Without properly functioning spark plugs you can't have a vehicle that runs properly. Spark plugs create that spark that ignites the fuel and air mixture in your engine to allow your vehicle to actually function. They are relatively inexpensive, but their importance can't be overstated. Because of the work that a spark plug does, firing up a tiny spark over and over again thousands of times every time you're out driving, they’re subject to a lot of wear and tear. That means that eventually your spark plugs are going to wear down and need to be replaced. While a mechanic may charge you $50 to $150 to get your plugs replaced, you can save yourself a lot of time and money by getting it done yourself.
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When Do Spark Plugs Need to Be Changed?
New spark plugs might cost you as little as $6. Your average spark plug tends to cost between $10 and $15 and some high-end ones are as much as $40, but you probably don't need anything quite as fancy as all that. Depending on the kind of spark plugs you have, you can expect that they're going to need to be replaced anywhere between the 30,000 miles and 100,000 miles. That's quite a big range and it does depend on the kind of plugs you have. More expensive plugs that are made with metals like platinum and iridium can last you 100,000 miles, but some cheaper copper plugs are definitely going to need to be replaced sooner.
Signs and Symptoms of a Bad Spark Plug
Aside from the routine maintenance there are some signs that your spark plugs are going to need to be replaced when they go bad. If you're experiencing any of these symptoms, you should definitely give your plugs a look to see if you need to swap them out or not. There are a number of reasons your spark plugs will go bad on you from corrosion buildup around the gap that produces a hard film of black carbon, to oil leaks accumulating debris that prevents proper sparking, to even things like a bad gap, where the space at the top of the plug where the spark is supposed to form being incorrectly machined and not working properly.
Engine Misfires
One of the most common symptoms of a bad spark plug is repeated engine misfires and engine hesitation. Because the spark plug causes the spark that produces the combustion reaction allowing your engine to work, if the spark plug is bad, you're going to be experiencing misfires that prevent the combustion reaction from occurring at all.
Bad Gas Mileage
If you're finding that you have to head to the gas station more often than usual and your car is actually performing worse than usual, you're having poor acceleration and so on, it's entirely possible that a spark plug is to blame. If the spark isn't occurring at the right time, then your injectors are going to be spraying fuel into the combustion chamber and wasting it because it's not being ignited correctly. That translates into more trips to the gas station for you.
Hard Starts
If your vehicle seems to struggle a bit when you're trying to get it started at first, as though it's thinking before it gets started and then it sort of lurches or jerks when it does get going it's possible that the spark plug is the reason for this as well. This happens because your combustion reaction isn't happening exactly the way it's supposed to in that cylinder to allow your engine to get started smoothly and run the way you expect it to the moment you turn your key in the ignition. The second cylinder may take over and start your vehicle, so you have that hesitation and jerky pause before your car gets going.
Changing A Spark Plug Step by Step
Changing the spark plugs in your vehicle it's not a very hard task at all. You just need to make sure you know exactly what you're doing and take your time, changing spark plugs in order one at a time. That should ensure that, going forward, your spark plugs are working fine and you're getting the kind of spark you need to allow your engine to function properly.
You need a few tools to make sure you can get this job done properly so if you don't have everything, make sure you pick it up or this job is not going to work out for you.
- A socket and ratchet set
- Some clean rags
- A spark plug socket which should be included with your socket set
- A gap gauge. This is the most specialized tool that you'll need for changing your spark plugs and if you don't have one you could pick one up at your local auto parts store. You can find them on Amazon for under $10.
When you are sure you have all the parts necessary you can begin the process of changing out your spark plugs.
Step 1: If you have never done this before then you need to make sure you know where your spark plugs are in your engine. Check with your owner's manual if you're new to this kind of work because it's not always obvious what you're looking for. Depending on the kind of engine you have you should see a series of wires, 4 to 8 of them, connecting to different parts on your engine. Spark plugs are going to be located under the plug covers of these wires on the engine. Again, this does depend on the type of engine you have.
For a 4-cylinder engine you're going to find the spark plugs on the top or the side of the engine, lined up in a row. In 6-cylinder engines they'll also be along the top or the side of the engine. if you're dealing with the V6 and V8 engines the plugs will be split up on either side of the engine. and of course, some car manufacturers hide the entire engine below engine covers. This is just a plastic case that makes your engine look nice and conceals everything beneath it. You'll need to use your socket set to loosen the bolts that hold it down and take it off before you can find your spark plugs.
Step 2: Make sure you pay attention to which of those electrical wires went to which plug. You’re going to want to reconnect them exactly as they were and if you just pull them off and toss them aside, they can get jumbled up and it can get confusing. Go ahead and label them if you need to.
Step 3: Working with a cold engine, you can start to remove your first spark plug. Start by removing the wire that covers the spark plug. Grab it near the base and pull it up as gently and evenly as you can. It may take a little work, but don't pull it too hard because you don't want to actually ruin that wire. Just work it up if it's not coming easily until the spark plug itself is exposed.
Step 4: You can now use your socket to remove the spark plug. If necessary, fit an extension bar on your ratchet to get a good grip on the spark plug and remove it.
Step 5: You can now give your new spark plug a quick inspection, and you're going to use your gap tool to measure the spark plug gap. The gap tool will look like a key ring of small metal pieces, a Swiss army knife or a coin depending on the kind that you get. The point of this is to, as the name suggests, measure the gap in your spark plug.
The spark occurs between those two little pieces of metal in the gap and it needs to be a precise size for this to happen. In fact, every time your spark plug fires, which can happen millions of times over the lifetime of a single spark plug, a tiny little bit of metal gets worn away. That's part of the reason spark plugs end up dying because, over time, all that sparking wears the metal away enough that the gap gets to be the wrong size and the spark can no longer properly form. That ends up causing things like your engine misfires and your car cranking but not turning over.
Depending on the spark plug there is a specific measurement that you're looking for with your gap tool. Typically, you are looking for something between .028 and .060 inches. You need to refer to the manufacturer standards to know for sure, however. While there is a tiny bit of wiggle room, there's not a lot. If it's off too much, then the spark plug definitely needs to be replaced. A brand new spark plug should be accurate and the measurement you want, but there are manufacturing defects that happen all the time so it's not impossible that the gap will be wrong. That's why you measure it before installing it.
There are some spark plugs that you can adjust if the gap is off. Adjustable gap spark plugs let you move the gap until you get the correct measurement. Just make sure you get it to the correct gap before you take the time to install it.
Step 6: At this point you need to put a little bit of anti-seize lubricant around the threads of the new spark plug to install it back in the engine. This prevents metals from reacting with each other and it will allow your spark plugs to work better. You want to inspect both the threads of the plug and the socket where you're going to plug it into. If necessary you can use a wire brush and some compressed air to clean that socket to make sure there's no buildup or debris in there that's going to get in the way of your spark plug being screwed In properly.
Step 7: You can start by hand tightening the spark plug into the socket. When you do use the ratchet, make sure you check the torque guidelines for how to install your spark plug. You never want to over tighten one of these because you could strip it and effectively ruin the work that you just did. If you strip the threads in the head of the engine, that's going to be a pricey repair job. You probably don't want to go more than one 8 to return past what you could tighten with your bare hand.
Step 8: You can repeat this process with your other spark plugs. If you're not replacing all of them, it's still a good idea to do a visual inspection. If you notice any white, crusty buildup like corrosion then you can use a fine wire brush and give them a quick clean if necessary Or just replace them.
Step 9: Make sure you replace the spark plug wire boots on the top of the correct spark plug. Spark plugs have to fire in the correct order, so you don't want the wrong wire connecting to the wrong plug. Make sure it gets installed firmly and securely so that it snaps into place. When this is done wrong, you may not be able to get the right spark and end up causing more engine misfires.
The Bottom Line
Spark plugs are relatively cheap and pretty easy to replace at home. You can save yourself a couple hundred dollars in labour cost by doing this yourself once you know what's happening. You just need to make sure you've got the right kinds of spark plug for your vehicle as there are many different options out there. Your best bet is to use the exact same kind of spark plugs that your car had installed to begin with. If that's not an option, make sure you buy some good quality replacements. There are a lot of cheap spark plugs out there, some are only two or three dollars, but better quality plugs are going to last you a longer time.