Your Guide to Reading Spark Plugs: Knowing What to Look for to Assess Engine Condition

New spark plug next to worn spark plug that’s past due for replacement

Though your fuel sensor, O2 sensor, CEL, and other dummy lights can tell you a lot about how your engine is running, both during a tune and during diagnostic & repair work, reading your spark plugs can arguably tell you more.

Many mechanics rely on the condition of the spark plugs as a telltale sign that either everything is a-ok, or that something could be tinkered with or fixed under the hood. But what exactly is “spark plug reading” and what should you be looking for when doing it?

Here’s what you need to know.

What is Spark Plug Reading?

As you likely already know, spark plugs are what ignite the fuel in your car’s combustion chamber. So they can tell you a lot about things like air-fuel mixture, ignition timing, and even oil or coolant leaks.

Spark plug reading is the process of looking at the current condition of a spark plug and critically evaluating what it means.

While experienced mechanics can look at a spark plug and immediately know what it tells them, for others, consulting a spark plug reading chart can be extremely helpful.

Mechanic holds used spark plug

What Does That Mean: Common Spark Plug Readings

There are a number of things you might see when reading a spark plug, most of which you can find in any spark plug reading chart. But for easy reference, here are a number of common things you might see when reading a spark plug, and what they mean. Since this is a highly visual activity, we recommend a few videos, which will give you an idea of what we’re describing actually looks like:

Center Electrode is Noticeably Worn

If the spark plug’s center electrode is noticeably worn away, that usually means that you waited too long to change your spark plugs. In other words, there are too many miles on it, and it’s past due for replacement.

Change them now, as an overly large gap will cause ignition misfires. It can even cause ignition coilpacks to fail entirely.

Spark Plug is Dry & Black

If the end of the porcelain looks very dry and is black or sooty, that normally indicates a rich mixture (too much fuel relative to air). If you notice this on one cylinder, you might have a leaky fuel injector. If spark plugs in multiple cylinders show this, then most likely, you’re dealing with some kind of overall fuel control issue.

Spark Plug is Suspiciously Clean

An extremely clean or spotless spark plug is not the same as an ideal or perfect spark plug reading. On the contrary, it often means coolant is getting into the cylinder and basically “steaming” the end of the spark plug.

Spark Plug has Some Discoloration

Some discoloration or browning from heat around the tip of the porcelain insulator, and some discoloration around the thread, but an otherwise fairly clean spark plug and electrode indicates a normal spark plug.

Spark Plug is Dry with Sandy-Textured Deposits

If the end of the spark plug is dry and you notice sandy-textured deposits on it, mostly on the outer electrode, this is probably the result of either bad valve seals or bad valve guides. These cause oil to be sucked into the engine in an atomized state, resulting in sandy deposits.

Spark Plug is Gunky, Oily, or Greasy

This is definitely not something you want to see. If the end of the spark plug has a lot of grime or gunky deposits on it, or if it looks extra wet, oily, and greasy, this usually means that liquid oil is coming from bottom of engine around or through piston and onto the spark plug. In most cases, this signifies the need for major engine repairs.

Spark Plug is Very Light White

Modern engines spend 99% of their time cruising, which means- operating at a very lean mixture. If the porcelain on all plugs from the engine are a light tan or even off white, that isn’t usually a problem. However, if you have a single plug that is far whiter than the rest, this often indicates one fuel injector that is not flowing as much as others. You need to take action immediately.

How to Read Rich vs. Lean Fuel Mixtures

If you have access to a spark plug cutter, you can cut the spark plug apart to expose the entirety of the porcelain insulator. Important note: everything you read on the internet about reading air fuel mixtures off of spark plugs is referencing full throttle operation. The only way to use this method is to put fresh plugs in, do a test run, and immediately shut the engine off and inspect them. Otherwise, you are looking at the cumulative effects of thousands of miles of mostly cruise power settings.

Ideally, you want a slightly rich condition, which is indicated by a black line at about a hundred thousands (0.1) inches from the bottom ledge/lip. The higher the black line is toward the tip and electrode, the richer the fuel mixture, and the less horsepower your car has.

If you only see white, your engine is running far too lean.

Learn More

Want to maximize your car’s performance? Replacing spark plugs routinely can help, but so can upgrading the type of spark plug used in your car – especially if you’re performing a tune or modding out your car in any other way.

Get spark plug suggestions for your VW or Audi by reading our blog “Spark Plug Wisdom: Maximizing Performance in Audi and VW Engines” now.

Or, if you’re ready to retrofit your car with performance-oriented parts, shop all IE collections.

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