This story was part of Counterman’s annual Technical Sales Seminars, which was published in the April 2009 issue.

Spark plugs are not the maintenance item they were years ago. But they still have to be replaced, and more often than may people realize. Most late model engines come factory-equipped with some type of long-life spark plug. The plugs might be platinum or they might be iridium. Both of these metals have a very high melting point and are very wear-resistant. Platinum is two to three times more expensive than iridium, but iridium has a melting point over 1,200 degrees F higher than platinum and is more corrosion resistant than almost any other metal.

100,000-mile life
The factory-recommended spark plug replacement interval for most platinum and iridium spark plugs is typically 100,000 miles — but this assumes normal driving with no other conditions that would accelerate plug fouling such as excessive idling, frequent short-trip driving, or worn valve guide seals or pistons rings that increase oil consumption. Under these kinds of operating conditions, any spark plug has an increased risk of fouling and misfiring.

One of the first clues that a spark plug may be fouled and misfiring is a Check Engine light. Since 1996, most cars have been equipped with engine misfire detection as part of the OBD II system. Misfires are bad news because they waste gas, reduce engine performance and cause a big jump in tailpipe hydrocarbon emissions. The catalytic converter can handle some misfires up to a point. But an engine with a steady misfire or multiple misfires can quickly overwhelm the converter’s ability to reburn the pollutants. Worse yet, misfires can make the converter overheat — and if it gets hot enough, it can suffer permanent damage.

A vehicle cannot pass a plug-in OBD II emissions test if the Check Engine light is on (regardless of the cause), and a vehicle with a steady or severe misfire problem won’t pass a tailpipe test. So motorists who live in areas where emissions testing is required have no choice but to replace the spark plugs if they are worn, fouled or misfiring for whatever reason.

One thing to keep in mind here is that misfires can be caused by problems other than bad spark plugs. A dead fuel injector or one that is dirty and is not supplying enough fuel to the engine can cause a condition called “lean misfire.” Replacing the spark plugs won’t solve this problem, but cleaning or replacing the fuel injectors will. Misfires can also be caused by compression leaks in a cylinder. The cylinder may not hold compression if an exhaust valve is burned, if a valve is bent, or if the head gasket is leaking. These are all major repair items, so a new set of spark plugs won’t help anything if compression is low.

BUILD-UP OF DEPOSITS
New spark plugs will make a big difference, however, if the old plugs are worn or fouled. Fouling occurs when fuel deposits build up on the electrodes. If the engine is driven more than a few miles, the plugs should get hot enough to burn off these deposits. But if the vehicle is never driven far enough or fast enough to burn off the deposits on the plugs, the plugs may foul and misfire.

Electrode wear is minimal with platinum and iridium because of their high melting points and corrosion resistance. Ordinary standard spark plugs, by comparison, experience a fair amount of electrode wear as the engine accumulates miles. By the time the plugs have 45,000 miles on them, the electrodes have worn to the point where replacement is necessary. Some people try to stretch their maintenance dollars by cleaning, filing and regapping their old worn out spark plugs, but the results are not the same as installing new plugs. Professional technicians say cleaning and regapping old plugs is a waste of time. It’s faster, easier and more reliable to replace old plugs with new ones. Internal problems such as bad resistors or a cracked insulator cannot be corrected by cleaning or regapping.

On older vehicles that came factory equipped with standard plugs, or on vehicles where plug access is difficult (like many full-size vans and the rear bank of plugs in front-wheel drive cars with sideways-mounted V6 engines), upgrading to platinum or iridium replacement plugs can extend the life of the spark plugs and reduce the need for maintenance.

In an older vehicle with a lot of miles on it, installing long-life plugs might even eliminate the need to ever change the plugs again (at least for the current owner).

Another application where long-life plugs make sense is on engines with coil-on-plug ignition systems. Removing all of the coils can be a time-consuming task.

Distributorless ignition systems that use a “waste spark” setup (that is, where two plugs share an ignition coil and fire simultaneously), double platinum or iridium spark plugs are a must. That’s because the spark plugs in these engines fire twice as many times as those in other engines. Consequently, if ordinary spark plugs or spark plugs with a single platinum electrode are installed, they won’t make it 100,000 miles because of electrode wear.