What are spark plugs and how do they work?
Spark plugs are one of the key components of your vehicle’s ignition system. They ignite gasoline and air inside of your engine. This gives your engine the power to propel you down the road. The spark plug takes high voltage electricity from the ignition coil and creates a spark at just right moment during the engine’s combustion cycle. This gives the engine the best mix of power, fuel economy and low emissions. However, the electrode at the end of the spark plug eventually wears out and can cause performance issues.
When should you replace spark plugs?
Replacement should follow the vehicle maintenance schedule. This will be listed in the vehicle’s owner’s manual. Some cars require regular replacement at 45k, 90k or 150k kilometers. It is possible in some cases that a spark plug might fail before the service schedule, but it’s not typically common.
Avoiding spark plug replacement can cause:
- Difficulty starting your car.
- Check engine light for a misfire code.
- Flashing check engine light during acceleration for misfire, this can cause engine damage.
- Reduced fuel economy and/or power.
- Rough idle and/or unexplained engine vibration.
- Failed State emission testing.
Even if only one spark plug is found to be faulty, they should always be replaced in sets. Most engines will require 4,6, or 8 spark plugs.
How do mechanics replace spark plugs?
Spark plugs are threaded into the engine’s cylinder head(s). Most cars have one plug per cylinder, so a four-cylinder engine will have four plugs, a six-cylinder engine will have six plugs, and so forth. The mechanic will:
- Have the correct spark plugs on hand for your vehicle
- Pull any diagnostic trouble codes that may be related to worn spark plugs.
- Remove the original spark plugs and look at removed spark plugs for any clues indicating engine mis-operation exists; such as oil burning, excessively rich mixture, pre-ignition damage etc. Inform owner of any found unusual conditions.
- Install new plugs using a torque wrench.
- Finally, a road test of the vehicle to confirm normal operation.
Is it safe to drive with worn spark plugs?
Yes, but spark plugs that are worn enough to cause engine-misfiring have the potential to cause engine damage and overheating of the catalytic converter. Regardless of the age of the plugs, or mileage, if the engine misfires or runs poorly, the plugs should be inspected for defects. Generally, spark plug failure will not keep the engine from running, but in cases of extreme wear, you may experience a limitation in the speed at which you can drive the car. If the maintenance schedule says the plugs should be replaced, it is best to do so promptly.
When having spark plugs replaced keep in mind:
- High-speed driving may accelerate spark plug wear. Consider earlier than required inspection and tune-up if you operate the car in severe conditions.
- Plugs should always be promptly replaced per the manufacturer maintenance schedule even if they appear to be working perfectly.
- If your vehicle is equipped with spark plug wires (as opposed to a coil-on-plug system), the wires should be replaced at the same time. This may increase the cost of your repair.
- It is possible that when doing a spark plug replacement, your vehicle may also need additional parts like ignition coils, this will add to the repair cost.
- Some engines require that the intake manifold be removed while replacing spark plugs. This may increase the parts and labor cost for the repair.
- Leaking valve cover gaskets, which have let oil get in contact with the spark plugs, coils or wires, may be the cause of ignition system issues. Valve cover gaskets will need to be replaced in addition to the spark plugs, wires or coils. This will add to the repair cost.