How to Inspect and Replace Spark Plugs

January 13, 2017

The spark plug uses the energy produced by the ignition coil to generate the spark necessary to ignite the air/fuel mixture in the combustion chamber.

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How to inspect your vehicle's spark plugs

Normal wear and tear: the firing end should be gray or a brownish tan in color and relatively clean, indicating a normal running engine.

Ash deposits: Too much oil in the combustion chamber or low quality oil/fuel can cause light brown deposits to form around the electrodes and insulator. This development leads to misfiring and hesitation.

Carbon Fouling: Resulting from an over saturated fuel/air mixture, a clogged air filter, or a malfunctioning choke operation, dry black residue on the plug can cause misfire and a weak spark.

Oil Fouling: Resulting from either valve guides (in 4-stroke engines), too much lubricant (in 2-stroke engines), or oil leaking beyond the piston rings, wet oily residue forming on the plug can cause misfire and a weak spark.

Overheating: A malfunction in the ignition or cooling systems or using the wrong type of fuel can cause blistering along the white insulator and glazed electrodes.

Worn Out: Electrodes subject to significant wear and tear will have trouble starting in cold or moist conditions.

How do I replace my spark plugs?

Not ready to do it yourself?

Here are the basic steps to replacing your spark plugs:

  1. Remove parts blocking access to the spark plugs - the air filter housing, for example.
  2. Remove ignition coils from spark plugs.
  3. Carefully remove electrical connectors so they don't break. (Or leave them connected if possible.)
  4. Loosen spark plugs with ratchet.
  5. Remove spark plugs from cylinder head.
  6. Be sure to inspect the old spark plugs for abnormalities.
  7. Check the gap of the new spark plugs. Refer to your owner's manual for the exact specification.
  8. Carefully thread your new spark plugs in by hand.
  9. After hand tightening, torque to manufacturer's specifications.
  10. Reinstall ignition coils and plug them in. When plugging in, feel and listen for a "click" and tug on it gently to ensure it's secure.
  11. Secure the coils with the bolts. (Careful not to over-tighten!)
  12. Reinstall air filter housing or any other blocking parts. (This is a good time to replace your air filter if necessary.)
  13. Secure any other parts removed during spark plug replacement.
  14. Repeat the process on the other side.
  15. After completion, start the engine to listen for any abnormal noises. If you hear anything wrong, shut off the engine immediately and recheck your work!

About the Author

RepairPal is the leading online source of auto repair resources and estimates. With many ASE Master certified mechanics on staff who have decades of experience, RepairPal knows all the fine points of car repair.

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