How to Diagnose a Bad Spark Plug

Have a hunch that one of your spark plugs has gone bad? You came to the right place. This diagnostic guide will help you determine if you need to replace any of the spark plugs in your car and pinpoint which one(s) they are.

The easiest DIY way to determine if you have any bad spark plugs is to remove each plug and then examine it. If you feel that this is a project you can tackle, this set of instructions is for you.

Tools to Grab Before Starting

To get pull out the spark plugs, you need the following tools:

  • Socket wrench set
  • Spark plug socket
  • Extension

(Important note: this set of tools and the instructions below are based on the V6 engine found in 2001-2007 Ford Escapes.)

Removing the Spark Plugs

To remove the spark plugs, follow the steps outlined here:

  1. Open the hood.
  2. Disconnect the negative battery cable from the battery.
  3. Unbolt the air intake boot and then move it out of the way.
  4. Remove the two bolts holding the throttle wire cover and then remove the cover.
  5. Remove the bolts holding the intake manifold in place.
  6. Disconnect everything from the intake manifold.
  7. Move the manifold slightly out of the way. There’s no need to remove it.
  8. Pick one cylinder and then remove the ignition coil from it.
  9. With an extension and spark plug socket, unscrew the spark plug and then carefully pull it out.
  10. Examine the spark plug (instructions below).
  11. Repeat with the rest of the plugs in your engine. Be sure to put the plugs back in the correct cylinders.

Examining the Spark Plugs

When you have a spark plug ready for inspection, you need to know what to look for. It’s okay for a spark plug to look a bit worn with a light tannish-grayish deposit. That’s normal wear and tear for spark plugs. You want to look for the signs of a bad spark plug, which are:

  • Black oil or soot
  • Excessive eroded electrodes
  • Cracks in the housing
  • Any melted areas

If you find any of these issues, then the spark plug is no longer good and it will need to be replaced. 

Got any questions about diagnosing a fouled spark plug? Contact us for answers!