When it comes to a clean-running engine, there are a few telltale signs beyond just a check engine light (CEL), that can alert drivers to potential issues. These symptoms may include hard starting, sluggish acceleration, engine misfires, or a rough idle. However, diagnosing problems that don’t trigger a diagnostic trouble code (DTC) can be as simple as pulling each spark plug to visually inspect its coloration, fouling, or damage.

From Spark to Symptoms

Before we dive into the discovery process, there are a few key things to understand to help you identify what you’re looking at and what you’re looking for.

Spark plugs generate the spark needed to ignite the air-fuel mixture inside the combustion chamber. This spark occurs when voltage arcs from the center electrode to the ground strap (or ground electrode), as shown below.

The color and condition of a spark plug’s ground strap can change depending on the air-fuel ratio (the mixture of air and fuel entering the cylinder). If that ratio is off, or if there are more serious issues like detonation, oil fouling, or overheating, it can leave telltale signs such as discoloration, deposits, or even damage to the electrodes.

Laying It Out

Before starting the discovery process, I recommend laying out a set of labeled napkins or paper towels, one for each cylinder. This makes it easy to keep track of which spark plug came from which cylinder and compare them side by side. It’s especially helpful if you’re diagnosing issues like a lean condition between specific cylinders.

Once all the plugs are removed and organized, visually inspect each one for tip damage, fouling, or unusual wear. This is also a good opportunity to double-check spark plug indexing, though to clarify, “indexing” refers to the orientation of the ground strap in relation to the intake valve, while “gap” refers to the distance between the center and ground electrodes, a much bigger topic for a later date. However, both can affect performance, so it’s worth verifying them during inspection.

Firing on All Cylinders

Once all the spark plugs are laid out on the table, start with a visual inspection. Look for damage to the ground strap (or straps—some plugs have more than one) and the center electrode. Next, examine the coloration on the plug tip. Most of the time, you’ll encounter one of seven common conditions (editor's note: please resist the urge to taste test your spark plugs)

• Normal
• Wet
• Burnt/Burned
• Carbon-fouled
• Oil deposits
• Worn electrodes
• Broken electrodes

Anything beyond these typical signs should raise a red flag and may warrant further inspection, likely with a borescope to check the piston crown and cylinder walls for damage.

So, you might be asking yourself: What does the color or shade of a spark plug mean? The good news is, it’s easier to interpret than you might think, as you’ll see below.

“Normal” Color

Now if you pre-bought a set of spark plugs thinking this might fix that annoying hesitation at mid-throttle, there's a solid chance you’ll think “normal” could mean it should appear metallic silver on the ground strap and white on the electrode. However, with every revolution of the engine (roughly 700-900 RPMs at idle) the spark plug fires and over these millions of sparks begin to have a light brown or gray-tan deposit on the outside. This is what is considered normal. 

Wet Plugs

A wet spark plug is often the result of engine flooding. These plugs will typically smell like fuel and, as the name suggests, be wet to the touch. This condition can stem from several issues, such as a stuck-open injector pintle or a coil that’s not firing—both of which allow unburned fuel to flood the combustion chamber.

If the plug isn’t carbon-fouled or showing signs of damage described in the other conditions, it can usually be dried out and reused. 

Burned Plugs 

Lean may be mean, but in factory vehicles, running lean can lead to serious damage, starting with the spark plugs. Burn marks, blisters on the ground strap, or even melted electrodes are clear signs the engine is running too hot. This can be caused by overheating, a loose spark plug, incorrect ignition timing, or an imbalanced air/fuel mixture.

Whatever the cause, you’ll need to replace the plugs. But, more importantly, you’ll have to dig deeper to identify and fix the root issue. 

Carbon-Fouled Plugs

If your spark plug looks like it’s halfway through transforming into a charcoal briquette, it’s probably carbon-fouled. This usually results from an overly rich air/fuel mixture – meaning too much fuel and not enough air. Causes can include a dirty air filter, excessive idling, or short trip driving that doesn’t let the engine reach full operating temperature.

In some cases, switching to a hotter spark plug (remember: a higher plug number typically means a hotter heat range) can help burn off excess fuel and reduce fouling.

Oil On Plugs

If your spark plugs have an oily coating on the tip, it’s a strong indicator that oil is entering the combustion chamber. This is due to worn piston rings or failed valve guide seals. Since maintaining proper oil quantity and pressure is critical to internal combustion (aka gas) engine’s longevity, this issue should be addressed as soon as possible.

Worn Electrodes

Spark plugs are a maintenance item, they’re not designed to last forever. In fact, most copper plugs may need replacement every 25,000-30,000 miles, while iridium or platinum plugs last 100,000. Because they can be tough to access, many owners try to stretch their lifespan as long as possible. Unfortunately, your plugs may have already lost their spark. Remember: if it looks like it needs to be replaced, it probably does.

Broken Electrodes / Improper spark plug gap

If the “previous owner” or, let’s be honest, you, just replaced the spark plugs and now you're seeing broken tips or a spark gap reduced to nothing, there's a good chance the plugs had the wrong thread length. Failing to check for proper plug length before installation can lead to internal engine damage on the piston. When in doubt, your local AutoZone can help you identify the correct spark plug for your specific engine.

When in Doubt, Pull them out

If your engine is hard to start, idling rough, misfiring, knocking, losing fuel economy, or just feeling sluggish, and you’re not getting any DTCs or a CEL, your spark plugs might be the culprit. These quick tips will give you some old-school insight to help diagnose issues and keep your engine running at its best without a trip to the mechanic or dealership!