Garbage in, garbage out.

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Ilene Williamson
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1 Garbage in, garbage out. Funny, but that old computer adage applies directly to repairing today s cars. Changes in national air quality standards have caused many areas to be declared in nonattainment, meaning stricter automotive emissions standards are required to clean up the air. In response, the states are implementing new, tougher emissions programs to meet the tightened specs. Like it or not, these new programs are here to stay, and they re pure opportunity for those who want and can do the work. The two charts shown on page 46 illustrate the relationship of the exhaust gases on a perfectly running engine. Only the air/fuel ratio has been changed. Fig. 1 illustrates the precatalyst gases with no air injection, no air leaks and no misfires. If a misfire were present, you could have high carbon monoxide and high oxygen at the same time, but as you can see, this condition never shows in the chart. Fig. 2 shows the exhaust gases after the catalyst has had a chance to clean things up. With those few points out of the way, let s take a closer look at those so-called garbage-out gases. The addition of NO X as an exhaust pollutant, combined with newer smog programs, has signaled the dawn of a new era in emissions diagnosis. Carbon Monoxide (CO) Carbon monoxide is formed in the combustion chamber when there is insufficient oxygen to complete the combustion process. For diagnostic purposes, CO is an indicator of richness (air/fuel ratios less than 14.7:1). In Fig. 1, notice that CO is over 15% with the air/fuel ratio at 8:1. It then drops to about.3% at 14.7:1 (stoichiometry) and stays at that level past 18:1. For this reason, CO can never be considered a leanness indicator. The red lines in Fig. 2 are postcatalyst CO percentages. There are two lines here, for systems with and without air injection (AIR). In shop-testing my van with a new catalytic converter, I found that the cat was capable of converting 8% CO to.10% without overheating. This van uses an AIR system that pumps air directly into the converter; newer vehicles without an air pump are less likely to convert this much CO because of the lack of oxygen necessary to complete the job. Without an air pump, a 2% drop in CO is about the maximum you can expect from the converter. CO failures are usually due to too much fuel (bad sensor input, high fuel pressure, evap failure, etc.) or too little air (dirty air filter, blocked intake, etc). Remember, however, that the whole emissions system is designed as a package, so a CO failure might not necessarily be a CO problem. For instance, some vehicles with air injection systems are designed to run at 2% CO at idle (preconverter), with the converter cleaning up the remaining CO without overheating because of the low volume of air introduced at idle. Systems with pulse air setups (one-way reed valves that open when the exhaust pressure goes negative) have a tendency for the valves to rust shut. Without the extra oxygen to catalyze the CO, tailpipe emissions will fail at that 2% CO level. The obvious fix is to repair the air injection system. Always consider driver habits in your diagnosis. Many of my elderly customers have older cars without heated O 2 sensors. They don t do a lot of driving, so these feedback systems rarely achieve closed-loop. Result: These cars fail their smog test every year. All we need to do to get them to pass is change the oil and heat up the converter to burn off the excess carbon accumulation. After repairing a CO problem, make sure you change the oil. Oil will become contaminated with fuel when a CO problem exists. Not changing it may cause the failure to persist, at least until the PCV system sucks the fuel out of the oil. Speaking of the PCV, it doesn t hurt to disable the system and look at the changes in the gases whenever you encounter an emissions test failure. On mass airflow (MAF) systems, you ll 42 May 1998

2 BY MARK WARREN need to clamp off or block hoses. If you just pull the PCV valve out, the MAF sensor will incorrectly think too little air is entering the engine. Hydrocarbons (HC) Gasoline is a mixture of many different hydrocarbons. So it doesn t take a genius to figure out that HC in the exhaust is indicative of unburned fuel. Some amount of HC will always be present in the exhaust due to fuel too close to a cool combustion chamber wall and large fuel droplets that burn on the outside only. Lots of HC means incomplete combustion due to engine misfire. Looking at Fig. 1, you can see that even at better than 15% CO (very rich), HC is only about 500 ppm (although there is no scale for ppm on this chart, HC goes from a high of 500 ppm to a low of 50 ppm). When an engine misfires, however, HC skyrockets (usually to about 2000 ppm per cylinder). I ve seen converters handle as much as 4000 ppm HC with the emissions relatively clean at the tailpipe, but the cat s operating temperature will go way too high and damage will result fairly quickly. The orange line in Fig. 2 shows the converter s ability to convert HC. Any condition that can cause a misfire can result in high HC. Things to look for include ignition problems, vacuum leaks, compression problems, injector deposits and out-of-whack cylinder balance. Keep in mind that an air pump makes little difference on a misfire, because the misfire supplies its own oxygen. Be careful when quoting prices for emissions repairs. As a case in point, a customer came to us with a high HC failure. My tech diagnosed a bad plug wire. The customer was quoted a price for diagnostics, a new set of wires and installation. After the repair, HCs were much better but still failing. Turns out the converter was dead from too much abuse. Now the quote doubled. Care to hazard a guess how that went over? When diagnosing emissions failures, leave yourself some working room and explain to the customer that repair is a process, not a single step. Like Yogi says: It ain t over till it s over! Oxygen (O 2 ) Fig. 1 shows oxygen (blue line) at about.5% when the air/fuel ratio is rich. Then at about 13:1, it starts to curve up to as high as 6% when lean misfire occurs. O 2 is an indicator of leanness; it gives no indication of how rich an engine is running below 13:1. Assuming that the air injection system is disabled, high O 2 readings can be caused by conditions other than lean mixtures. An oxygen reading of 20%, for example, is a pretty good indication that the sampling hose has fallen out of the tailpipe. This may sound crazy, but I ve actually seen more than one tech proclaim a car fixed with a reading like that. A high O 2 reading may also be caused by a leak in the analyzer or its sampling hose. After a number of emissions failure comebacks in my shop, I had to find out what was going on. The last car tested had low CO, HC and CO 2, yet O 2 was 10%. By the way, this car ran great. After double-checking that the air injection was disabled, the only conclusions I could come up with were either a large exhaust leak or a problem with the sampling hose. I removed the sampling hose from the exhaust and plugged the holes with my fingers. After treating my burned fingers and cooling the hose, I repeated the test. No low-flow indication on the gas analyzer. Moving back through the system showed no improvement even with the hose removed and plugged at the inlet. Turns out the problem was a missing O-ring on the filter housing. Apparently, when we replaced the filter earlier, we had mis- May

5-GAS ANALYSIS 14.7:1 (Stoichiometry) CO 2 14.7:1 (Stoichiometry) CO (without AIR) CO NO X NO X CO (with AIR) Charts: Mark Warren HC O 2 Fig. 1 Fig. 2 placed the O-ring.

3 5-GAS ANALYSIS 14.7:1 (Stoichiometry) CO :1 (Stoichiometry) CO (without AIR) CO NO X NO X CO (with AIR) Charts: Mark Warren HC O 2 Fig. 1 Fig. 2 placed the O-ring. Remember, it never hurts to test the tester! Always write down the gas readings before and after disabling the AIR system, then compare them to see if the AIR system is functioning properly. I recommend pulling and plugging air injection lines for testing; switching the diverter valve can be complicated and may not give you the results you desire. For example, I once worked on a Land Cruiser for three days trying to solve a marginal CO failure, only to find an old piece of air pump vane lodged in the diverter valve. The valve I thought was closed leaked just enough air onto the O 2 sensor to shift the mixture rich. Oxygen represents about 20% of the atmosphere. If a four-cylinder engine has a complete misfire on one cylinder, expect the tailpipe O 2 level (air injection plugged) to be about 5%. How did I come up with this percentage? Well, one cylinder represents 25% of the total number of cylinders, right? So 25% x 20% O 2 = 5%. On a V8, a single cylinder misfire would spew about 2.5% O 2 out the tailpipe (12.5% x 20% O 2 = 2.5%). No need to go into six cylinders; you get the picture. Carbon Dioxide (CO 2 ) We want carbon dioxide as high as possible on a percentage basis (see the section on Mass vs. Percentage Emissions on page 50). CO 2 and H 2 O are the perfect byproducts of combustion from a carbon-based fuel. You can see in Fig. 1 that the maximum percentage for CO 2 (green line) is about 14.7%. You may encounter higher percentages on your analyzer. That s because most analyzers don t measure CO 2 or O 2 nearly as accurately as they do CO, HC and NO X. Note that the CO 2 level will be lower than 14.7% if an air injection system is diluting the exhaust gas with air. HC Nitrogen Oxides (NO X ) Nitrogen oxides is the new kid on the block for most state emissions programs. NO X is formed under high-temperature and high-pressure conditions in the combustion chamber. NO X failures can be attributed to EGR system failure, overadvanced ignition timing, lean-running conditions, a failed reduction portion of the converter, high compression due to combustion chamber deposits, overheating, overly warm intake air and valve timing and cylinder balance problems. The good news from the field is that most NO X failures are pretty easy to diagnose and fix, although some, such as valve timing problems, can be pretty far down the process of elimination. Look at NO X (black line) in Fig. 1. While CO and HC are fairly low in the 14.7:1 ideal mixture range, NO X is quite high. Obviously, the converter has to work pretty hard to convert NO X in this range. Notice in Fig. 2 that the converter is very efficient at converting NO X at ratios of 14.7:1 and below. Above stoichiometry (leaner mixtures) the reduction portion of the converter stops working on NO X due to the presence of too much O 2 (greater than 2%) in the exhaust. To properly reduce NO X, the exhaust must be low in O 2 46 May 1998

The typical scenario is that a car comes in failing CO, you fix the problem and now it fails with high NO X! What gives? Fig. 3 Fig.

4 5-GAS ANALYSIS Screen saves: Mark Warren and the converter must have some CO to work with. Even so, a converter can handle only about 2000 ppm NO X ; much more than that needs to be addressed by EGR or engine design. Watch out for a CO to NO X failure! The typical scenario is that a car comes in failing CO, you fix the problem and now it fails with high NO X! What gives? Fig. 3 Fig. 4 Preconverter O 2 Sensor In Closed Loop Postconverter O 2 Sensor Sweeping With Front Sensor Revved to 2000 RPM to Warm Converter Converter Light-Off Postconverter O 2 Will Follow Front on Rapid Throttle Changes Preconverter O 2 Sensor In Closed Loop Postconverter O 2 Sensor At Converter Light-Off While it s difficult to have a NO X failure with a lot of CO present, high CO also causes carbon buildup in the combustion chamber and in the reduction portion of the converter. Even if you fix the CO problem, this carbon buildup remains. With CO low and the carbon buildup still there, you have the perfect recipe for NO X formation. It s interesting to note that Denver has a CO problem, not a NO X (ozone) problem. But as their emissions program cleans up the CO-failing cars, NO X may become an air quality issue in the future. When performing a compression test, don t overlook cars with higherthan-normal compression. I ve seen many 3.8-liter GM engines with a 135- psi compression spec check out at 180 psi or higher from combustion chamber deposits. Decarbon that engine or you ll have a NO X problem! Note that some GM engines with taper-seat spark plugs have had problems with carbon buildup around the plug threads. When you go to spin in new plugs, the carbon ramps up in front of the threads and the plug feels like it s tightened, although the taper never landed. Result: The spark plug overheats, the engine pings and NO X is created. We have removed these plugs and have seen no witness mark on the tapered part of the plug from seating. Moral: Clean those threads! Testing Converter Efficiency When you get a car that fails an emissions test, you need to know the general health of the converter. Is it dead, inefficient or simply trying to handle too much? The first test I like to run is called a cold converter test. To do it, bring the engine to operating temperature and then shut it down for a halfhour. This allows the converter to cool but still keeps the fuel system pretty close to closed-loop operation. In Fig. 3 we re monitoring the front O 2 sensor (top chart), the postconverter O 2 sensor (second chart) and engine rpm (lower chart) on a 1997 four-cylinder Chevy Lumina. Notice that after 300 seconds at idle, the converter still didn t light off. At the 350-second point, we revved the engine to 2000 rpm and light-off occurred about 50 seconds later. As you can see, the rear O 2 sensor flattens out and quits following the front O 2 sensor like it did before light-off. The later movements on the rear O 2 sensor are relative to rapid throttle changes. Fig. 4 is the same recording only zoomed in to get a better look at the 48 May 1998

5 sweep of the O 2 sensors and light-off. This is exactly how OBD II systems test converter efficiency by comparing the pre- and postconverter O 2 readings. Back to the cold converter test. I record the 5-gas readings before and after converter light-off. This gives a good indication of what the converter is trying to deal with. Although this test works great for CO and HC, it s not nearly as effective for NO X because by the time you adequately load an engine to test for NO X, the converter will already be lit off. Another option is to perform the dual O 2 OBD II test, which checks the ability of the converter to store O 2 and oxidize HC. Like the cold converter test, however, this test won t indicate a problem in the NO X reduction portion of the converter. Okay, so how else can we test converter efficiency? How about measuring the temperature difference at the front and rear of the converter? While this may tell you if the converter can light off, it gives no indication of true efficiency. Also, some vehicles run so clean under no load that the converter may not even light off for lack of enough to burn. GM has an excellent but complicated test to determine converter efficiency. You first warm up the converter by running the engine at 2000 rpm. Then shut the engine down and disable the ignition system and injection. Next, connect propane to the intake with a flow gauge attached, set the flow to the prescribed amount from a chart on engine size and crank the engine at wide-open throttle. Finally, record the CO 2 percentage from the tailpipe and compare the reading to the chart. A quicker way is to warm up the converter, disable the ignition and crank the engine over, checking for CO 2 out the tailpipe. This won t give you percentage efficiency, but with practice, you can get some good data. The best way to know what s going on with a converter is to test the gases before and after it. So drill a small hole in front of the converter (this can be a problem on some cars that have the converter built into the manifold) and compare the readings. This takes some time but it s a sure-fire test. And because of the inability of the other methods to check the reduction (NO X ) portion of the converter, it may be your only hope. Converters die from overheating, poisoning from sulfur, lead, zinc, phosphorus, silicone and carbon and physical damage. Don t replace an overworked converter and expect the new one to last. Also consider the possibility of poisoning before completing or quoting a repair. If a car is a major oil burner, the new cat won t have much of a chance. If you have a blown head gasket, be sure to warn the customer of possible converter and O 2 sensor damage. A bunch of O 2 sensors and a new converter may cost almost as May

6 5-GAS ANALYSIS much as the cylinder head job itself! Some converters get coated with contaminants, yet aren t dead. Remember the story of the old folks who never drive long enough to get the feedback system into closed-loop? Most shops will take that car on the freeway for a while to relight and clean the converter. We ve had good results creating a misfire to heat the converter nice and toasty. This procedure may also revive a converter with sulfur coating. There is a great amount of caution and discretion associated with the misfire rejuvenation procedure. We never take the shell temperature over 800 F, and we usually perform the procedure only on a converter that has been declared dead. Mass vs. Percentage Emissions We were all trained in percentage and ppm gas readings. This worked great for years, so why did the government switch over to grams-per-mile? Do they do this stuff to make our lives harder? Not at all. The feds need to predict the total vehicle emissions inventory to calculate air quality. A 2-liter engine at 1% CO doesn t spew nearly the same total emissions as a 7.5-liter engine at the same CO level. Grams-per-mile is the only effective way to measure this. Also, keep in mind that grams-per-mile changes the way some gases appear. Remember I said we want CO 2 as high as possible on a percentage basis? Well, in grams-per-mile, a high CO 2 reading may indicate dragging brakes. In this scenario, the engine could be in closed-loop and the percentage/ ppm gases could look good, yet fuel mileage is poor and CO 2 emissions excessive. Using grams-per-mile allows us to take HC, CO and CO 2 readings and predict fuel mileage. That s why many state emissions programs give an estimated mpg rating on the printout. Finally, grams-per-mile readings measure mass emissions and are affected by air injection. Percentage readings are diluted by air injection, mass readings are not. As you can see, 5-gas analysis isn t the easiest thing in the world to comprehend. It s knowing how each exhaust gas relates to the others. It s knowing what catalytic converters can and can t do to clean things up at the tailpipe. It s knowing how driver habits can affect emissions. Above all, it s putting all these things together in an effort to achieve the cleaner air we and our families deserve. Gentlemen, start your analyzers! For a free copy of this article, write to: Fulfillment Dept., MOTOR Magazine, 5600 Crooks Rd., Troy, MI Additional copies are $2 each. Send check or money order. 50 May 1998

Although catalytic converters. Keeping the

Although catalytic converters. Keeping the 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,