When should you change your spark plugs?

For gasoline engines, spark plugs are essential for your gasoline engine to work. Without them, there is no combustion and nothing will propel the piston down and turn the crankshaft. However, like your oil and your other general car maintenance consumables, spark plugs also have a lifespan. So here is a handy guide to help you figure out if you need a new set of plugs and what to get. 

On a side note, if you have a diesel engine in your car and someone tells you that you need new spark plugs, just nod and say yes because diesel engines don’t need spark plugs. 

Like a lot of maintenance practices, better to be safe than sorry. Spark plugs play an integral role in the combustion cycle of the engine, and if that component were to fail, it would lead to an expensive fix. 

The first sign that you should pay attention to is your odometer reading. Your service manual will include the interval for a spark plug change. That will depend on what plugs you use. If you hit a certain mileage with your car, then it’ll be time to swap out the old plugs for new ones even if they are in working order. Again, better to be safe than sorry. 

Judge how your engine idles. Rough idling when your car is warmed up is a sign that you need to have your plugs replaced. To monitor this, check your car's tachometer and see if the needle jumps up and down noticeably. Also, if you feel the car's not running smoothly and there is an excess amount of vibration, that's another telltale sign. 

Loss of power or hesitation while on the accelerator can be attributed to a number of other factors, but a bad spark plug can be part of the problem or the only problem you need to fix. Again, this is another sign that you need new plugs. 

Poor fuel economy might not be noticed right away, but if you notice that your fuel economy is way below what it should be, then it may be time to check your plugs. Loss of power and fuel economy go hand in hand, and you may not notice your foot pressing deeper and deeper into the accelerator thus consuming more fuel than needed. 

If you can’t get your car to start, it is likely that there’s more than one problem plaguing it, or that a spark plug is a culprit if the engine is not turning over. 

If you have the tools to take a look, usually you will need a specific tool to be able to take the plug out of the socket, and it is also a question of accessibility. For example, boxer engines like the one 2.0-liter found in the Toyota 86, have their plugs mounted off to the side, perpendicular to the opening of the hood, however, most cars with inline engines such as the Mitsubishi Xpander have easily-accessible spark plugs that will allow to access and check for yourself, with the right tools of course. 

You will need a deep socket and a socket wrench in order to loosen the plug from its seat. The size of the socket that you will need will depend on the car you have and the spark plug that is installed. Once free, take a good look. 

Now go to the electrode. You will notice that a well-used plug will have a darker tip compared to a new one. This is called fouling, and is caused by the carbon buildup that is part of the combustion cycle. When a plug is too fouled up and looks like it is burnt, then that means that you will need a replacement. 

Depending on the number of cylinders your engine has and depending on the technology accompanying it, you will have at least one spark plug per cylinder. 

Now, it's important to make sure that you have the right spark plugs in the right spec. Consult your manual or service guide of your respective automobile to get the right plug for your motor. Take note of the diameter and the gap of your plug to ensure proper fit in your engine. 

Also, pick the material of your ground electrode. There are three main types that vary in price, conductivity, and longevity. 

• Copper 
• Platinum 
• Iridium 

Copper is the most affordable and lasts about 20,000 kilometers or a year before you will need to change them out. The tradeoff is that copper plugs have a strong spark which will give you a better ignition. If you frequently change oil or if your car has a short maintenance interval then a copper plug would do you well.

A platinum plug, meanwhile, will give you a slightly weaker spark but last a lot longer than a copper plug. It is also a bit pricier. Expect it to last about 30,000 to 50,000 kilometers before it needs changing. As the middle-ground option, it does well to balance spark and longevity. 

However, the last type on our list is a commonly recommended plug because of its longevity. It will spark well enough, but the real draw is the longevity. You get from 50,000 to 100,000 kilometers on a single set, but changing earlier if you drive your car hard is recommended but this comes at a price since iridium plugs are the most expensive among the three. 

Above all else, make sure that you change your plugs in a set. Changing only one plug may mess up your service cycle, so when one goes bad, it would be wise to change out all the other as well.