Auto Service World
Feature   June 1, 2013   by Andrew Brooks

Hanging Tough

Spark plugs and sensors are subject to extreme stresses - so looking after the basics is even more important


The working conditions facing spark plugs and O2 sensors aren’t getting any better. Smaller engines with higher operating temperatures, a bewildering array of fuels and additives, and onboard diagnostics and monitoring systems that are paying closer attention than ever to every phase of engine operation mean that plugs and sensors are expected to perform more precisely and consistently than ever, while delivering premium performance under tighter and tighter tolerances.

“Car manufacturers have much tighter specifications today than they used to,” says Tom Potter, a senior service engineer with DENSO Products & Services Americas Inc. When the basics are looked after and an engine is well maintained, spark plugs can survive the most stressful conditions much longer. “The better the engine, the better the plug life,” Potter says. “If you have a car running on good fuel, where the engine oil and air filters are changed out when they should be, the plugs will last you a long time, especially with good driving.”

Potter worked as a tech in Southern California for some 20 years and he knows the importance not only of extending spark plug life though proper maintenance, but also how much a good tech can tell by removing and examining the plugs at service intervals.

“You can tell a great deal about the engine condition,” Potter says. “You can tell the fuel being used, and also if you have an imbalance in the fuel mix. A good tech will carefully look at each spark plug as they’re taken out. If they’re burning correctly and all look the same you know you have a balanced engine, and you won’t expect problems after the tune up – but if you have one plug darker in colour, or signs of rich fuel, you have a fuel injector problem, or else something’s going on in that one cylinder.”

Potter recalls that he was able to identify the fuel brand being used by distinctive spark plug colouring. And there’s more information to be gleaned by close observation. When a plug is white or powdery, Potter says, the vehicle probably has a vacuum leak near that intake port, which will cause the engine not to run correctly and will create valve problems later on. “Almost everybody’s spark plug site has the same reference pictures,” Potter says. “They’ll tell you how to ‘read’ spark plugs. It’s taught in all the trade schools.”

Since about 2000, spark plugs have shown dramatic improvements in lifespan and much better tolerance for temperature levels and variations, says Richard W. Hibbs, project manager for ACDelco Canada – but good maintenance is still vital in maintaining high levels of engine efficiency. “The plugs don’t create the abundance of misfires they may have in the past,” Hibbs says, “so this is how they could get overlooked and go beyond the OEM service intervals.” Even plugs that have lasted as long as 160,000 km still need to be replaced in order to boost the efficiency of fuel burn. Also, the metals in the plug and in the engine itself are subject to huge temperature changes, and there’s always the possibility that the plugs will become very difficult to remove – or even wind up seized in the engine.

As spark plugs evolve, diameters are shrinking while length is increasing, says Sukhneet Mavi, product manager, spark plugs for NGK Spark Plugs Canada. “The older ones were 40mm diameter with a ¾-inch reach for the threads,” he says. “The newer ones are mostly 12mm diameter – thinner but longer – and we’ve seen some that can go up to a 26.5 mm reach.”

New plugs are able to accommodate a better flame kernel, thanks to the fact that now the actual electrode protrusion inside the combustion chamber is much greater, Mavi says. “We’ve seen plugs going up to 7 mm so they fire much deeper inside the chamber. This gets you more efficient combustion.” But it also imposes greater strain on the plug, which impacts service life, he says. “The electrodes retain more heat during combustion, and they draw that heat away from combustion.” Under higher RPM conditions this increases erosion of the electrodes.

The industry has responded with higher-grade metals. “Advances in plug technology have largely been focused on the use of precious metals to reduce gap erosion and extend service intervals,” says Michael Kollenberg, product manager, Champion Spark Plugs, Federal-Mogul Corp. “This has been driven by OEM concerns about the access to and serviceability of their increasingly sophisticated and tightly packaged engine bays.” Kollenberg notes that today’s plugs employ iridium, a dense and highly durable metal. “The old nickel plugs won’t do anymore, and in response the industry has seen a succession of progressively higher-tolerance metals being employed.” Iridium is preferred due to its high durability, which helps slow gap erosion, and its terrific ability as an electrical conductor.

Despite advances, however, the importance of regular maintenance – and the short span of some maintenance intervals – is still not as well understood as it should be, says Jay Buckley, technical training director for Fram, Prestone and Autolite. “We find most drivers simply ignore spark plug maintenance until they either notice a drop-off in fuel economy or the check engine light is on and the engine is misfiring,” Buckley says. “When the check light is flashing, the catalytic converter can be damaged.” Buckley adds many technicians continue to assume all cars have 100,000-mile plug change intervals, and while that is true in many cases, some automakers still use copper core technology and specify 32,000- or 60,000-mile plug changes.

Sense ability

When servicing and maintaining oxygen sensors, a consistent understanding of differences in operation due to differences in design is required, says Hibbs. “A technician must have regard for year, make and model of a vehicle because there could be different O2 sensors in different vehicles, models and engine displacements. There are two, four and six-wire applications, air/fuel, wide band designs, and O2 sensors can be anywhere from having two to six minimum to an application.” It’s vital to understand scanner data in order to interpret correctly the different designs and functions for proper diagnosing or to prevent misdiagnosis, Hibbs says. “Oxygen sensors also have a lifespan for efficiency, but do require regular interval changes to keep fuel efficiencies at their best.”

At the basic level, O2 sensors benefit from the same basics as spark plugs: they’ll deliver a longer service life if the environment they operate in is in good running order, Potter says. “They’ll last virtually forever if conditions are optimal. But if you start getting oil through the engine or you’re running weird fuel additives, then you shorten the service life.” Potter notes that it’s very rare for the sensing part itself to fail. The sensing part usually gets coated or poisoned from foreign chemicals. The weak spot is the heating element, he says.

“The actual service life of a sensor varies a lot,” Mavi says. “You could have failing engine components that lead to the check engine light coming on – in that case the sensor is simply saying that some component has failed or is failing. Or the issue could be that the actual electrical components that led to the sensor varying have now corroded, and now there’s extra resistance in the circuit and the sensor is failing. Or the sensor has degraded to the point that it can’t function properly.”

Compounding the problem for the te
chnician is the fact that the technology has evolved beyond the point where the response is simply to change out a failing sensor and erasing the codes. “That’s not going to work anymore,” Mavi says. “Now the vehicle has to go through a certain set of drive cycles or procedures that are different for each manufacturer for the sensor to get reset. Otherwise that check engine light will come on again after a few days.”

Technician training is essential to avoid misdiagnosis based on sensor readings, says Carey Banks, product manager for Robert Bosch Inc. “It’s important to understand that oxygen sensors could also alert to problems with other components,” Banks says. “Contamination found on the O2 sensor tip can be signs of a bigger problem. Considering the advances in O2 technology, it’s highly recommended that technicians use diagnostic equipment to understand the root cause of ECU codes.”

Banks says that while past service life for sensors averaged out at about 30,000 miles, nowadays that figure is well over 100,000. In fact Bosch says it produces O2 sensors that can last 150,000 miles or more in some cases.

“Given the intelligence of the ECU, newer heated O2s should only be replaced when the ECU deems the O2 is defective,” Banks says. “There will be no performance advantage in replacing O2 sensors when not defective.” The basics still count, too, Banks says – “we do recommend consulting your vehicle manufacturer’s guide for further information on maintenance schedules.


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