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Spark Plug Diagnosis: 7 Conditions To Check For (+ 4 FAQs)

April 18, 2022

Spark plugs offer a window into your engine’s operating conditions.

But what to check for when “reading” a spark plug?
And how do you perform a spark plug diagnosis?

In this article, we’ll answer those questions and more, including what the symptoms of a defective spark plug are and how urgently you’ll need a spark plug replacement if yours is faulty.

This Article Contains:

Let’s begin.

7 Conditions To Check For During Spark Plug Diagnosis

A spark plug that has been used for some time can exhibit grayish-tan deposits on its surface and slight electrode erosion. Sometimes, you may see a red coating on the spark plug due to the additives in unleaded fuel.

All that’s normal and doesn’t indicate engine damage or issues with your ignition system.

However, your engine could be in trouble if you notice any of these conditions when “reading” your spark plug:

1. Carbon Fouled Spark Plug

Dry, black soot on the insulator tip and electrodes reveals a carbon fouled spark plug. 

Carbon fouling can result from:

While carbon fouling can arise due to different factors, its root cause is often that the combustion deposits on your spark plug aren’t burning off properly.

2. Oil Fouled Spark Plug

An oil fouled spark plug shows oily, black deposits on its insulator tip, center electrode, and ground electrode.

If the spark plug is oil fouled, a worn valve guide, valve cover, or piston ring may be the culprit. 

Oil is likely leaking through the damaged valve guide, valve cover, or piston ring into the engine cylinder, depositing on the spark plug.

Oil fouling can adversely affect your fuel quality and the engine’s operating duration.

3. Fuel Fouled Spark Plug

Wet gasoline on the spark plug is a sign of fuel fouling.

It happens due to:

You need to be wary of fuel fouling since it can lead to engine misfires (a situation where one or more engine cylinders don’t produce power), poor engine performance, and higher exhaust emissions.

4. Spotless Spark Plug

An unusually clean spark plug points to a coolant leak.

When the leaked coolant reaches your ignition system, it can have dire consequences — your spark plugs may get damaged, and the engine could misfire. It can also wear down your engine quickly, leading to expensive repairs.

5. Overheated Spark Plug

Melted electrodes and blisters on the spark plug insulator tip are signs of overheating.

An overheated spark plug is a result of:

If your car has an overheated spark plug, you may lose power or experience excessive vibration at high speeds.

6. Extremely Worn Spark Plug

A worn-out spark plug has eroded electrodes and a wide gap between them, creating a weak spark.

It’s generally a consequence of:

You can expect poor starting and delayed acceleration if your vehicle has excessively worn spark plugs.

7. Physically Damaged Spark Plug

Physical damage in a spark plug manifests as indentations on the center or ground electrode and a broken insulator tip.

The damage occurs because:

A mechanically damaged spark plug brings down your gas mileage.

Now that we’ve covered the conditions to look for, let’s go over how a professional mechanic would perform a spark plug diagnosis:

How To Perform A Spark Plug Diagnosis

Before troubleshooting your spark plug issue, your mechanic will first gather the right tools — a socket, a spark plug socket, a socket extension, a ratchet wrench, a spark plug tester, etc.

Then, they’ll:

Next, we’ll answer some common questions car owners ask about spark plug diagnosis.

4 Spark Plug Diagnosis FAQs

Here are answers to four frequently asked questions related to spark plug diagnosis:

1. What Are The Main Functions Of A Spark Plug?

Your car’s spark plug plays three crucial functions:

2. What Are The Symptoms Of A Faulty Spark Plug?

If you experience any of the following symptoms, chances are you’ve got a bad spark plug:

3. How Urgently Do I Need To Replace A Defective Spark Plug?

A defective spark plug can hinder normal combustion, reduce fuel economy, decrease engine performance, and lead to costly engine damage.

That’s why you need to replace a defective spark plug ASAP. 

If you don’t have enough automotive experience or need help changing the spark plugs, get in touch with RepairSmith.

RepairSmith is a mobile auto repair solution offering a wide range of services at upfront, competitive prices.

4. What Does Spark Plug Heat Range Mean?

A mechanic may suggest buying a hotter or colder plug when replacing your spark plugs.

This distinction is based on the spark plug’s heat range (not type), indicating its ability to dissipate heat. So, your platinum or iridium plug could be either a hot or cold plug.  

Some factors influencing this ability are:

A colder plug has a high heat range since it can dissipate heat quickly and stay cool. So, if your car has a high-RPM engine operating at high temperatures, you’ll need to opt for a cold spark plug.

On the other hand, if there are fuel or carbon deposits on your spark plug, the mechanic may suggest using a hotter spark plug. A hot spark plug retains heat for longer and burns off any oil or carbon build-up.

Final Thoughts

Diagnosing your spark plugs helps you understand the internal health of your engine. You’ll know if there’s a rich air/fuel mixture, a worn-out valve guide, or a coolant leak into the combustion chamber.

But for an accurate diagnosis, you need ample automotive experience. Otherwise, you could get the diagnosis wrong and be oblivious to critical engine issues.

That’s why it’s best to leave the diagnosis part to a professional mechanic.

Contact RepairSmith today, and our ASE-certified technicians will come to your driveway to diagnose and replace your bad spark plugs.