Let's Find Out If Your Spark Plugs Need Replacing
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Don’t worry, it’s not just you. Everyone dropped their first, or likely fortieth, F-bomb when their dad had them hold the spark plug while he hit the kick starter on their dirt bike. While it might have seemed cruel and unusual, followed by his raucous laughter and a lifetime of vengeful plotting to get him back, the power of that shock is something you kept in mind every other time you worked on a vehicle’s electrical system.
Luckily, that’s the kind of lesson you only need to learn once and, thankfully, it’s not the only way to test a spark plug. Like all other things mechanical, spark plugs will see their day. But just because a plug isn’t firing doesn’t mean it’s the problem. Lack of spark can be on account of plenty of other faulty electrical components.
That said, it pays to know how to test a spark plug so you can pinpoint the issue. There’s more than one way to skin a cat, though. So, it’s a good thing you’ve got The Drive’s brainiac team to teach you the ways that won’t leave your hair standing up straight.
Spark Plug Testing Basics
Estimated Time Needed: 5 minutes
Skill Level: Intermediate
Vehicle System: Ignition
What Is a Spark Plug Test?
A spark plug is a simple device that turns the ignition system’s energy into spark, whichignites the fuel mixture. The short version of how they work is by sending electricity through the plug, so an arc breaches the gap between the center electrode and the ground electrode.
There’s more to it than that, but that’s all we need to know to understand the tests we’re performing as we’re just trying to ensure the plug can perform that very task.
Why Test a Spark Plug?
Why are we performing a spark plug test? Spark plugs are cheap. So, it’s not a big deal to replace them if they go bad.
There are plenty of contributors to a misfire. It can be a lack of fuel, compression, or spark. And if it’s a lack of spark, it can be due to a bad plug, coil, ignition module, distributor, or spark plug wire. So, if you fall into the habit of throwing parts at a car until it works again, you can wind up deeper in the hole than you could ever imagine.
As a rule of thumb, you always want to start with the simplest and easiest solution and gradually work your way up the latter when troubleshooting. In the case of a misfire, the easiest place to start is by checking the spark plugs.
Sometimes a visual inspection is enough to determine if a plug is faulty. Over time the center electrode will wear away, and it can’t perform its job. But what if it’s not? How can you tell for sure if the spark plug isn’t firing?
We’re going to talk about two tests you can perform to test out spark plugs. Both are relatively easy and require little more than basic hand tools. By performing these tests, you can quickly determine whether or not the spark plug is the source of your problems.
A spark plug set in the cylinder head.
Spark Plug Test Safety
Because we are highlighting two different ways to test plugs, there are variable safety considerations. One of which only requires a multimeter, but the other involves working with an exposed plug under the hood. For that second method, you want to keep these safety tips in mind.
- Protect the essentials. Plastic surgeons will welcome any services, but you’ve only got one layer of skin and two eyeballs. So, unless you want to risk a botched surgery, throw on a set of safety glasses and gloves.
- Let the engine cool. Don’t attempt to pull plugs when the engine is hot. There are plenty of parts under the hood waiting to barbeque you. That includes plugs that aren’t exactly in tip-top shape. So, be sure to let the engine cool off before diving in.
- Be mindful of moving parts. The engine isn’t running for the grounding test, but it is cranking over. Be mindful of that and steer clear of any moving parts.
- Don’t be a moth to a flame. Don’t touch the spark plug when the ignition is engaged. The low amperage probably isn’t enough to kill you, which is good. However, on average, about 20,000 volts are running through the spark plug. So, even it isn’t likely to kill you. It’s not going to feel good—just take my word for it.
Everything You’ll Need To Test Your Spark Plugs
The good news is that you’re only going to need a few tools to perform both of these tests. There are more professional ways of doing it, but since we’re talking about a home job, we’re going to keep it on the cheap. That said, you probably have some of these tools on the ready, but we’ve gone ahead and hyperlinked each in case you don’t!
- Spark Plug Wire Puller (For older vehicles without coil packs)
- Multimeter (for resistance test)
- Spark Plug Socket
- Sockets (for cars with coil packs)
- A spark plug that’s known to work
Organizing your tools and gear so everything is easily reachable will save precious minutes waiting for your handy-dandy child or four-legged helper to bring you the sandpaper or blowtorch. (You won’t need a blowtorch for this job. Please don’t have your kid hand you a blowtorch—Ed.)
You’ll also need a flat workspace, such as a garage floor, driveway, or street parking. Check your local laws to make sure you’re not violating any codes when using the street because we aren’t getting your ride out of the clink.
Here’s How To Test a Spark Plug
Let’s do this!
A spark plug arcing
Cut Fuel Supply to the Engine.
Remove Spark Plug Wire/Coil Pack
Remove Spark Plug From the Cylinder Head
Connect Spark Plug to Plug Wire/Coil Pack
Place the Spark Plug on a Grounded Surface
Crank the Ignition and Inspect for Spark
(Optional) if There Is No Spark, Connect a New or Known Working Plug
Using a multimeter.
Perform Steps 1-3 of the Previous Method
Set Multimeter to Ohms
Test Resistance Between Probes
Test the Plugs
Sometimes You Need a Certified Mechanic
As much as The Drive loves to put the “you” in do-it-yourself, we know that not everyone has the proper tools, a safe workspace, the spare time, or the confidence to tackle major automotive repairs. Sometimes, you just need quality repair work performed by professionals you can trust like our partners, the certified mechanics at Goodyear Tire & Service.
Pro Tips to Testing Spark Plugs
Here are a few of The Drive’s pro tips.
- You can check to see that spark is present at the coil pack or spark plug wire without a spark plug. By placing the tip of the screwdriver in place of the plug then bringing the shank within 1/8th of an inch to the ground, it can conduct a spark. Just don’t hold onto the screwdriver during tests. If it isn’t isolated, you’re likely to get zapped.
- If you’re working with an older car, you can place a screwdriver to ground out the starter solenoid or relay. This is a way to work around having to call in a friend for help.
- A multimeter is your best friend for ignition troubleshooting. Testing everything from the spark plugs to an ignition module is possible with this tool. Each car is different, as are the test procedures for the ignition system’s remaining parts, but this tool is a must-have essential for any application.
- If the plug isn’t the issue, work your way in sequence through the ignition system. This is the best method to pinpoint the source of your problems.
- If the spark plug checks out, double-check your spark plug gap. Spark plug gap needs to be to spec in order for an engine to run properly.
- Always make sure your plug wires aren’t grounding out somewhere else. If you’re running big headers, they can melt the plug wires and rob the spark plug of energy if they make contact.
FAQs About Spark Plug Testing
You’ve got questions, The Drive has answers!
Q: How Many Ohms Should a Spark Plug Have?
A: How much resistance a plug should have ultimately depends on the manufacturer’s specifications. While most spark plugs for passenger vehicles should have somewhere in the 4,000-8,000 ohms neighborhood, your exact plug may call for different values.
Q: How Can You Check for Bad Spark Plug Wires?
A: Usually, visual inspection is enough to determine if a plug wire is bad. Any signs of wear or rot are telltale signs that it’s time to replace them. You can use a multimeter to test the resistance to be sure.
Q: How Can You Tell if a Spark Plug Is Firing?
A: The grounding test is performed to determine if a plug is firing. You want to watch the center electrode to see if a strong, blue spark is created. Alternatively, you can use a spark plug light. However, this only shows if a signal is being sent to generate a spark and not that the plug is, in fact, firing.
Q: What Happens When One Spark Plug Is Not Firing?
A: If you have a misfire, your engine will lack in power and economy and run rough. But that’s the least of your issues. Unburnt fuel washes the walls of any lubricant and eventually pollutes your oil. So, it’s worth fixing the misfire ASAP.
Q: Can a Spark Plug Shock Kill You?
A: The energy sent to spark plugs has high voltage but low amps. Amps are what’ll kill you. So, even if the shock hurts, it probably won’t kill you unless you have a serious heart condition.
Let’s Talk, Comment Below To Talk With The Drive’s Editors!
We’re here to be expert guides in everything How-To related. Use us, compliment us, yell at us. Comment below and let’s talk! You can also shout at us on Twitter or Instagram, here are our profiles.
Jonathon Klein: Twitter (@jonathon.klein), Instagram (@jonathon_klein)
Tony Markovich: Twitter (@T_Marko), Instagram (@t_marko)
Chris Teague: Twitter (@TeagueDrives), Instagram (@TeagueDrives)
Toni Scott: Twitter (@mikurubaeahina), Instagram (@reimuracing)
Hank O’Hop: Twitter (@HankOHop), Instagram (@HankOHop)
Fluke 117/323 Multimeter and Clamp Kit
Gearwrench 5-Piece Magnetic Swivel Spark Plug Socket Set
ARES 70053 – Spark Plug Pliers
Williams 50666 3/8-Inch Drive Socket and Drive Tool Set
Got a question? Got a pro tip? Send us a note: [email protected]
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