How to Repair a Faulty Car Spark Plug
A simple step-by-step guide to inspecting, cleaning, checking and replacing a faulty spark plug
What Is a Spark Plug?
A spark plug is an important component of a petrol engine. It delivers an electrical spark inside the combustion chamber to ignite the air-fuel mixture.
A worn or fouled spark plug can cause difficult starting, rough engine operation, misfires, poor acceleration and increased fuel consumption.
Tools and Materials
- Correct replacement spark plug
- Spark-plug socket
- Ratchet and suitable extension
- Spark-plug gap gauge
- Soft brush
- Clean cloth
- Torque wrench
- Vehicle service manual
- Safety gloves and eye protection
Step-by-Step Procedure
Allow the Engine to Cool
Park the vehicle on a level surface and switch off the engine. Allow the engine to cool sufficiently before working around the spark plugs.
- Park safely and engage the parking brake.
- Switch off the ignition.
- Allow the engine to cool.
- Keep loose clothing away from moving components.
Locate the Spark Plugs
Locate the spark plugs on the engine. Depending on the vehicle, they may be underneath ignition coils, plug wires or other components.
Remove the Ignition Connection
Carefully disconnect the ignition coil or spark-plug wire from the spark plug according to the manufacturer’s procedure.
- Do not pull wiring by force.
- Keep connectors clean.
- Remember the original positions of components.
Remove the Spark Plug
Use the correct spark-plug socket and ratchet to carefully loosen the plug. Remove it without damaging the surrounding components.
Inspect the Spark Plug
Examine the removed spark plug carefully. The appearance of the electrode and insulator can provide useful clues about engine operation.
- Check for cracked ceramic.
- Look for damaged electrodes.
- Check for excessive carbon deposits.
- Look for oil or fuel contamination.
- Inspect the threads for damage.
Clean a Lightly Fouled Plug
If the spark plug is only lightly contaminated and is otherwise serviceable, carefully clean deposits using an appropriate soft brush and clean cloth.
Check the Spark-Plug Gap
Check the electrode gap using a suitable gap gauge. Compare the measurement with the vehicle manufacturer’s specification.
- Use the correct specification for your engine.
- Do not guess the required gap.
- Adjust only if the plug manufacturer permits adjustment.
Replace the Spark Plug if Necessary
If the plug is damaged, worn or unsuitable for further service, replace it with the exact type specified for the vehicle.
- Check the plug part number.
- Confirm the correct heat range.
- Confirm the correct thread size and reach.
- Check the manufacturer’s installation instructions.
Install the Spark Plug
Carefully start the spark plug by hand to avoid cross-threading. Tighten it using the torque specification provided by the vehicle or spark-plug manufacturer.
Reconnect and Test
Reinstall the ignition coil or spark-plug wire and make sure all connectors are properly seated.
- Reconnect the ignition component.
- Check that tools have been removed from the engine bay.
- Start the engine.
- Listen for abnormal engine operation.
- Check for persistent misfire or warning lights.
Common Spark-Plug Conditions
| Condition | Possible Meaning | Action |
|---|---|---|
| Light deposits | Normal contamination or service-related deposits | Inspect and clean if appropriate |
| Heavy carbon deposits | Possible combustion or operating issue | Inspect plug and investigate the cause |
| Oil contamination | Possible oil-control or engine problem | Diagnose the underlying problem |
| Damaged electrode | Plug may be worn or damaged | Replace the spark plug |
| Cracked ceramic | Physical damage | Replace immediately |
Final Inspection Checklist
- ✓ Engine is cool before work begins
- ✓ Correct spark-plug specification confirmed
- ✓ Spark plug inspected for damage
- ✓ Electrode gap checked against specification
- ✓ Cylinder-head threads kept clean
- ✓ Spark plug installed without cross-threading
- ✓ Correct tightening torque used
- ✓ Ignition coil or plug wire reconnected
- ✓ Engine tested after installation
- ✓ Persistent problems diagnosed by a qualified technician
Conclusion
A spark plug is generally not a component that can be permanently repaired. Minor surface contamination may sometimes be cleaned, but a plug with damaged electrodes, cracked ceramic, damaged threads or significant wear should be replaced.
Correct identification, proper installation and the correct tightening specification are important for reliable engine performance.
