Some thoughts on spark plugs

Advances in technology have lengthened the maintenance intervals for many pieces on our cars. One of those pieces – the one I am writing about today – is the spark plug. All gas engine cars have them.

When I started in the car business, it was common for plugs to need cleaning every year, and replacement by 15,000 miles. As technology improved and engines got cleaner, the plugs started lasting longer. First it was 30,000 miles, then 50,000, 60,000 miles.

Today, many of the new cars we service have 100,000-mile spark plugs installed at the factory. That 100,000-mile rating was derived by installing the plugs in test vehicles, and then driving them hard and fast to pile on the miles. At various intervals the plugs were removed and inspected for wear. After a number of engineering tweaks, Bosch, Beru, NGK, and other spark plug manufacturers came up with a plug that would last the 100,000 miles and still perform acceptably. Based on that, the carmakers established the current change interval for spark plugs.

So the question today is: How often should you change your plugs, if you have a car with the 100,000-mile change interval? Should you follow the manufacturer’s recommendation, or do something else? Why?

The first thing I point out to new clients when we discuss maintenance is that there is a time component to service as well. Spark plugs may hold up fine for 100,000 miles if driven on the highway every day, but short trips and occasional use will wear them out a lot faster. A person who drives 10-12,000 miles per year may not hit 100,000 miles for almost ten years. That’s way to long to leave a set of plugs in the car.

Carmakers recognize that. If you look in most owners manuals you will see a time specification for plugs. They’ll say something like five years or 100,000 miles. I strongly suggest you pay attention to this time limit when considering long-life wear items like spark plugs.

I have read of spark plugs breaking off in the cylinder head when removed after many years. I’ve never experienced that on a five year old car, but there may well be parts of the world where corrosion is worse (near the ocean, as an example) and it you live in such a place, you’d be wise to consider that fact and adjust your service intervals accordingly.

When changing plugs, the next decision a motorist faces is what plug to buy. If you are at the dealer, the decision is simple: you’ll get original equipment plugs. If you’re at a Bosch Car Care Center, you should get the correct Bosch plugs, and if you’re at an independent or chain store, you best find out what they propose to install to be sure you are comfortable.

Thirty years ago, selection of spark plugs was simple. A dozen part numbers would service a majority of the cars on the road. Today many cars have special plugs and it’s important to install the right one. Every parts store has cross-reference catalogs, all of which lead you to believe any “crossover” plug will work. In my experience, that’s often true for older cars but often wrong on newer vehicles. We’ve seen Land Rover, Mercedes, and BMW cars with ignition misfires that were ultimately traced to “supposedly correct” but off-brand spark plugs.

If you have a late model car, be sure you fit the right plugs.

The last point I’d like to address with spark plugs is what happens if you don’t change them in time. As plugs age, the voltage to fire them increases. A plug that needs 20,000 volts to fire when new may need 80,000 by the time it’s used up. If you go beyond that, the voltage may rise to 100,000 volts or higher. This increased voltage puts much greater stress on ignition coils and wires. Premature ignition failure is the usual result of running plugs too long.

Six spark plugs might cost $80 for your BMW or Mercedes. Six spark plugs and six coils (because you waited too long) could cost $700, maybe more. As you can see, changing plugs before the ignition fails makes very good economic sense!