Spark plugs are incredibly important to the overall functioning of your engine. Without a spark plug, you will not have a combustion reaction in your engine and you simply can't drive your car. As important as spark plugs are, they're also often an overlooked and underappreciated part of your vehicle as well. They're usually very affordable to pick up, so many people don't give them the second thought. However, when your spark plugs aren't working you'll know it because your vehicle's performance is going to greatly suffer as a result. When you need to get your spark plugs replaced, a mechanic is probably going to charge you anywhere between $50 and $150 to get the job done.
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Spark plugs themselves are not that expensive. As with any repair job, the majority of the cost you're paying to mechanic use for the labour. You can go to a site like AutoZone right now and you'll see that you can pick up new spark plugs for as little as $6. The majority of spark plugs cost about $10 a piece. You can find higher-end the spark plugs that might cost you $40 a piece as well.
You need to know that a more expensive spark plug doesn't necessarily make it a better spark plug. If a dealer or a mechanic is doing a spark plug replacement for you and they charge you $40 or $50 for a spark plug, you should be making sure that's exactly what you want. There's no reason to pay that much for a spark plug and unless you are driving some kind of performance automobile that requires a very rare and expensive kind of spark plug, it shouldn't be that necessary to pay that much for the part. Always get a cost breakdown for a repair job, so you can make sure that you're not overpaying for a part that should be pretty cheap.
What is a Spark Plug?
It's worth noting off the bat but not every vehicle actually even uses spark plugs. Obviously electric vehicles work much differently than traditional gas-powered automobiles and as such they don't require a spark plug because there is no fuel to ignite. Likewise, diesel engines don't use spark plugs the same way that your gasoline power engine will. But every other standard combustion engine, including those in hybrid vehicles, uses spark plugs to ignite the fuel and air mixture in the combustion chamber so that the combustion reaction can occur, and the engine can actually perform its function.
In the simplest terms a spark plug is an electrode with a ceramic insulator and a metal shell. The electrode inside connects to the output terminal of your ignition coil. you screw the shell into the cylinder head which grounds it and the electrode sticks out through the insulator into the combustion chamber. There is a tiny gap at the top in which the spark reaction occurs between two electrodes.
Although it depends on the individual spark plug, they usually operate on a voltage of 12,000 to 25,000 volts. Some spark plugs will get as high as 45,000 volts. When the actual spark occurs, the temperature gets up to 60,000 Kelvin. This allows the fuel and air mixture to ignite creating the explosion that is the combustion reaction that causes your engine to function.
The electricity comes from the ignition coil and creates that tiny spark in the gap to ignite the aerosolized fuel and air mixture. If you could watch the reaction as it happens you would see a tiny arc of electricity go between the tip of the spark plug and the tiny electrode that hangs over the top of it. Anytime someone that refers to the gap in their spark plug, that small space is what they're referring to.
Most spark plugs are made from a copper core so that they can efficiently conduct electricity. Copper is remarkably good at this, which is why most household wiring is made out of copper. It's remarkably good at conducting electricity but it is not very resilient. When you expose copper to high temperatures it will melt which is why most of the copper is coated with a much stronger metal like nickel in order to make it durable. There are all higher-end spark plugs that you can buy that are made of rarer and precious metals like iridium or platinum. These are great conductors of electricity and they can also resist corrosion and heat damage for a long time as well. As you know however a metal like platinum is much more expensive than one like copper
When spark plugs work, they do so extremely fast. When you read it in text like this it seems like a very slow process. The power goes from the coil through the electrode and then creates a spark in the gap. However, in your engine this happens blindingly fast. To use a common example, if you have a 4-stroke engine that's operating at 3000 RPMs you're going to be having 1500 sparks every minute in the engine. This is why the precision timing of your engine is so crucial. For a spark to happen 1500 times a minute, everything needs to be timed down to the fraction of a second. If it's off even slightly you can see how you may be having combustion at the wrong time, or not at all. It would take barely any variance in the timing for this to occur.
One single spark plug should be good for millions of sparks over the course of its lifetime before it needs to be replaced. And it will need to be replaced. Operating at the temperatures that spark plugs operate, and at the frequency they operate, it's almost unimaginable that one could be expected to last forever.
Signs and Symptoms of a Bad Spark Plug
No spark plug can be expected to last forever and after about 20,000 miles to 40,000 miles if you haven't replaced your spark plugs, you should really take a look at them to see how they're doing. That's the average lifespan of your typical spark plug these days.
If you happen to be using some higher-quality spark plugs, the kind that are made with iridium and platinum, then you might be getting a little more life out of your plugs. These ones have life spans that can extend from about 60,000 miles to 100,000 miles. There are some very rare circumstances in which drivers have had spark plugs last them 150,000 miles or more, but you should never expect that to be the case. If you're experiencing any of these symptoms, it's a good sign that you need to at least inspect your spark plugs, and probably have them swapped it for new ones.
Bad Gas Mileage
When you notice a sudden drop in your gas mileage which is to say you're having to go fill up more often than usual but you're not doing any difference driving that it's possible your spark plugs are to blame. This is because as the fuel is injected into the combustion chamber, if the spark plug doesn't ignite it that fuel is just wasted. Your engine will struggle to make up for it in the cylinders that are still working, and you'll continue to waste fuel until you get the issue addressed.
Misfires
Engine misfires and engine hesitation are both symptoms of a bad spark plug. Of course there are a few reasons that you could experience an engine misfire but a poor spark plug is definitely one of them because it's not sparking when it's supposed to and the combustion reaction can't occur as it's required. This is actually the origin of the term misfire, because the combustion reaction is not happening, and it is literally misfiring.
Poor Acceleration
When your spark plugs become dirty or corroded, they're not able to create the spark at all or at the right time for your combustion reaction to occur. This is going to manifest as an inability for you to get up to speed the way that you're used to. Remember that if you have a four-cylinder engine and one of the cylinders doesn't fire, you're losing 1/4 power right there. That happens each and every time that cylinder doesn't fire correctly. If you have more cylinders not firing, then you're going to lose power equal to those as well. This can really make your vehicle struggle to get moving at all.
Hard Starts
There are times when starting your vehicle seems like a bit of a chore. You put the key in the ignition or press the button and it's like something's getting started but then it doesn't actually happen. It may take a moment before the vehicle to start and when it does it kind of surges on you. That can be the result of the combustion reaction not happening the way it's supposed to in the first cylinder that would normally get your engine started. If that spark plug isn't working, then the next plug in the next cylinder has to take over so you'll have that brief moment where things aren't working the way they're supposed to.
Why Do Spark Plugs Go Bad?
As we said, spark plugs have a limited lifespan and there is a good reason for it. Remember that a spark plug has to create many millions of sparks at a remarkably high temperature over and over again. There are a number of things that your plug can experience as it's trying to do this job that can limit its ability or cut it off entirely.
Bad Gap
As we mentioned, the tiny space for the spark occurs is called the gap. You may not think much of it at first glance, but that gap is a very precisely engineered piece of technology. The space between the tip of the spark plug and the electrode is scientifically calculated to create the proper spark to ignite the fuel in your engine. If a manufacturing defect occurred and the gap size is off or some kind of corrosion or debris got in there and is altering the gap in some way then the gap size will be incorrect and the timing of your sparkly off as a result.
Overheating
Because spark plugs have a copper core and are not able to handle very high heat without being mixed with another metal like nickel, the heat that spark plugs operate at can sometimes have a negative effect on how the plug works. When properly made of operating at proper temperatures your spark plug should be able to handle everything, but if your engine begins to overheat even the nickel alloy may not be up to the task of keeping the spark plug working properly. If you're having issues like pre-ignition or the cooling system has failed in your engine begins to overheat this can cause excessive wear and tear on your spark plug and make it wear out much faster than we normally would.
Corrosion Buildup
Because of what spark plug does it's very susceptible to getting covered in a black carbon build up over time. This is a very hard film of carbon that coats the end of the plug and ends up causing it to perform worse over time. It eventually can stop it from working at all.
Oil Leaks
If you have an oil leak somewhere in the combustion chamber it quill likely build up around the spark plugs especially on the tip where the spark is supposed to occur. This problem gets worse as dirt and other debris in your engine will get stuck to the oil preventing the spark from occurring. This will cause you to have engine misfires and potentially the inability to start your car at all if it happens in all the plugs.
The Bottom Line
For all the work that a spark plug does to get your car going it's impressive that you can still get them so cheaply at stores. That's a good thing considering how often you need to replace spark plugs to keep them working properly which means that it's not going to break the bank to get this job done. Luckily, if you don't want to pay for a mechanic to swap them out at a cost that could be well over $100, it's also fairly simple to do this job yourself. All you need to do is head over to YouTube and check out a video that can walk you through the process if you've never done it before and you can save yourself a lot of money by changing your own spark plugs at home.