The spark plugs inside your car's gas engine are remarkably durable and could last for dozens of oil changes before needing to be replaced. For components that withstand intense heat, pressure, high-voltage electrical currents, and vibrations, the spark plugs typically remain unnoticed until something goes wrong.
They are rather small, but they do big things for your motor, and the combustion process wouldn't be complete without them. Despite their hard-wearing nature, all spark plugs will eventually break down. The most common signs that your motor needs new plugs are starting problems, poor fuel economy, lackluster acceleration, and a general feeling of lethargy from the engine.
Spark plugs wear out at different rates, so claims that engines need new plugs every 20,000 or 30,000 miles are nonsense without knowing what type of spark plugs are in the cylinder head. Moreover, you can clean and reuse old spark plugs, but don't expect them to last as long or deliver the same level of performance as new spark plugs. Just like how there are several motor oil myths that won't go away, there are also various pervasive myths about spark plugs. Here are the spark plug myths that are just that — myths.
It may not look like it, but spark plugs come in many types and sizes. The most common types are copper, platinum, and iridium, and the difference lies mostly in the material utilized for the ground and/or center electrodes. Installing the wrong type could result in hard-to-diagnose misfires and mediocre acceleration.
For instance, downgrading to incompatible platinum or copper plugs in an engine that needs iridium will surely lead to performance variations, idling issues, excessive engine vibrations, and premature spark plug wear. Meanwhile, the size of the spark plug has to do with the thread diameter, reach, and the socket size, so you can't just swap the spark plugs from a Toyota V6 to a BMW inline-six and expect them to fit and work flawlessly.
And even if a random spark plug does fit in the cylinder head, it doesn't mean that it'll keep on sparking without bringing with it a host of problems. Spark plugs have hot and cold heat ranges, and using the wrong type could either foul the spark plug tips with carbon deposits or pre-ignite the air-fuel mixture, which are both bad news for your engine and bank account. There is no one-size-fits-all solution when it comes to spark plugs, so your best recourse is to consult the owner's manual to determine the correct type, size, and heat range for your engine.
There is no such thing as a forever part in a car. For instance, most new cars with no dipsticks in their automatic transmissions are purported by the manufacturer to have lifetime CVT or transmission fluids. However, the truth is that flushing and refilling with fresh fluid every 30,000 to 60,000 miles is necessary to prevent damaging the gearbox, especially when consistently driven in extreme hot or cold weather or when towing or hauling heavy cargo.
It's a similar deal with spark plugs. They don't last forever, although some can deliver more miles before wearing out completely. The plugs will mainly wear out and fail from overheating or carbon buildup, but some can last longer than most. Basic copper plugs need replacing every 20,000 to 30,000 miles, but platinum and iridium spark plugs are hardier and can last upward of 100,000 miles or so. Yes, spark plugs are long-lasting components, but they won't last forever (because nothing does).
It does make sense to leave the other spark plugs alone if only one is acting weird. Despite their relatively low cost, replacing eight or 16 iridium or platinum plugs can still dent your budget, but what's the point of spending all that money if only one spark plug is faulty, right?
Wrong. You can replace just one spark plug if you like, but it doesn't ensure a smooth-running and reliable engine. For starters, mixing new plugs with old ones can imbalance your car's ignition system, potentially causing misfires and poor fuel economy. Moreover, a single worn plug means the others are most likely on their way out, so it's wiser and more economical to replace them all at once rather than one or two at a time.
Of course, it's perfectly fine to replace just one spark plug if you're in a pinch or in an emergency, but keep in mind that doing so is just a temporary fix and is not a long-term solution. Replacing all the plugs at the same time is a surefire way to prevent future engine issues and costlier headaches.
If it were true that high-performance spark plugs can add horsepower, everyone would be scrambling for an upgrade. And when we say performance plugs, we're talking about platinum or iridium spark plugs with advanced materials and longer lifespans. If the owner's manual recommends platinum spark plugs, upgrading to iridium is possible without any negative consequences, other than the cost. For reference, platinum spark plugs cost around $4 to $8 each, while iridium plugs can cost upwards of $15 each or more.
But in doing so, don't expect your engine to magically churn out more horsepower and torque, since performance spark plugs won't necessarily provide noticeable horsepower gains. Iridium plugs are more about efficiency than adding power, and you're essentially paying for longevity rather than a power-boosting mod.
Engines didn't just appear from thin air with copper, platinum, or iridium plugs. The type of spark plugs was specifically chosen by the engineers who designed and built the engine, and they did so to ensure the motor's reliability, serviceability, and performance. Hence, there's some hard-backed science as to why your engine has that type of plug, and no myth should get in the way of that.