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How To Care For Your Car: Spark Plugs

December 12, 2019

What are spark plugs?

Spark plugs are the small but important engine components that receive high voltage electricity and create the spark that contributes to powering the engine. Depending on the age of the vehicle and the style of its ignition system, each spark plug is connected directly to an ignition coil, or to a wire leading to the central coil/distributor that provides the voltage needed to create a spark. When an engine is running properly, a spark is produced once per engine cycle (once every two RPMs on a normal car’s engine). The spark is produced by the electrodes at the end of the plug (shown below on the left) and then enters the cylinder to start combustion of the air and fuel present.

Do different cars have different types?

All spark plugs have the same basic design and function. There may be slight variations in plug length or size between vehicles. The main differences, though, lie in the style of the center electrode as well as the size of the electrode gap. These factors are specified by the manufacturer to optimize engine performance and efficiency. Diesel vehicles are one exception, in that they do not have spark plugs (the fuel is ignited by compression instead of spark).

Why do they fail?

For several reasons, spark plugs are a maintenance item on every vehicle and should be replaced at the intervals specified by manufacturer before they reach the point of failure. Over time, spark plugs become contaminated and fouled (primarily by carbon buildup, but sometimes by oil or fuel if an engine is not running well). The electrodes can also wear down, or the gap can become excessive. In some instances, the insulation portion of the spark plug can crack from age and heat and lead to misfires. Additionally, fluid leaks from the top area of the engine can make their way to the spark plugs and contaminate them.

How will I know if I have a problem with my spark plugs?

Ideally, spark plugs will not pose a problem if they are replaced according to the manufacturer’s schedule. However, spark plug issues can present themselves in the form of a slightly rough idle, decreased fuel efficiency, or even misfires and a check engine light. In extreme cases, a vehicle may have such worn or fouled spark plugs that it will not start.

What if I don’t replace them?

If spark plugs are not replaced when they are due by the vehicle’s maintenance schedule, the symptoms described above may begin to occur. As the rough running conditions and misfires worsen, one or more spark plug may cease operation completely. When this happens, the fuel in that cylinder is not being used so it is exiting the engine into the exhaust, where it can damage the catalytic converters. By replacing spark plugs as a maintenance item before they fail, these issues can be minimized. If problems do arise, addressing them as soon as possible will help to ensure minimal damage to other parts of the vehicle.

What does replacement cost, and why?

Spark plug replacement can be very simple on certain engines and may cost as little as $100 (such as on a domestic vehicle with a four cylinder engine). However, some engines require removal of the upper intake to access the plugs, adding time and complexity. Other vehicles may have a simple engine layout but require higher performance spark plugs or special tools for access and removal. A very small number of vehicles have two spark plugs per cylinder, leading to an increase in parts cost. The upper price range for spark plug replacement can be up to $1000 or more depending on the vehicle, but on most cars it is far less than that.

Is there anything else I should replace at the same time?

If spark plugs are being serviced purely for maintenance, they are often the only component that needs to be replaced. However, on a car with higher mileage, it often makes sense to replace old ignition coils at the same time due to the overlap in labor. This helps to ensure that failure of these components is avoided, and prevents having to pay for labor to replace the coils when they do fail. If there are any engine problems that are damaging the spark plugs (excess fuel, or oil leaks, for example), they should be addressed to prevent further damage to the new spark plugs.

Is there anything I can do to lower the cost of repair?

Replacing spark plugs as a maintenance service before they fail may seem more expensive, but waiting until they fail can lead to damage of other engine components. Therefore, replacing the plugs when they are due is the most cost-effective path to choose. There are many spark plugs available and some options may be cheaper than others, but it is always recommended to install the type specified by the vehicle manufacturer to ensure the highest efficiency and performance.