Let's Find Out If Your Spark Plugs Need Replacing

Don't worry, we're not going to shock you like our shop teacher did to us...

Hank O'Hop

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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, which ignites 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.

Hank O'Hop

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!

Tools

  • Spark Plug Wire Puller (For older vehicles without coil packs)
  • Multimeter (for resistance test)
  • Spark Plug Socket
  • Sockets (for cars with coil packs)

Parts

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! 

Hank O'Hop

A spark plug arcing

Grounding Test 

  1. Cut Fuel Supply to the Engine.

    For fuel-injected cars, simply pull the fuse for the fuel pump. On carbureted engines, you need to disconnect the plumbing to the fuel pump. After this, run the engine until it burns off all of the fuel in the system.
  2. Remove Spark Plug Wire/Coil Pack

    For cars with coil packs, simply undo the mounting bolt then pull the coil from the plug. With an older engine, gently pull the plug wire free from the spark plug. Spark plug pliers make this much easier for you and the wire.
  3. Remove Spark Plug From the Cylinder Head

  4. Connect Spark Plug to Plug Wire/Coil Pack

  5. Place the Spark Plug on a Grounded Surface

    There are plenty of grounded metal surfaces that you can lay the plug on. You always want to make sure to keep the plug away from fuel sources and the connections clear of moving parts. Trust me. The engine fan does not care about your fancy spark plug wires. 
  6. Crank the Ignition and Inspect for Spark

    Have someone turn the ignition so that you can inspect the plug. Make sure the spark is strong and blue in color. If the spark is weak and dull orange, it's likely that it's not strong enough to ignite the engine's air and fuel mixture.
  7. (Optional) if There Is No Spark, Connect a New or Known Working Plug

    Connect a good plug to the coil pack or spark plug wire and repeat the test. If there is spark, you know the old spark plug is bad. If there is not or the spark is still weak, you can troubleshoot other ignition system components. To be sure if a plug is bad, you can also perform the multimeter test.
Hank O'Hop

Using a multimeter.

Multimeter Test

  1. Perform Steps 1-3 of the Previous Method

  2. Set Multimeter to Ohms

  3. Test Resistance Between Probes

    Always start by ensuring there is no resistance present in the probes to ensure you are getting an accurate reading.
  4. Test the Plugs

    Touch one lead to the terminal on the end of the plug and the other to the center electrode
  5. Inspect Reading

    You want to ensure that the resistances that appear are to spec. What is acceptable does depend on the manufacturer’s specs. But, for the most part, a reading of 4,000-8,000 Ohms is usually acceptable. 

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.

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