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How to Check Spark Plugs – Here’s What You Need To Know

Signs of Bad Spark Plugs

In order for the combustion reaction to occur in the engine of your car, you need properly functioning spark plugs. The whole idea of a combustion engine is contingent on the spark produced by those plugs igniting the fuel and creating the combustion that the engine is named for. If your spark plugs don't work, your engine doesn't work. With that in mind, it's a good idea to know how to check your spark plugs in case there's a problem with them so you can identify what it is and fix it. Spark plugs are fairly cheap if you need to buy new ones, in fact you can get a new one for as little as $6, although they average between about $10 and $15 a piece. Heading to a mechanic will set you back around $50 to $150 if you need to get your spark plugs replaced. However, if you know how to handle the job yourself, you're obviously saving a lot of money.

Auto Repairs Are EXPENSIVE


There are a number of signs that your car will give you if you're having a problem with your spark plugs. Knowing how to diagnose them and then what to do about it can save you a lot of time, money, and frustration in the long run. With that in mind, let's take a look at what can go wrong with your spark plugs and what you can do about it. 


How Long Do Spark Plugs Last?


There's no hard-and-fast rule for exactly how long a spark plug is going to last. A number of factors can affect the life span of any given spark plug. This includes the kind of spark plugs that you bought, the make and model of your vehicle, and how you drive the car as well. Cheaper spark plugs are obviously not going to last as long as higher quality ones. Spark plugs made with metals like platinum, iridium, and so on are designed to handle high heat and last much longer than cheaper copper and nickel ones.


In general, you're probably going to have to replace your spark plugs anywhere between 30,000 miles and 100,000 miles. Obviously, that's a really big range to deal with, but those factors we mentioned earlier really play into how it works out for you. On the higher end of things, those platinum and iridium plugs are the ones that are going to last upwards of 100,000 miles. If you're not sure what kind of plugs you have, it's a good idea to check and see so you have an idea how long you can expect them to last before they start giving you problems.


Regardless of the projected lifespan of your spark plugs, there are a lot of factors that can also greatly reduce the life of your plugs. Check out a number of the signs and symptoms that will indicate your spark plugs are having problems and need to be replaced. 


Remember that your car has as many spark plugs as it has cylinders. There are actually some cars that will have more than one spark plug for a cylinder but that's not the normal design for engines. Regardless, if you have a 4-cylinder vehicle, you have four spark plugs that could have problems. When you're checking your spark plugs you need to do each one in turn to see how many, if any, are not working correctly. 


Signs and Symptoms of Bad Spark Plugs 


If you are experiencing problems with your spark plugs there are signs you can be on the lookout for to let you know something needs to be done about it. You should get a good idea of what a spark plug looks like when it's brand new so that if yours are having problems you can easily identify it just from a quick visual inspection. Aside from the visuals, there are also a number of performance issues that can help you identify problems with your spark plugs as well.


Engine Misfires


One of the most common problems with a bad spark plug is that your engine is going to misfire. That's almost a textbook definition of a poor spark plug in fact. If your spark plugs can't produce a spark, that cylinder of your engine will not be able to have a combustion reaction, and that is essentially just what a misfire is. If it happens on a routine basis you can almost guarantee that it's a spark plug causing it and will also lead to some severe damage over time.


Poor Gas Mileage


When spark plugs aren't working then the gas that your injectors are spraying into your engine is essentially just being wasted. That's going to require your engine to push a little harder and use more gas to try to compensate for what's missing, which in turn means you're going to be heading to the gas station more often. Even worse than that, you'll end up getting a worse performance as a result so you're paying more to get less.


Hard Starting


When you first start your vehicle, if you're finding that the engine seems to struggle to get going, like it takes a moment before it actually engages, that can be a problem with your spark plugs. This is due to the combustion reaction not occurring exactly right when you turn the key in the ignition. If the spark doesn't work right away, the second cylinder in your engine may have to take over and get things started. That causes a hesitation and jerking and lurching motions when you try to start the car. 


Carbon Coating


If the top of your plug and the sides are black like they've been on the barbecue, that's a sign you've got carbon build-up. This comes from running with too much fuel, bad wiring, leaking injectors, and other potential problems with the combustion reaction. The carbon coating makes it extremely difficult for your spark plug to actually spark properly. It s possible to clean this coating off and get the plug working again.


Shiny Coating


If the tip and side of your spark plug is shiny that might be an indication that you have a rich fuel mixture. This could also indicate problems with ignition or the heat range. In any event, it's a symptom of fuel fouling your spark plug. That could potentially be fixed by adjusting the fuel injector of your vehicle or checking the heat range of your spark plugs.


 Oil Ash


If engine oil is getting on your spark plugs it could be coming from worn piston rings and damaged valve guides or seals. That's a sign of a much bigger problem in your engine that's going to need to be repaired very quickly before your engine fails on you entirely.


Pre-Ignition Damage


If the side electrode looks like it's been burned away that means your engine is running too hot. The plug is likely firing too soon or there's not enough fuel being burned. This may be related to a problem with your fuel injectors and the timing of your engine. The spark plugs are likely to die pretty soon after you notice this kind of damage.


Red Coating


If you notice that there's a mysterious red coating on your spark plugs that's a sign that you have lower quality unleaded fuel that has resulted in the additives washing the spark plugs. This isn't necessarily a bad thing and is not the kind of thing you really need to worry about. Your engine won't be performing poorly as a result. 


How to Check Your Spark Plugs


Obviously, it's not always going to be as easy as simply taking a look at the spark plug to tell that it's bad. There are some ways you can check your spark plugs to determine if they work properly or not.


Take a Close Look: A visual inspection of the tip and sides of the electrode of your spark plug are the first and most important thing you can do to see if it's still working right. That's how you're going to notice the things we mentioned like carbon buildup or the shiny fuel coating. If you're seeing a lot of black buildup on your spark plug then you may need to change it for a new one, but also remember that it could be indicating a bigger issue you need to look at. You might have leaking fuel injectors and some bad wiring as well.


Check the Wires:  A multimeter is a handy tool to use when you're checking things like your battery and your spark plugs to make sure they're working properly. You can get a cheap multimeter for $15 or $20 on a site like Amazon.com. The multimeter can measure the resistance of your spark plug wires to let you know if it's still functioning properly. The foot of the wire should have a resistance of 10,000 to 15,000 ohms. The higher the resistance is, the worse your wiring is.


Spark Color:  The colour that your spark plug produces can tell you a lot about the condition of the spark plug itself. Checking that can be a little intimidating but it's not very difficult once you get the hang of it. What you need to do is remove the spark plugs but keep it plugged to the cable and grounded to the frame of your vehicle. Just any exposed metal that's not part of the battery or the fuel system would be a good idea. You can start the engine at that point and take a look at the plug to see what kind of spark is producing.


  • If a spark plug produces blue sparks, then your spark plug is working the way it's supposed to.
  • A yellow spark means that your spark plug is not working the way it's supposed to right now and it's a good idea to get it changed.
  •  No spark obviously means that there is an issue and your spark plug isn't working at all. One thing to keep in mind about checking the spark in this way is that it's not being subjected to the same pressure that it would be experiencing in the combustion chamber in your engine. That means it may not give you a one hundred percent accurate result, but if you've been experiencing other symptoms indicating you have a spark plug problem, this may help you diagnose it.


Spark Plug Testers:  You can buy a tool called a spark plug tester for under $10 on a site like Amazon.com. This is used to test whether there is an electrical current capable of generating a spark. You simply plug it in to your engine where the spark plug goes and turn on your engine. A light will turn on if it's receiving the correct electrical signal. If there's no light, then there's a problem that needs to be dealt with. 


The Bottom Line


As you can see there are a whole host of ways that your spark plugs can go bad and manifest problems in the way your vehicle operates. Depending on the level of damage you may be experiencing misfires, stalls, bad fuel economy and more but still having a functional vehicle. If it gets bad enough your car won't drive at all. So even though spark plugs are a relatively inexpensive and small item, and replacing them does not take a ton of effort by any means, they are of crucial importance to making sure your engine works the way it's supposed to.


As cheap as spark plugs are, you may be tempted to not take them very seriously when something goes wrong but often a spark plug is your first indication of a bigger problem in your vehicle. If something has caused your spark plug to die prematurely, that could let you know that you have a bigger issue in your engine such as problems with the valves, the pistons, fuel injectors and so on. If these problems are allowed to go on for too long then repair bills could skyrocket from just a few dollars you're spending on a spark plug to many thousands of dollars to get some serious engine repairs done. For that reason, make sure you're checking your spark plugs when you need to and replace them if necessary. 

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