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:
- 7 Conditions To Check For During Spark Plug Diagnosis
- How To Perform A Spark Plug Diagnosis
- 4 Spark Plug Diagnosis FAQs
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:
- Dirty air filters
- Excessive driving at slow speeds
- A rich airfuel mixture
- Incomplete combustion
- Idling your car for too long
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:
- Engine flooding
- An excessively rich airfuel mixture
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:
- Improper spark plug tightening
- Engine overheating from poor cooling
- Pre-ignition (the air/fuel mixture ignites before your spark plug fires)
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:
- Bad maintenance
- Spark plugs exceeding their service life
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:
- You’re using a spark plug of the wrong size in your engine cylinder. As a result, the spark plug’s firing end gets flattened by the piston during engine operation.
- There’s foreign matter in the engine’s combustion chamber.
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:
- Open the car’s hood.
- Remove the engine cover (if your car has one) with a socket.
- Pull up the ignition coil and disconnect it from the electrical connector.
- Use a spark plug socket to remove your spark plug from the cylinder head.
- Examine the spark plug for spark plug fouling, wear, physical damage, etc.
- Check the spark plug wire (if your car uses one) for scorch marks, corrosion, etc.
- Repeat the same process with your car’s other spark plugs.
- Replace the damaged or worn spark plug with a new spark plug (as needed).
- Re-install the spark plugs, reconnect the ignition coil, and turn on your car to see if everything works.
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:
- Ignite the air/fuel mixture using an electrical spark to power your engine
- Remove heat from your engine’s combustion chamber, acting as a heat exchanger
- Burn off any combustion deposits, like soot, fuel, residues from additives, etc
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:
- Rough idle
- Excessive vibration
- Slow acceleration
- A weak spark
- Poor fuel economy
- Incorrect ignition timing
- Difficulty starting your engine
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 AutoNation Mobile Service.
AutoNation Mobile Service 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:
- Insulator nose length
- Gas volume around the insulator nose
- The materials used to build the insulator, ground electrode, and center electrode
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 AutoNation Mobile Service today, and our ASE-certified technicians will come to your driveway to diagnose and replace your bad spark plugs.