Down and Dirty

Remember when emission control equipment was an add-on nuisance that was better cut off a smog-choked engine? Today it’s so integrated into engine control systems that no one would think about removing an air pump or defeating an EGR valve. Those under hood components and others nearby, are active components, but out back, under the floor pan is a part that few consider anymore: the catalytic converter. Usually relegated to exhaust system work, the converter and these days, converters, are worth a second look as a potential service item when bringing a dirty vehicle into compliance.

What are we dealing with in the exhaust stream? It’s a soup of chemical compounds, but the main culprits from an emissions test standpoint are oxides of nitrogen (NOx), Carbon monoxide (CO) and unburned hydrocarbons (HC). Unburned hydrocarbons are simply fuel that hasn’t properly burned in the combustion chamber, while NOx is the result of an unfortunate fact about the air we breath: It’s 16 percent oxygen and 78 percent nitrogen. Nitrogen is useless to an internal combustion engine, but at very high temperatures and pressures, like those found in the combustion chamber, it combines with oxygen to form the dreaded oxides of nitrogen. CO is a natural byproduct of the combustion process. Control of HC, CO and NOx is the reason why converters are sometimes labelled as “three way” units. Good engine control (read, ‘computer’) helps, but no matter how efficient the process, HC and CO need aftertreatment, which is where the catalytic converter comes into the picture.

Modern converters control the three main pollutants by using two different processes: oxidation and reduction. Oxidation/reduction converters have two separate catalyst beds enclosed in a common body. The front half, the reduction catalyst, deals with NOx by pulling the nitrogen off the oxygen and allowing the two elements to recombine as O2 and N2 the same as they existed in the air naturally. CO is handled by the rear-mounted oxidation catalyst, which uses the newly-remade O2 to combine with CO to make CO2. More oxygen boosts the efficiency of the process, so a little air injection here is sometimes used, which is why the air tube is commonly seen halfway down the converter body. It also suggests that a test that shows excessive CO might have an air pump or air injection “plumbing” problem.

Heat is essential to good efficiency; it’s another reason to properly pre-condition a vehicle before a smog check. Future versions will use electrical-preheating for even faster light-off, especially since the upcoming 42-volt architecture will provide plenty of power.

Outside of a failure in the emissions test lane, Catalytic converters rarely make themselves noticed, even when they fail. But when they fail, it’s important to go after the engine problem that caused the failure first. Likely DTC’s will point at oxygen sensors, which can tell a story immediately if there’s evidence of coolant contamination. Odd deposits, especially following recent engine work, can also suggest use of non-sensor safe sealants, or non-sensor safe fuel additives. When you remove the converter or pipes, examine the interior carefully. Look for obvious contamination and a broken or powdered catalyst bed. Impact damage can also dislodge the substrate. As the honeycomb breaks up it can partially clog the converter, limiting engine RPM and killing gas mileage through excessive back pressure. Unburned fuel in the exhaust is a major cat killer, so burned exhaust valves, oil or fuel-fouled plugs or even jumped timing belts can do damage. For older machines without downstream O2 sensors, but between smog tests, it could be worthwhile to suggest a test, or at least to get ready for a possible cat replacement at testing time.

And don’t forget the heat shielding. The heat and corrosive undercar environment makes these sheet metal guards early casualties, but the solution for that annoying rattle is to repair or replace, rather than remove. As a technician, you can’t control what the consumer parks on, or over, nor can you stop him or her from having the vehicle undercoated with any one of a number of foul-smelling oil sprays that will burn in contact with the hot converter.

In theory, catalytic converters should last the life of a vehicle. If they don’t, the detective work can lead to service as simple as spark plugs, or as complex as a software upgrade to the ECU’s fuel map. In either case, there’s no mystery to the diagnostic procedure. Even if you work in a non-I/M jurisdiction, it’s still a high-value service … and you’re doing the Earth a favour.

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OPTIONS FOR BETTER PERFORMANCE

It’s a given that as an engine flows more gas, it will need a catalyst bed that can provide enough reactive surface for a long enough time to work its oxidation-reduction miracle. But what if you just can’t quite get that well tuned engine into compliance? Consider installing a specialty converter. High-performance versions of a similar model often have bigger catalyst beds, and some universals could be selected as a converter upsize. For example, Walker offers their California-OBD II-spec Clean Air Ultra converters across the continent. According to Walker Catalytic Converter Product Manager Pat Haynes, “CleanAir Ultra converters are ideally suited to non-California vehicles that require the increased conversion efficiency made possible through the units’ higher capacity catalyst loading. They’re engineered specifically to address the requirements of the California Air Resources Board (CARB) but are also an excellent upgrade option for vehicles needing lower tailpipe emissions and improved NOx conversion efficiency”.

If you’re changing converters as part of a modified exhaust system, however, don’t move them to a radically different location. Cats are engineered to live in a happy zone where they’re warm enough to light off quickly and work efficiently, without dying from the blowtorch heat of the exhaust ports. Upstream converters that are now appearing at the exhaust manifold are designed to take the extra heat, but secondaries are at home under the floor pan.

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TROUBLESHOOTING CONVERTER ISSUES

When a converter goes bad, it’s rarely because of a failure of the catalyst itself. According to Hans Borneby, senior project manager, ArvinMeritor LVA (Light Vehicle Aftermarket), “It’s essential that the technician do what we call a root cause analysis.” Borneby refers to a systematic diagnostic check that determines which engine parameters or component failures took the converter outside of its normal operating conditions.

“The most common cause of failure is a rich condition that causes excessive converter temperatures”, he declares, adding, “that kills the catalyst bed. A faulty coolant temperature sensor, for example, can drive the ratio rich by fooling the engine into thinking it’s cold.”

Borneby notes that sometimes the root cause can damage more than just the converter, too: “Coolant or other foreign substances in the exhaust stream that can poison the catalyst can also coat sensors such as the O2 sensor, so it should be checked as part of the total system analysis.”

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WHAT ABOUT SEALANTS?

Sealants, and the solvents that evaporate out of them as they cure, are sure-fire sensor killers in the wrong applications. According to Todd Hassard, marketing coordinator for Loctite AAM & Industrial, Henkel Canada Corporation, “the use of a sensor safe sealing product is critical on any vehicle equipped with an oxygen sensor.” Traditional silicones leave a telltale vinegar smell. According to Hassard, “the vinegar odour is in fact acetic acid vapour, a curing by product of an “acetoxy” silicone. Although the acetic acid given off by traditional silicones can create localized corrosion on steel parts, the low molecular weight silicones used in the base materials are the contamination culprits.

Oxygen sensor contamination can occur in two different situations. In the first case, volatile silicone fumes could travel downstream (internal) and contaminate the oxygen sensor tip as they pass by. The other possible method of contamination is for fumes to reach the outside of the sensor and contaminate the ambient air reference port. One condition will lead to a full rich signal (the fuel economy drops, eventually the catalyst suffers) and the other to a full lean signal (the engine runs hot, risking critical engine damage), neither being acceptable. In either case the only solution is to replace the sensor. These conditions lead to a lot of troubleshooting by the technician, and work that can not be billed. The solution is to ensure that the gasketing material is rated as “sensor safe”. There is a small additional cost, but it is insignificant next to the cost of a single customer problem.”