Auto Service World
Feature   November 1, 2006   by Jim Anderton, Technical Editor

The Year of the CAT

Emissions regulations are tightening. Are you up to speed with modern catalytic converter service?

Way back in the 70s, a revolution in emission control started in a place where few would have imagined: in the tailpipe. “Smog” engines had charcoal canisters, PCV, idle mixture limiter caps and little else. Converter technology had been around for years. By the fall of 1974, however, the widespread availability of unleaded fuel made converters standard equipment in most passenger cars. Other emissions system technology, like combustion chamber air injection and exhaust manifold afterburners have gone away; but the catalytic converter is here to stay. And as emissions rules tighten (recent Conservative legislation is rumored to shift Canadian emissions specs to California’s strict California Air Resources Board rules) lazy or dead converters simply won’t be legal.

What’s in the Can

Catalytic converters live in an environment with three main pollutants: oxides of nitrogen (NOx), carbon monoxide (CO) and unburned hydrocarbons (HC). Catalytic converters are basically scaffolds that hold expensive metals like platinum, palladium and rhodium. The metal surfaces on the ceramic honeycomb “brick” act as a sort of “workbench” on which the oxidation and reduction chemical reactions on the pollutants take place.

The metals themselves aren’t used up in the process, but they have to present a clean surface for the exhaust gases to react properly. Converter evolution has gone from primarily oxidation types to oxidation/reduction catalysts. In the mid-80s, it became common to feed additional air downstream of the reduction catalyst to boost oxidation efficiency in the second brick.

Today, they’re three-way catalysts with oxidation pre-cats to burn the HC and CO, adding heat to the secondary reduction cat where it’s most needed for that heat consuming (endothermic) reaction. With computer engine controls, there is a common perception that mixtures have trended leaner, with implications for converter life expectancy. The real reason was tighter control around the 14.7:1 stoichiometric air fuel ratio without the wide swings between lean and rich that pre-computer systems endured.

“The misconception that we went to a leaner system is really the result of better control, closer to the 14.7:1 ratio. In fact, it tended a little to the rich side because of EGR, declares CATCO’s national training manager Corey Smith, adding, “As fuel delivery got more sophisticated, exhaust went from high HC and CO and low in NOx to high NOx and low HC and CO”

Another side effect, for emissions at least, was that improved control allowed higher compression ratios which meant higher combustion temperatures and with it, NOx.

And for higher mileage and poorly maintained engines, carbon buildup can raise the compression even higher, worsening NOx levels. The result of all this? According to Smith, “At 160,000 km, the owner comes in with a P0402 code, poor converter efficiency, and the tech thinks the converter is no good. By definition, that code means inefficient of low threshold. The CAT can’t do its job.”

Too rich a threshold will burn out a CAT, but the code can also be set for NOx, which won’t damage the converter, but will confuse the rear O2 sensor. Replace the converter and the failure will happen again. An HC/CO failure causing a melted converter is a fuel delivery issue, but the NOx failure can be as simple as carbon on the piston crowns. Combustion chamber cleaning is always a good service technique when converters are suspect. Ever peek into a failed converter and find it carbon coated? Decarbonizing the engine might have saved the unit. In fact, defects in new converters are exceedingly rare. Dean Clarke, Central Canada regional manager for Tenneco Inc., describes a typical situation: “When we get a blued converter, we know that the unit has been overheated, usually due to raw fuel dumped into the exhaust system. We request the emissions test printout and in the majority of cases the converter failed because of engine failure issues. The rare exceptions are when the converter is damaged in shipping or handling.”

Universal or direct fit?

Both universal and direct fit converters are installed across the country and each has both advantages and disadvantages.

Jim Fox, national sales manager for Maremont relates: “You have a price savings with a universal, because you’re just buying a shell and substrate, but you’re adding your time and labour to build it up into an assembly with OE fit. Direct fit is a much faster, easier install. For a highly skilled exhaust shop, a universal may put more profit onto the bottom line, but those skills are scarcer and scarcer these days. For most installers, the time savings of direct fit are more important.” Fox sees a shift toward OE-type units.

“For 95 and up applications, universals are more difficult to use,” Fox adds. “There are often multiple ports, flex joints and precise placement between the O2 sensors is very important. Converters don’t die; they’re killed by something upstream, like O2 sensor issues, fuel delivery, oil or coolant.”

Both universal and OE fit converters can do the job if they’re installed correctly and in the right application. If you’re going the universal route, it’s very important that the unit is specified for the application. The external size or shape of the can isn’t a reliable indicator, and if reducer fittings are needed it’s time to check the part number. The air injection tube is a dead giveaway in fit up, but where the shell looks symmetrical, there is a right way around, indicated by markings on the unit. Direct fit units are usually easier to fit the right way around, but given a stubborn enough tech, any converter can go on backwards. If the air tubes don’t fit easily, recheck your work. If you need a two way converter for an older application, and you find a three way unit in the box, don’t assume the worst. Some manufacturers back list newer units as replacements for out-of-production cats, often requiring the tech to plug the air injection port or ports.

How do you know if a universal is a decent unit? There are no current Canadian standards, so look for US EPA or CARB certification. With imported White Box parts proliferating in the brake and chassis segment, are questionable converters out there?

According to Dean Clarke, Central Canada regional manager for Tenneco Inc., “We haven’t seen it yet, but at some point it’s going to arrive. There’s a lot of air in a converter, unlike brake rotors, so that has shipping cost implications, but I’m sure they’ll find a way. In converters, it’s all about how much precious platinum, palladium and rhodium metal is on the ceramic brick substrate. There are no regulations, so less expensive products may not be durable.”

Universals should be installed with the body of the converter the same distance from the rear O2 sensor as the original unit, even if clearance considerations suggest a little for or aft movement.

Floor pan heat shielding is usually trashed by the time a CAT needs replacement, but resist the temptation to cut the rattling shielding away. The replacement converter will run hot as hell, as it should, so any undercoating that had built up over the cool, dead converter will melt and drip, so keep it on the tin and off the CAT’s shell. It’s unlikely that the car will burn down, but it’s highly likely that you’ll get the vehicle back with an owner complaining about the smell.

At press time, the Conservative government’s Clean Air Act is still before Parliament, and if passed, it could result in emissions cut points that are half current levels. That means more failures, and perhaps I/M programs across the country. It also means that converters will appear on everything from lawn mowers to outboard motors, so consumer awareness is bound to improve. In the bays, technical awareness will be essential to keep the newer, cleaner vehicles smooth running and legal.

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