There was a time when spark plugs were designed to last for a couple of years.
Bi-annually, vehicles would be brought into the service bay for a tune-up, which often included replacing the plugs, possibly the ignition wires as well as inspecting the distributor cap, rotor and coil.
Today, aside from classic vehicles, the concept is almost obsolete. Modern iridium or platinum spark plugs are designed to last up to 100,000 miles (160,000 km) and hard starting or rough running issues are often caused by very different factors than in the past.
Mark Lemay, who owns and operates Auto Aide Technical Services in Barrie, Ont., provides technical training and diagnostic services for the aftermarket.
Thing of the past
“Fouled plugs, which would make the vehicle hard to start, are a thing of the past,” he says. “If you go back 30 years, carbon build ups on the plugs would often cause the engine to not start.”
Today thanks to far more powerful ignition coils, Lemay says he rarely runs into these kinds of issues. “If the coil is bad on a modern vehicle, you’re more likely to get a misfire,” he adds. If the vehicle does not start, it is more likely the result of a faulty cam or crank sensor rather than a bad plug or coil.
And while modern sparkplugs do have a much longer shelf-life, with OEMs suggesting they will last 100,000 miles (160,000 km) Lemay does caution technicians to, in some cases, inspect plugs on vehicles at half that mileage, particularly those with aluminum cylinder heads.
He says the reason for this is due to the steel on the plug can react with the aluminum, causing the plugs to seize in the head over time, so in those cases, he recommends pulling the plugs out and inspecting them at around the 50,000 mile (80,000 km) mark. Overall, he says, spark plug manufacturers have improved the product so much that isn’t as much of an issue.
“The coatings on the threads are pretty good now and they use higher-grade steel, so there is less risk of the plugs breaking like they might have done in the past.”
Poor maintenance, bad fuel
At Norlang Auto Service in Langley, B.C., Corey Doell notes that while spark plug technology has improved immeasurably, he still sometimes runs into issues. “Fuel contamination from additives, as well as oil consumption due to excess mileage and poor maintenance, as well as low-tension piston rings they use in modern engines, can still cause plugs to foul and fail early,” he explains.
When plugs do need to be replaced, Doell is an advocate of using what the OEM originally recommended. “We want to ensure we match the heat range and tip design to maximize the fuel burn,” he says. “This improves fuel mileage and extends the life of emissions system components.”
At Green Tree Auto Care in Orangeville, Ont., shop owner Eric Mileham says he does come across plugs that are frequently worn out before the OEM recommended service interval. “Sometimes we see this at 50% of the expected odometer reading,” he explains.
For optimum performance, Mileham says its good practice for technicians to replace the coil boots at the same time as the plugs, to avoid the risk of a faulty coil which can trigger a misfire. In cases like this, he recommends checking the Mode 6 misfire history data. “Occasionally this can provide a clue that the ignition system is struggling and can warrant removing a plug for inspection.”
Other factors such as clogged mass-air flow sensors (due to infrequent cleaning and servicing) and using poor quality, lower octane fuel can also impact ignition system performance, so these are also things that need to be considered if a vehicle is experiencing a rough running condition.
One issue that does seem to be popping up related to spark plugs is carbon tracking. In order to maximize fuel economy and minimize emissions, OEMs are calibrating modern engines to run as lean as possible, which leads to carbon tracking. Mark Lemay explains.
“Electricity always wants to take the shortest route possible, so what happens is that it burns a carbon trail down the side of the spark plug on the porcelain.”
Lemay says that a technician will pull out the sparkplug, see the carbon track on it, and replace the plug, only to find that carbon quickly builds upon the new plug and the car is back in the shop with a drivability issue.
To get around this, technicians should replace the plug, boot and coil all at the same time. Lemay also recommends that for aftermarket shops, it’s important to specifically choose a spark plug designed for that vehicle, whether it’s an actual OE plug or equivalent from an aftermarket supplier. “There needs to be some care taken when you install the new plug. It must be physically the same and it must have the correct threads.”