Tips & Advice

Changing Spark Plugs

Gas-powered cars run on what are essentially controlled explosions of energy, controlled in part by the spark plugs. The spark plugs channel the electrical current from the ignition, igniting the fuel. It’s an essential part of any working combustion engine and all modern cars. Like anything, they wear out, and are a relatively easy-to-fix issue on your car that you can learn to diagnose and correct with the right tools and the right know-how.

Removing Old Spark Plugs

  1. Locate the spark plugs in your car. When you open the hood of your car, you should see a bundle of 4-8 wires leading to different points on the engine compartment. The spark plugs are located at the end of these wires, under the plug covers that attach them.
    • On a 4-cylinder engine, spark plugs will be located on the top of the engine in a row.
    • On 6-cylinder and 8-cylinder engines, plugs should be separated evenly on each side of the engine.
    • Some cars have engine covers you’ll have to remove to see the spark plug wires, tracing them back to find the plugs themselves. Check your owner’s manual and look up where your spark plugs are, how many you have, the correct “gap” and the size socket needed to remove them.
  2. Let the engine cool before removing your spark plugs. If you’ve been running the car for a while, the plugs can get pretty hot. Only remove them when the engine is cool. In the mean time, assemble your tools while you wait for your engine to cool. To change your spark plugs, you’ll need:
    • A ratchet wrench
    • An extension
    • Spark plug socket, usually included in any ratchet set
    • A gap gauge, available at any auto parts store
  3. Remove the first spark plug. Pull the wire plug from the engine by gripping it as closely to the bottom as possible and working it gently off to reveal the spark plug. Don’t yank the wire out of the plug, or you’ve got a bigger job on your hands. Fit your wrench with the extension and use the ratchet to remove the spark plug from its housing.
    • If you’re checking to see if the plugs need changed, just remove one and check the gap. If it’s burned out, go to the auto parts store and get new plugs before removing any more. You need to remove the plugs one at a time, keeping track of the order. Spark plugs fire in a specific order, and crossing a wire with the wrong plug will make the engine run rough.
    • If you need to remove all the plugs at once for some reason, consider using small pieces of masking tape to mark each wire with a numbered location so you can remember.
  4. Measure the gap of the spark plug. This number should be a specific measurement anywhere between .028-.060 inch, with a bit of wiggle room depending on your particular set of plugs and your car. Check your owner’s manual to find out the optimum distance for your spark plug gap and use a gap checker to check the distance.
    • If the distance is higher than it should be, but the plug is still of a relatively high quality and is an adjustable-gap plug, you can either try and change the gap with a vice while the measure is set at the correct measurement, or you can buy new plugs. It’s usually recommended that you just replace the plugs. Spark plugs are not terribly expensive, and it’s a good idea to keep them new and firing well.
    • If you’re going to start changing your own plugs, invest in a tiny gap checker. It’s basically a metal ring you can use to see if the electrodes are close enough to fire properly.
  5. Check the plugs for wear. It’ll be a little dirty even if the plug is working properly, but you need to change your spark plugs if you see any white, limey build up around the electrodes of the plugs, or if you see any evidence of burning. Sooty build-up suggests that you need to change your plugs.
    • If the plugs are bent, black, or broken, you could have a bigger problem and should consult a mechanic.

Installing New Plugs

  1. Get the correct replacement plugs. You can either consult your owner’s manual or the booklet at the auto parts store that corresponds to your particular make and model. There are literally hundreds of different combinations of spark plugs and measurements, ranging in price from less than two dollars to 15 bucks, made of platinum, yttrium and iridium. Plugs made of precious metals are typically more expensive and the coatings resist wear slightly better. Talk to your auto parts dealer for advice.

    • A good rule of the thumb is get the same plugs you’ve got in the car currently. Never downgrade to a less expensive plug and don’t think too hard about fixing something that works already. The manufacturer installed those plugs for a reason, probably, so uncomplicated the process and get the same ones.
    • You can typically buy fixed gap or adjustable gap spark plugs, so it’s up to you if you want to check your plugs regularly and make minute adjustments. If you do, get adjustable plugs. Above all, you need to make sure the gap measurement is the correct measurement for your car. If you check it yourself, you’ll know for sure. Remove it from the package and do a quick check to verify the measurement.
  2. Consider cleaning around the threads before reinserting the new spark plugs. When you’re changing your plugs, it’s also a good opportunity to check the wires for wear and to clean up around the wire terminal. Use a wire brush or compressed air to clean up around the wire connections and make sure you’ve got a good, clean port. Replace the wires if necessary.
  3. Insert the new plugs and tighten with your ratchet. Using the spark-plug socket, remove each plug from the engine, and replace each with a new spark plug. Tighten a bit (say, 1/8th of a turn) past hand-tight. Replace the spark-plug cables on the same plugs they originally came from, and remove the masking tape.
  4. Consider lubricating the plugs as you install them. Try putting a very small amount of anti-seize lubricant on the plug threads if you are installing them in an aluminum engine. The anti-seize prevents a reaction between dissimilar metals. You can also use a small amount of dielectric silicone compound on the inside of the spark plug wire boot to make the plugs easier to remove in the future.


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