module in cold coolant mode (left) and warm coolant mode (bypass mode shown on right) courtesy of the mechanical temp actuated valve. Drawing courtesy: Toyota
GEN I Prius models (2001-2003) and some older Lexus hybrids use an exhaust hydrocarbon adsorber for cold starts. A vacuum / electrically controlled ‘flaper’ reminiscent of an old heat riser is used to direct exhaust gasses into an outer section of the converter containing a HC adsorber (think Evap canister) to lower emission on cold starts. These ‘flappers’ can get rusted up and stuck in certain areas prone to rust, sea air or road salt. Drawing Courtesy: Toyota
Diagnostics - Exhaust leaks
Exhaust leaks can at times be very difficult to find. Cracked pipes/manifolds, porous welds, bad donut gaskets, loose clamps, loose mounting bolts and warped manifolds are just a few common leak sources. Beyond using your ears and/or automotive stethoscope that are numerous methods out there to find leaks. A gas analyzer works to locate less than obvious leaks if you have one. Most full service shops have a smoke machine for evap leaks which can also be used for exhaust leaks. Since smoke machines put out very low pressure you may want to supplement this tool with a leak detection machine designed for turbo vehicles. Smoking an induction system with higher pressures makes for a realistic test on turbo and supercharged engines. If you don’t have a smoke machine, there are some imaginative techs that have used everything from a leaf blower fastened to the exhaust to shop air (funneled into the exhaust pipe via a smoke machine cone exhaust adapter). Of course you’ll need to couple your rubber cone and blow gun to the exhaust with a pressure regulator dialed down to no more than 10 PSI. That’s enough for realism and not too much to create leaks that were not there before you worked on it. Adding some Sea Foam into the intake while the engine is running will create some smoke (don’t overdose it though). Keep in mind besides audible leaks (and smells) exhaust system leaks can throw off the engine management system’s O2 sensor readings.
Diagnostics - P0420 / P0430 Catalyst Efficiency DTCs
Engine management systems use the Pre-Cat and Post-Cat O2 sensor activity to pass or fail catalyst efficiency DTCs. (P0420 and P0430) Exhaust dilution via exhaust leaks make the O2 sensors prone to tell the ECM there is a lean fuel mixture problem or false air. Or the ECM might see the false O2 sensor signatures as a sign the cat is not working. In addition to checking for exhaust leaks, here are a handful of diagnostic tips to help you with these codes.
1. Do Your Electronic Research
Pulling DTCs (all of them) and researching the web (TSBs, software updates, tech tips, etc.) become the essential first steps on cat efficiency diagnostics.
2. Check for Proper Fuel Control
The converter’s peak operating efficiency range is 14:62 to 14.77 (Lambda .995 to 1.006) which means if the A/F is not right the cat will struggle to work correctly. Scoping the O2 sensors, observing fuel trim numbers, looking for false air and mixture imbalances (1 lean cylinder creating 3 rich cylinders after fuel trim) can wreak havoc on a converter’s performance. It is easily spotted on an ignition scope pattern, however. Obviously any ignition misfire is your ‘Job 1’ before installing the replacement cat. Air and fuel w/o spark means a cherry red converter, a misfire symptom and flashing MIL to go with the P03XX DTC. This is a HUGE killer of cats. Other misfire causes are less damaging to the cat. Due to the chemistry of air / fuel mixtures, fuel trim will go up slightly as the unburned O2 from an ignition misfire is sampled by the O2 sensor. On the other hand, a lean misfire will also result in higher fuel trim adjustments. Both misfires have unburned O2 sampled by the O2 sensor. Remembering this will help you solve misfire problems quicker.
3. Contamination (poisoning the cat)
As discussed previously, always make sure the oil consumption, coolant leak or customer’s fuel additive addiction is fixed FIRST before a replacement converter is installed.
4. Make sure your replacement converter is correct
Quality and construction often go with price and as you’ve probably experience before, some vehicles just are not going to stop setting the P0420. Some lesser expensive replacement converters literally have less noble metal in them. They may pass a tail pipe test but still fail the OBD II efficiency test. Today’s exhaust systems are neither simple nor cheap. As Ben Franklin once said, “The bitterness of poor quality is remembered long after the sweetness of low price has faded from memory.”
The new Corvette Stingray allows the driver to change inputs such as Cylinder Deactivation, Active Exhaust, Magnetic Shocks, Stability Control System, etc. via the Drive Mode Selector. Photo Courtesy: General Motors
The exhaust and uses a pair of butterfly valves to adjust the sound when the cylinder deactivation system is in V4 mode. Optionally, a dual-mode active exhaust with two extra electro-vacuum actuated valves to open for even MORE noise! Photo Courtesy: General Motors
