How to Test a Spark Plug [With and Without a Spark Plug Tester]

The spark plug is an important component of your vehicle’s engine. To ensure it is always working correctly, it is important to test the spark plug regularly. How to test a spark plug?

You can test a spark plug with or without a spark plug tester. If you have a spark plug tester, you attach it to the ignition wire. Then, connect the other end of the tester to the spark plug. Turn the engine on and see if there is a glow on the spark plug tester.

Knowing how to test a spark plug is a fundamental part of troubleshooting any small engine. Doing so only takes a few minutes and may only cost you little or nothing at all. The tools you need include a spark plug wrench and a spark plug tester. But the latter is optional.

Read on to learn more about how to test a spark plug with and without a spark plug tester.

Also, to make the job easier, take a look at the easy-to-use Lisle 20610 Inline Spark Tester on Amazon to see its price and great reviews:

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How to Test a Spark Plug

how to test a spark plug

Knowing how to test a spark plug is a fundamental part of troubleshooting any small engine. It will only take you a few minutes to do so and may only cost you little or nothing at all. The tools you need include a spark plug wrench and a spark plug tester. But the latter is optional.

We don’t always have the time to visit a mechanic to help us check our vehicle’s spark plugs. In which case, it is important to know the basics of inspecting and testing this important part of our vehicle.

Below, we’ll look at the following steps:

  1. Removal of the Spark Plug Wire
  2. Conducting a Test Using a Spark Plug Tester
  3. Performing a Test Without a Spark Plug Tester
  4. Assessment of the Findings

Let’s discuss each of these steps:

1. Removal of the Spark Plug Wire

You will notice the spark plug on a chainsaw or outboard motor. Once you see this, remove the shroud that covers it. If you are testing the spark plug of your lawnmower or lawn tractor, they typically don’t have spark plug shrouds.

The next step is to pull off the ignition wire or detach it from the end of the spark plug. Make sure not to pull it off carelessly to prevent causing any damage. Carefully reach the sides of the spark plug and gently lift the wire’s end fitting.

If you find it hard to pull the cap and ignition wire, carefully rotate the rubber fitting one way or the other as you try to remove it.

2. Conducting a Test Using a Spark Plug Tester

If you have a spark plug tester, attach it to the ignition wire. Then, connect the other end of the tester to the spark plug. You may place the tester on top of the engine.

The next step is to turn the ignition on. Then, try to start the engine. It’s alright if the engine won’t successfully start, but make sure to turn the key.

Once done, try to observe the transparent sides of the spark plug tester. If there is no visible glow or spark present, then it means that there’s no spark occurring at the tip of the plug. 

So, what does this mean? This means that the spark plug is probably in bad condition. However, this is not the case all the time. The lack of glow or spark may also result from a fault involving the ignition system.

The best spark plug tester we’ve used is the Lisle 20610 Inline Spark Tester. See its price and great reviews on Amazon:

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3. Performing a Test Without a Spark Plug Tester

You can still test the spark plugs without a spark plug tester. The first step is to use a spark plug wrench to loosen and detach the plug from the engine. Do this by rotating it counterclockwise. However, please remember that you should remove the ignition wire before you start loosening and removing the plug.

Once done, you may reconnect the ignition wire onto the top of the loosened spark plug. Position the plug so that the lower metal body touches a metallic part of the engine’s cylinder head.

Carefully turn the engine over and inspect the gap at the tip. Remember that the engine is not capable of running with the spark plug removed. However, it will freely turn over.

You would know if the spark plug and all the other ignition system parts are properly working. A blue spark will appear between the curved metal top and the tip of the inner central electrode of the spark plug every time you try to turn the engine over.

If there is no visible spark, check if the ignition is turned on. Then, try once again. If there’s still no spark appearing after several tries, then there is a big probability of engine trouble.

4. Assessment of the Findings

Observe the tip of the removed spark plug. Once the spark plug is detached from the engine, it should come out with a central electrode with a brown or light gray residue. The plug should not have any fluffy or oily black deposits. Those deposits may be the reason for the absence of spark. It prevents the engine from running properly or from starting.

Start addressing the issue by replacing the spark plug. Make sure that the new one that you will install is exactly of the same kind.

Once the spark plug has been replaced, start the engine of your vehicle. It is supposed to start or run smoothly this time. Otherwise, there is most likely an issue with the ignition or fuel system. This is because, after conducting the test, you have finally confirmed that the spark plug is not the issue.

When Should You Replace Your Vehicle’s Spark Plug?

how to test spark plugs

You should look out for several signs to determine whether you should already replace your spark plug or not. Among these signs are the following:

  1. Check Engine Light Comes On
  2. Car Has Trouble Starting
  3. Higher Fuel Consumption
  4. Rough Engine Idling
  5. Vehicle Doesn’t Accelerate Quickly
  6. Manufacturer’s Recommendation

1. Check Engine Light Comes On

If a spark plug starts to fail, the most obvious indication is when the Check Engine symbol lights up or starts flashing. So, when you notice it flashing, don’t just ignore it. It definitely means that something’s wrong with your engine and might cause you a lot of trouble if left ignored.

As part of preventative maintenance, it is always a good practice to replace the spark plug. Your vehicle’s manufacturer also emphasizes this through your Owner’s Manual. Doing so will not only make your engine up and running smoothly once again. It will prevent further damage to the engine as well. Most importantly, it will prevent you from shelling out more money for costly repairs.

2. Car Has Trouble Starting

We have mentioned this several times earlier. Nonetheless, I choose to reiterate this. Be alarmed once your vehicle begins to encounter some trouble when you start the engine. Always consider the fact that there probably is a problem with the spark plug. So, the best thing to do is to conduct a test to confirm this.

Although aside from an issue with the spark plug, the problem may be with the battery. Check how long you have been using the battery. For all you know, it might already need a replacement too.

3. Higher Fuel Consumption

Observe how often you gas up. If your vehicle seems to consume fuel more frequently than usual, then the spark plug may be worn out.

This symptom was also confirmed by the National Institute for Automotive Service Excellence. It once stated that a misfiring spark plug could noticeably decrease fuel efficiency by roughly 30%.

4. Rough Engine Idling

As a driver, you would definitely know when there’s a problem with the vehicle by merely listening to the sound it makes. If and when you notice some knock-like, pinging, or rattling noises, the chances are that there’s an issue with the spark plug.

5. Vehicle Doesn’t Accelerate Quickly

If you’ve been driving your vehicle for quite a long time now, you have surely become familiar with its ride and handling capability. You would immediately know if it doesn’t normally respond every time you try to accelerate it.

Once you encounter this, then there must be a problem with the spark plug. You better replace it immediately to bring back that smooth driving experience.

6. Manufacturer’s Recommendation

For sure, you want your vehicle to last long. So, you better take good care of it to the best of your abilities. But if you are not very knowledgeable about taking care of it, the best way is to turn to your Owner’s Manual. You will never go wrong if and when you follow all the recommendations of your vehicle’s manufacturer.

Consult your owner’s manual to know the best time to replace your spark plug. Typically, a standard spark plug can last for up to 60,000 miles. However, this is not the case all the time.

If you start to notice something unusual with your vehicle’s engine even before you reach 60,000 miles, it’s best to get the spark plug tested already.

The Purpose of the Spark Plug

how to test a spark plug wire

Now that we have discussed how to test a spark plug wire, let’s discuss why it’s important to test them.

The function of the spark plug, which is found in gasoline engines, is to ignite the air/fuel mixture in the engine. A spark plug that doesn’t spark normally can be attributed to several factors. Among these are age, use, and poor fuel quality.

The two main symptoms of a bad spark plug are hard starting and failure to start at all. If your vehicle starts to consume excessive fuel but still has less power, these are symptoms too. Once your vehicle experiences these, you better opt to get your spark plug tested. Of course, with basic knowledge, you can conduct the test on your own.

It is important to test the spark plug because it is such an important part of your vehicle. If the spark plugs aren’t working properly, your car‘s motor will be rough and will lack power.

If you’re unsure where the spark plug is located, it is always at the top of the engine’s cylinder head. The cylinders are among the largest parts of small gasoline-powered engines. They have cooling fins on the outside.

The spark plug threads into a hole in the head of the cylinder. A thick wire and fitting that are pushed over the end of the spark plug provide electric power. This power is required to create the spark at the hidden tip of the plug.

In terms of the engine’s ignition system, it delivers a pulse of a very high voltage current. This current passes through the wire and then into the spark plug. The current typically range between 20,000 and 30,000 volts for a small engine.

As for the tip of the spark plug, it sits inside the engine’s combustion chamber in the cylinder head. It also includes a small gap.

When the high voltage electricity reaches the gap, it creates a spark. It ignites the air/fuel mixture stored inside the engine. This is what causes the engine to start running.

If there is any issue with the spark plug, there is a possibility of deficiencies in other parts of the engine.

Conclusion – How to Test Spark Plugs

You must know how to test a spark plug on your own. Besides, it is a fundamental part of troubleshooting any small engine. It will only take you a few minutes to do so and may only cost you little or nothing at all. The tools you need include a spark plug wrench and a spark plug tester. But the latter is optional.

Once your vehicle’s engine starts to show symptoms, don’t even think twice about testing the spark plug. If you are not sure if you can conduct the test independently, you can always seek professional help. Let a mechanic check it out for you.

The bottom line is, have your spark plug tested immediately. It doesn’t matter if you do it on your own or ask a professional to do it for you. The important thing is you get to pinpoint the issue and address it as soon as possible.