ASM TESTING GETTING A PASSING GRADE

ASM TESTING GETTING A PASSING GRADE 22 May 1999

While less demanding than an I/M240 drive trace, successful ASM testing still requires a thorough understanding of feedback systems and the relationship among exhaust gases. BY LUDLOW BLACK Photos: Ludlow Black Acceleration Simulation Mode, or ASM, is an emissions test that is being used in a number of states instead of the more costly I/M240 test. And there s been lots of debate as to how effective it is when compared to I/M240. Remember, I/M240 is run while a vehicle is being driven through a driving simulation, which requires acceleration and deceleration like the Federal Test Procedure (FTP). With ASM, even though the vehicle is tested under load, it s run at a steady speed. Current ASM tests are run at a speed of 15 or 25 mph. In some cases, both speeds are used. The amount of load placed on the vehicle is calculated to represent the same resistance to the vehicle as light acceleration. In other words, the load is greater than what it would be to maintain the speed. Fortunately, all of the magical calculations are done in the test computer. During the FTP or an I/M240 test, the sampling hose captures the entire exhaust output of the vehicle. The flow through the hose is greater than the amount of exhaust the vehicle produces, however, and is constant. So air is allowed to enter the hose to make up the difference. A portion of the exhaust and air mixture is then pumped into the analyzer for testing. This method allows vehicles with differentsize engines to be tested using the same limits. Instead of being measured in concentrations, the gases are measured in grams per mile. During an ASM test, only a portion of the exhaust gas is measured. To make the test fair for different-size vehicles, the limits are adjusted. During the test, the GVWR from the vehicle sticker is entered. Once this information is known, the limits automatically adjust for the test. You have to be careful to enter the correct vehicle weight when you run an ASM test, says Mike Decowski of Friendship Automotive in Woodbridge, Virginia, who has owned and run a test-and-repair facility since the start of that state s program last year. If the wrong weight is entered, the car may incorrectly pass or fail. Mike goes on to say that when he gets a vehicle May 1999 23

Table 1 from another station in for repairs, he always verifies that the correct weight was entered on the report before beginning diagnosis. The biggest difference between idle testing and ASM is that NO X levels are measured. Automobile manufacturers have been given the task of reducing this gas almost to the point of eliminating it altogether. Reduction of NO X is a design criterion from the beginning of new-engine development. Consequently, new vehicles produce very little of it. Older fleets are a different story. These vehicles use less recent engine designs, and produce a tremendous amount of NO X. (It s not unusual to see NO X readings of 2500 to 3500 ppm before the catalyst.) Some manufacturers rely on an EGR valve to reduce these levels to less than half. That still leaves over 1000 ppm to deal with. This is where the catalyst comes into play. A latemodel three-way catalyst can reduce Table 2 the remaining NO X to less than 200 ppm. But what about those vehicles without an EGR valve? To understand how things work, we need to go back to the process that creates NO X in the first place. In the Beginning During emissions training we learned that NO X is created under high cylinder temperatures and pressures. Only then will nitrogen molecules bond to oxygen molecules, producing one of our worst pollutants. Let s break down each contributing factor and see what can be done about it. High Temperature. Temperature can be controlled with the design of the engine and cooling system. The best engine temperature is a delicate balance between NO X -producing heat and temperatures that are too cool for complete combustion. If the engine runs too cool, fuel can condense inside the cylinder and exit unburned as HC. If the engine runs too hot, NO X can form rather easily. Remember, the ignition system must fire at the precise moment that will allow time for complete combustion during the power stroke, but not allow detonation. An EGR valve is used to dilute the air/fuel mixture with an inert gas that lowers peak temperatures in the cylinder. Other factors also must be taken into consideration for example, the load placed on the engine. Axle ratio, horsepower-to-weight ratio, electrical loads and aerodynamics come into play here, as well. Once an engine is designed to run at temperatures that produce the least amount of NO X possible, what can go wrong? Several things. For one, the thermostat can stick partially closed, increasing temperature and thus NO X formation. The thermostat can also stick open, causing the engine to run too cool, increasing HC and CO. Also, the timing could be set incorrectly, increasing heat and causing detonation. These conditions can easily double the NO X output of the engine. Other causes of detonation, such as carbon deposits, for example, must also be considered. While today s fuels burn very clean, leaking valve seals can create carbon buildup in the cylinder. In addition, the EGR valve or tube might be plugged with carbon, or the valve may not open properly, causing cylinder temperatures to increase. High Pressure. Car manufacturers have reduced the compression on their engines as much as possible. The balancing factor is power, economy and complete combustion. Problems in this area include excessive cylinder head or deck machining, carbon deposit buildup and fuel that burns too fast, all of which can lead to detonation. Detonation causes a rapid increase in combustion pressure, so anything that can cause it must be considered when there is excessive NO X present. Oxygen and Nitrogen. The atmosphere contains over 70% nitrogen, and since no air filter on the market today will keep it out of the intake, we re stuck with it. Oxygen, on the other hand, can be controlled by adjusting the fuel mixture. Running the engine 24 May 1999

Table 3 at the ideal air/fuel ratio will use almost all of the oxygen present during the combustion process. This eliminates one of the needed elements in the NO X formula. On late-model vehicles, once the engine is at operating temperature, the fuel mixture remains at stoichiometry (14.7:1) almost all of the time. During heavy acceleration or deceleration, however, the mixture is modified. For instance, when the pedal is pressed close to the floor, the air/fuel mix is enriched for maximum power; when it s released, the fuel mix is leaned out to reduce emissions. But remember, neither of these conditions occurs during an ASM test. Certain carmakers play some pretty interesting tricks with the fuel mixture to reduce NO X. For example, on some vehicles, if the engine is running with a rich mixture, oxygen will be eliminated so that the cylinders run cooler. What about the increases in HC and CO that result? A very large converter is typically used, with an air pump or gulp valve providing secondary air to the converter. This allows the HC and CO to be oxidized before it leaves the tailpipe. Another approach used by one import manufacturer is to run the engine very lean. Remember, NO X production begins to fall off at an air/fuel ratio of around 17:1. A slightly lean mixture causes an increase in combustion temperatures, but once the mixture gets really lean, there simply is not enough heat produced to cause NO X formation. This manufacturer uses a special oxygen sensor to accomplish this lean fuel control. As you can see, the fuel mixture plays an important role in NO X production. If the engine is running lean, there will be lots of leftover oxygen to make NO X. In addition, a lean mix leads to high cylinder temperatures, aggravating the situation even more. If the engine is running rich, NO X will be reduced, but at the cost of higher CO and HC. A word to the wise: If you re repairing a vehicle that failed for high CO, don t forget to check for NO X before a retest. Even if the NO X numbers were good on the test report, the high CO concentration could have been temporarily reducing NO X. Once the CO goes back to normal, any NO X problems the vehicle has will show up. So far we ve been looking at factors that cause or prevent NO X production in the engine. Now let s turn our attention to the device used to reduce tailpipe emissions of NO X after it s formed and leaves the engine. The Big Cat The process used to remove NO X from the exhaust is called reduction. With reduction, the nitrogen is split away from the oxygen by the catalyst, a process that requires precise levels of oxygen in the catalyst bed. This is where fuel control comes into play. If the engine is running too lean, NO X reduction cannot occur. A lean-running engine, therefore, can fail an ASM test for NO X and pass for HC and CO. However, close examination of the test results might yield some clues to problem. More on that later. Besides the possibility of a lean-running engine, there are other factors that might prevent the catalyst from doing its job with regard to NO X control. For example, the catalyst might be empty or may have been overheated to the point that there are no active sites in the substrate. Another possibility is that there is too much oxygen in the catalyst, preventing reduction. A lean mixture is not the only thing that can cause this An ASM test lane, complete with dynamometer and gas analyzer, is a major expense for most independent repair facilities. Many shops rely on sheer test volume to help recoup some of the investment. May 1999 27

problem. For example, there might be a secondary air system malfunction that continues to send air into the exhaust ahead of the catalyst. Other possibilities are cracked manifolds and leaking EGR tubes or air injection pipes. Remember, anything that adds oxygen to the exhaust will have a detrimental effect on the reduction of NO X in the catalyst. Just how important is the catalyst when it comes to NO X reduction? In testing an engine under load with a 5-gas analyzer, I ve found precatalyst NO X readings of 3200 ppm on a properly adjusted, normally running engine. I ve seen the same vehicle reduce that to less than 400 ppm as indicated by tailpipe readings! Don t forget there are many vehicles that don t have an EGR valve. These vehicles are designed to let the catalyst reduce all of the NO X. How do you know if the catalyst is working properly? This seems to be the million-dollar question to many technicians. There are several clues that might lead you to suspect the catalyst. Let s start with the emissions test report. The HC, CO, NO X and dilution numbers from this report can help lead you to a successful diagnosis. Let s review the information in Table 1 on page 24. First of all, notice that this vehicle is not equipped from the factory with an EGR valve, so the catalyst is responsible for reducing all of the NO X. It obviously isn t doing a good job, but is it empty? Probably not! If it were, HC and CO would be higher. The dilution number is the sum of CO and CO 2 at the tailpipe. If excess air were entering the catalyst, this number would be lower. The second clue is that there is almost no CO present in the exhaust. This is a strong indication that the mixture is lean. Armed with this information, the technician checked the fuel control system and found a carburetor problem. Once it was repaired, the exhaust readings came into line. The results are shown in Table 2. Notice the slight increase in CO in Table 2? That means the catalyst is doing its job on all three gases. Also notice the increase in CO 2. (To calculate CO 2, simply deduct the.3% CO from the 15.2% dilution.) In this case, CO 2 is 14.9% pretty good. Now let s look at the readings in front of the catalyst on the same car, after the repairs were completed (Table 3 on page 27). As you can see, this engine runs slightly rich since O 2 and CO are not balanced. Still, the converter had no trouble cleaning up the HC and CO. NO X is high at 2645 ppm, but this is to be expected on an engine with no EGR valve. The catalyst reduced this number drastically, by 1670 ppm. Note that these readings came from a 5-gas analyzer, so they include CO 2 and O 2. What would it mean if you saw these numbers on a failed test report? Most likely it would indicate a bad or nonexistent catalyst. Remember, if all three gases are high, always suspect the catalyst. Unfortunately, most emissions test reports fail to include O 2, even though it s being measured during the test. However, you can measure it when you baseline the vehicle before repairs with your 5-gas analyzer. If your shop is a repair-only facility and you don t have a 5-gas analyzer, you really should consider investing in a portable unit. Difficulties can arise when you try to repair a vehicle without using an analyzer and send it away for a retest. Some states allow only one free retest. If the vehicle still fails, who pays for the next test or tests? The investment in time needed for a shop to take a car to a test station over and over again is prohibitive. It would be far better to know whether the vehicle will pass before it s returned to the customer. Some manufacturers of portable 5-gas analyzers allow you to record scan data and gases simultaneously while road-testing the car. With that much information available, you could be directed to the problem quickly once you analyze the data. If you work at a test-and-repair facility, you have equipment at your disposal to run diagnostic tests under the same conditions as the emissions test. This allows you to pretest the vehicle before a registered retest. However, some locations do so many emissions tests that the technicians have a hard time getting on the machine for diagnosis. One tech told me his shop started a second shift to address this issue, but quickly filled the new time slots with more emissions tests. A second dynamometer setup for diagnosis is an ex- 28 May 1999

pensive solution, since it would not generate the revenue from multiple tests. A portable 5-gas unit is a reasonable solution for these shops, as well. Baseline or Verification Road Test If you intend to verify your repairs during a road test using a portable analyzer, you need to determine the conditions at which to operate the vehicle during the test. One way is to hook up your scan tool during the test and record the data. By looking at the rpm and load factors, you can approximate A portable 5-gas analyzer, like this one from Vetronix, allows repair-only facilities to check their work prior to a retest. Test-and-repair shops can also benefit from these units because it frees up the dyno to do other smog checks. the needed drive cycle. Experience has shown, however, that it s not extremely critical to duplicate the test conditions when repairing a vehicle that has failed by a wide margin. Just record the gases while gently accelerating from 15 mph to around 30 mph. Remember to account for the delay between when you press the record button and the time it takes for the gases to be displayed on the screen. Likewise, expect a peak to occur in the NO X reading after you let off the accelerator. It s also important to know what portion of the recording contains the readings that occurred during the period of acceleration. It s critical when using an analyzer s averaging function that the low readings before accelerating and the high readings that come after are excluded from the calculation. Step-By-Step Diagnosis A gas analyzer is used to test a vehicle from back to front, as they say. Diagnosis can also be performed the other way around, however. By doing baseline tests of the systems responsible for the control of exhaust gases, a technician can find the root cause of a failed test. Another important item that should not be overlooked is how well the engine runs. Believe it or not, some customers are more than willing to drive an imperfectly running vehicle. When it fails an emissions test, they re not worried about how it runs, they just want it to pass. Because of this, they may neglect to inform you about problems that have been developing. Don t underestimate the importance of the customer interview as well as your own observations during the road test. Basics, basics, basics we ve all been beaten up pretty hard about this subject, either by a trainer or our own experience. A while ago, I visited a shop that was having trouble getting a car to pass an emissions test. They recalibrated their machine, called tech lines and used every tool and tester they had. I walked behind the vehicle and discovered chunks of converter substrate on the floor behind the car. The catalyst was nearly empty! The point is, don t get so caught up in the technology that you forget to use your head. Fuel Control Most leading emissions techs use a lab scope these days for detailed diagnosis. Careful analysis of the O 2 sensor waveform can identify fuel system problems or rule them out as the cause of an emissions failure. The first step is to verify sensor integrity. A faulty O 2 sensor will cause the feedback system to issue an incorrect fuel command. With the engine at operating temperature, force the mixture rich and then lean, to be sure the sensor can reach the proper high and low voltage limits. A good O 2 sensor should exceed.8 volt when it sees a rich mixture and less than.3 volt when the mixture is lean. These readings are under actual test conditions. The next step is to see how fast the sensor reacts to changes in the fuel mixture. Observe the waveform when the throttle is snapped and released. The voltage should rise abruptly and, when the engine coasts down, fall quickly to the minimum voltage. Now that sensor integrity has been verified, it can be used as a window into the fuel control system. The voltage should be averaging around 450mV at cruise. Use the record function on your scope or have someone else drive! Also, watch for hash in the waveform when the engine is under load; this may indicate a misfire. If the voltage seems to stay on the rich side, look for problems in the fuel delivery system or load sensors. If it stays lean more often than not, suspect restricted fuel flow, vacuum leaks or faulty load sensor indications. Remember, the feedback system will try to compensate for these problems, so it might be a good idea to clear the computer memory and test the system again to see what steps it s taking to compensate. Clearing the fuel control information from the memory of vehicles capable of long-term fuel correction can give you a quick check of the fuel control system. Here s how to do it: 30 May 1999

Record the baseline readings, then clear the memory. Next, disconnect the O 2 sensors (front only on OBD II vehicles) and check the readings again. If the numbers get worse, the feedback system is trying to correct for the problem. If they get better, the feedback system is the problem! Once you determine what the vehicle needs in order to pass its test, you ll have to see if the repairs exceed the waiver amount in your area. In the past, the amount that the owner had to spend in order to make his vehicle pass was severely limited. Today, many programs that have been running for a while have raised the limit to a more effective amount. Replacing the Converter Be sure to follow all regulations when replacing a catalytic converter. When in doubt, find out! For instance, it s against regulations to install a used, noncertified converter. Also make sure that any items that could damage the new cat are repaired. A misfire, for instance, can damage a new unit in short order. If the vehicle has air injection, remember that it s supposed to vent to atmosphere during periods of prolonged idle. Mike Decowski brought up another concern: New catalytic converters are the three-way type, so they reduce NO X. Older vehicles came with two-way units that didn t affect NO X. So a car can have EGR or fuel control problems and still pass an emissions test with a replacement converter. I m not sure how long this type of fix will last. I d rather repair the cause of the high NO X. Like Mike says, the best solution is to find the root cause of the high emissions. Many states are implementing a so-called report card system to identify shops by average repair costs and success rate. Make sure your shop gets a passing grade! For a free copy of this article, write to: Fulfillment Dept., MOTOR Magazine, 5600 Crooks Rd., Troy, MI 48098. Additional copies are $2 each. Send check or money order. May 1999 31