How to Inspect Your Spark Plugs

If your engine isn’t performing quite as well as it should, you may need to check your spark plugs. Typically, spark plugs last between 50K and 100K miles. When a spark plug goes bad, you’ll see a reduction in engine performance and fuel economy.

You don’t need any fancy diagnostic tools to determine if one of your spark plugs is bad. All you need to do is to simply inspect your spark plugs. It’s easy to tell a bad spark plug apart from a good one.

In this guide, we’ll show you how to take a spark plug out of the cylinder head and then correctly determine whether or not it’s still good.

Symptoms of a Bad Spark Plug

In addition to reduced engine performance and fuel economy, you might notice the following symptoms brought on by a bad spark plug:

  • Rough idle
  • Misfiring engine
  • Engine hesitating (or surging)

If you experience more than one of these symptoms, then chances are good that you have at least one spark plug that needs to be replaced.

How to Access the Spark Plugs

Image Credit: PaulsTravelPictures

The spark plugs are pretty easy to access. It’s generally easier on older cars because there usually isn’t an engine cover or many other parts in the way. Here is a basic overview on accessing the spark plugs on all types of GMC models:

  1. Make sure the engine is cool to the touch.
  2. Disconnect the negative battery cable from the battery.
  3. Move all the necessary parts out of the way. Depending on your GMC model and how old it is, there may be an engine cover, an air intake boot, and/or some tubes. On some models (such as an early 2000s GMC Sierra 2500), you don’t have to move anything. The spark plugs are directly accessible as is.
  4. If there’s an ignition coil on the spark plug, remove it. Otherwise, unplug the wire connected to the spark plug.
  5. With a spark plug socket, remove the spark plug.
  6. Inspect it, and then put it back into the cylinder if it’s still in good shape.
  7. Repeat with the rest of the spark plugs in your engine. The total number of spark plugs there are depends on how many cylinders are in the engine.

What a Used Spark Plug in Good Condition Looks Like

Normal wear and tear is common for a spark plug that has been hard at work for a while. What does normal wear and tear on a spark plug look like, though? A used but still good spark plug has some or all of the following:

  • A light tannish-grayish deposit
  • Reddish coating on the ceramic insulation (which is common when you use low-quality unleaded fuel with additives, but the plug is still fine)
  • A shiny coating on the tip and ground electrode (which is an indicator of an air/fuel mixture that’s too rich; in such a case, you would just need to adjust the air/fuel mixture)

What a Bad Spark Plug Looks Like

Look for the most common signs of a bad spark plug:

  • Excessive erosion on the ground electrode
  • Cracked housing
  • Black oil
  • Black soot
  • Melted materials anywhere on the plug

If you’re not sure whether one of your spark plugs is bad upon inspection, you’re welcome to contact us for assistance.

What if One of the Spark Plugs is Bad?

Genuine gm plugs

A bad spark plug won’t ignite the fuel mixture fully, or perhaps not at all. So the best course of action is to replace it as soon as possible. We advise against using aftermarket spark plugs for the reasons listed hereOEM spark plugs are pretty affordable on our website, as we offer the lowest total transaction prices available online for genuine OEM GMC parts.

Here’s a comprehensive set of instructions on replacing your spark plugs.