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How to Diagnose a Failing Spark Plug

Have a hunch that your engine has lost its spark? Spark plug failure is one of the most common engine problems and the good news that it’s easy to diagnose.

Tell-Tale Signs of a Failing Spark Plug

Spark plugs

The following symptoms are commonly associated with a failing spark plug:

  • High fuel consumption
  • Rough idle
  • Misfiring engine
  • Sluggish acceleration
  • Starting your engine takes more cranking than it used to

These symptoms, however, could mean that there’s something wrong with another part (such as a bad oxygen sensor), so it’s a good idea to officially confirm that the culprit behind these symptoms is a bad spark plug before replacing it.

How to Remove Your Spark Plugs

Back in the old days, you could just pop the hood and easily see the spark plugs. Engines these days are so loaded with emissions tubing and sensors and plastic engine covers that you might not even be able to see your spark plugs at first. However, physically inspecting the spark plugs is the best and easiest way to confirm spark plug issues.

Old plugs

Here’s a basic list of steps you can follow:

  1. Open the hood.
  2. Remove the main plastic engine cover. In many cases, this requires no tools. If you see no obvious fasteners, the cover will usually lift straight up.
  3. Locate an ignition coil and unplug it from the wiring harness. On most newer VW's, the spark plug is directly underneath the coil. If you are going to remove all of the plugs, it is probably easiest to unplug the wiring harness from each plug at this time.
  4. Pull the ignition coil straight up. You may need to use a screwdriver to pry the coil up a bit to get it loose enough to pull up.
  5. Remove the spark plugs with a 5/8” spark plug socket and ratchet.

How to Diagnose Your Spark Plugs

If you’re able to inspect your spark plugs, then you should know how to identify a bad one. First, let's describe a used spark plug in good condition:

  1. The porcelain portion of the spark plug should have no visible cracks.
  2. The J shaped electrode should have no obvious signs of erosion. It should be the same thickness from the base of the J to the tip.
  3. The electrode should have a very light deposit that is a grayish-tan color.

If the electrode is eroded, or the porcelain is cracked, you should definitely replace your plugs. Sometimes the condition of the plugs will provide clues to other engine problems. You may need to see a mechanic or do some more research if your one or more of your plugs show any of the following conditions:

  1. Dry black sooty deposits
  2. Wet black oily deposits
  3. A glazed or glossy appearance
  4. Heavy greenish or whitish deposits
  5. Excessive erosion

Need to replace your spark plugs? It's important to do it the right way. Find out how to do it here!