Although catalytic converters. Keeping the

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1 Keeping the BY JEFF MASTERMAN The catalytic converter isn t only an exhaust and emissions component, it s now an intricate part of OBD II systems. Here s how to keep your customers cats purring along, and what to do when you suspect they ve seen better days. Although catalytic converters have been around a long time, they ve received very little attention by the average tech working in a shop outside of an enhanced emissions area. Where I live, for example, it seems the only converter work that s done is to yank em off and dual em out! This is changing with the spread of enhanced emissions test programs, but the primary issue that will bring the most attention to converters across the country is OBD II. Remember, the OBD II system monitors the catalyst for efficiency, so a worn out unit will turn on a vehicle s MIL. As a result, technicians in areas with no emissions test program in place are now replacing converters as a matter of routine. If we set aside the converter s ability to reduce emissions for a moment, we re left with a component that should not leak, rattle or restrict the flow of exhaust. In fact, these criteria must be met wherever a vehicle is tested. Any undercar tech should be adept at finding exhaust leaks, so I ll move on to the rattle test. The converter contains a ceramic substrate that can melt, crack or break into pieces. When striking the unit s inlet or outlet pipe with a rubber or plastic hammer, listen for evidence of loose chunks inside. (Don t strike the cat s shell, or you could become the source of the damage yourself!) If damage is discovered, don t forget to determine how much of the 28 June 2001

2 This backpressure gauge has all the features you ll need for checking out exhaust restrictions. The adapter fits in the O 2 sensor hole. substrate has come out of the converter...and find out where it went! A muffler plugged with converter innards can cause as much restriction as a busted up cat. A manifold vacuum test will help provide clues about a restriction in the exhaust, but measuring backpressure is the only way to confirm it. To perform a manifold vacuum test, connect the gauge to a large vacuum source, like the brake booster, and allow the engine to idle. Record the Photos & screen saves: Jeff Masterman

3 Fig. 1 vacuum reading, then raise the rpm to around 2500 for another recording. The vacuum reading should be the same or higher at Next, hold the throttle there for a minute or so, to see if the vacuum starts to drop. Also snap the throttle a few times and check for a rapid response on the gauge. If the vacuum drops at 2500 rpm or the gauge responds slowly to changes in throttle angle, it s time to hook up the backpressure gauge. Get the Right Gauge There are several sources for these gauges. Look for one that s calibrated in low pressure, because mechanical gauges are generally most accurate near the center of their range. A 0-50-psi gauge is unsuitable for this test. Another consideration is where the gauge gets tapped into the system. The type used to research this article screws into the oxygen sensor mounting boss after the sensor is removed. This is an ideal test location and requires no pipe drilling or other modifications to the exhaust system. The gauge should have high-temperature tubing to prevent damage if it comes into contact with a hot exhaust component. Once the backpressure gauge is installed, bring the vehicle to operating temperature. With the engine idling, the pressure will usually be below 1 psi on a good system. A backpressure reading over 1.5 psi is cause for concern. Next, bring the rpm to 2000 and check the pressure again. A good system will usually show less than 2 psi. If the pressure is over 3 psi, the catalyst is most likely going south. If the cat fails the pressure test, save time by checking the pressure again, this time with the outlet pipe disconnected from the converter. If there s a significant drop in pressure now, then the substrate has probably made its way into the rest of the exhaust system. We all know it s easier to sell a job once, so now is the ideal time to report this to the customer. Note that techs have seen cases where a restriction gets worse after they ve driven the vehicle. So if a customer comes in with a won t go over 30 mph complaint and you drive the vehicle down the road with good results, don t assume the problem is behind the wheel! Run or drive the vehicle with the backpressure gauge connected until you re sure the problem lies elsewhere. A Chemistry Lesson Let s get back to the purpose of the catalytic converter. A cat is designed to cause a chemical reaction that gets 30 June 2001

4 Fig. 2 rid of harmful combustion byproducts. The newer cats are called threeway catalysts because of their ability to lower the concentration of three different gases at the tailpipe. It takes some pretty fancy chemistry to make this happen, which can be the subject of another article. However, I should mention that the feedback system must be in control and the air/fuel ratio must be very close to stoichiometric (14.7:1) for the three-way cat to do its job. The exhaust gases that are controlled by three-way catalysts are hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NO X ). HC is raw, unburned fuel that goes through the engine intact. It s generally caused by a misfire. CO is partially burned fuel left over when a cylinder runs out of oxygen during combustion. A rich mixture is usually to blame. High combustion pressures and temperatures that cause oxygen molecules to attach to nitrogen cause NO X formation. An inoperative EGR valve or a lean mixture can also cause increased NO X levels. HC and CO are eliminated by the cat through a process called oxidation; in other words, they re burned up. This process requires that oxygen be either present in the exhaust gas or pumped into the converter. NO X presents a different problem. It must be reduced or split apart. This reduction process requires low oxygen levels in the catalyst. Notice something interesting here? It looks like these two processes are at odds with each other. We need relatively high oxygen levels for oxidation and low levels for reduction. How does one catalyst provide both? Take a look at a typical O 2 sensor data graph in Fig. 1 on page 30. Notice how the sensor switches rich/lean about once per second? This action is not random; the PCM drives the mixture this way to create periods of high and low oxygen content in the engine-out gas to the converter. When the mixture is slightly rich, the oxygen level in the gas is low; when it s lean, the oxygen level increases. The last part of the process that makes a three-way cat work is a compound in the converter that stores and releases oxygen. Applied in a wash coat over the substrate, the Oxygen Storage Component, or OSC, is what allows the converter to reduce NO X during the low-o 2 (rich) period without causing an increase in CO or HC emissions. It s also used by the OBD II system to monitor catalyst efficiency. With O 2 sensors mounted before 32 June 2001

5 Fig. 3 and after the catalyst, the PCM can determine whether or not the catalyst is storing and releasing oxygen. Fig. 2 on page 32, a graph of pre- and postcatalyst O 2 sensors, demonstrates very little activity at the rear sensor due to the OSC in the converter. Some manufacturers test an OBD II converter at a steady cruise, while others perform the test at idle. If it s done at idle, the PCM quickly switches the mixture from rich to lean. Look at Fig. 3 above. Notice that when the mixture goes lean, the precat O 2 sensor switches first, with the postcat sensor switching after the stored oxygen is depleted. If the switch points between the two sensors occur at the same time, chances are the OSC in the converter is shot. The temperatures needed to destroy the wash coat are higher than the melting point of the substrate. Because of that, the OBD II system infers that if the OSC is okay, the substrate is, too. A visit to the Emissions Forum on iatn will tell you that this is not always the case, however. Cars that fail an emissions test with the MIL off are not uncommon. Test to Be Sure As mentioned previously, a three-way catalyst requires closely controlled oxygen levels for proper operation. If the mixture is lean, combustion chamber conditions will favor the production of NO X. In addition, the catalyst will be flooded with oxygen. As a result, tailpipe NO X levels will increase dramatically. If the mixture is rich, there ll be an increase in CO and HC levels, and there won t be enough oxygen for the catalyst to oxidize them. Always test the O 2 sensor with a lab scope to be sure it has the proper response. Be aware that even if the sensor passes a snap test in the bay, there are cases where it ll stop switching or won t produce a proper signal at cruise. After you test the O 2 sensor, check fuel trim with your scanner to see how far it is from center. If the PCM has to add or remove a lot of fuel to achieve correct O 2 sensor response, it s operating on the edge and may fail to control the mixture under certain conditions. Also, test the EGR system dynamically to make sure that not only the valve and controls operate, but that there s also actual flow of exhaust gas into the intake when it s open. If timing is adjustable, set it to spec to reduce NO X. If an engine is equipped with secondary air injection, test it for proper 34 June 2001

6 Fig. 4 Graph: Danny Iwama operation, as well. Secondary air, or AIR, can cause elevated NO X levels if it s flooding the catalyst with oxygen under the wrong conditions. These systems are usually disabled after the engine reaches operating temperature. If the AIR system is pumping all the time, it can overheat and damage the converter, as well. Temperature-testing a converter is commonly used, but it s not always conclusive. Always follow up with another test before condemning the catalyst. To perform the test, run the engine until it s at normal operating temperature, then shut the engine down and ground a plug wire. Restart the engine, run it slightly above idle, then take temperature readings at the converter inlet and outlet. There should be a significant temperature rise across the converter. Caution: Do not run the engine for more than a few minutes or damage to the catalyst can result! You may want to perform this test on vehicles that are known to have low emissions, to gain some experience. Another procedure that s being used in the field is the Cranking CO 2 Test. To perform this test, first make sure that the engine is at normal operating temperature and that you precondition the catalyst by running the engine at 2500 rpm for two minutes. Next, shut the engine down and quickly disable the ignition system so that there s no spark available. (The injectors must work for this test, which must be performed while the catalyst is still hot.) Hook up your gas analyzer and crank the engine for 10 seconds while watching the gas readings. Note: It ll take several seconds for the sample to get to the gas bench. CO 2 should reach 10% to 12% by the end of the 10 seconds. If the CO 2 does not reach at least 10% after 10 seconds and the HC goes over 500 ppm, the cat may not be working at total efficiency. The graph in Fig. 4 above shows a converter that effectively converted the HC into CO 2 and passed the test. For my money, the best way to test for catalyst efficiency is to take gas readings in front of the converter and compare them to the readings at the tailpipe. Several companies make kits for just such a procedure. To perform the test with repeatable results, an emissions dynamometer is useful. Install the probe, then run the vehicle under load while the gases are collected. The formula for calculating efficiency is: precatalyst reading minus postcatalyst reading divided by the precatalyst reading times 100, or (Pr Po) Pr 100. What you want to see is an efficiency rating around June

7 After installing this little fitting, a gas analyzer probe can be placed upstream of the converter so you can compare the precat readings with what s coming out the tailpipe. The results you obtain ultimately determine how efficient the converter is at cleaning up exhaust pollutants. Here is a typical dual-bed converter. Reduction is performed in the front bed, while air is pumped in after it to oxidize HC and CO. 80%. The following table shows an example of a tired catalyst: 15 MPH WITH ASM LOAD Precat Tailpipe Efficiency CO % % O % % HC ppm ppm % CO % % % NO X ppm ppm % Once a bad catalyst is found, you need to determine what caused the failure. While catalysts eventually do wear out, they can withstand lots of abuse before they fail. Always check for improper fuel control and AIR operation, misfire and oil consumption if a converter goes down before its time. Many vehicles today have over 100,000 miles on the clock and probably use some oil. As far as the catalyst is concerned, oil is just another form of HC, and it will be oxidized. However, crankcase oil also contains chemical elements that will eventually poison the cat and cause premature failure. Check the inside of the inlet pipe for oil residue and ask the customer how often he needs to add oil to the engine. Today, a catalytic converter is an exhaust component, an emissions component and part of the OBD II system. If you re not replacing many of them yet, don t worry. There is a car with a glowing MIL in your future! I would like to thank Herb Barry, Danny Iwama and other iatn members for their help with this article. Visit to download a free copy of this article. Copies are also available by sending $3 for each copy to: Fulfillment Dept., MOTOR Magazine, 5600 Crooks Rd., Troy, MI June 2001