Learn2 Replace Spark Plugs

Firing on all cylinders

Spark plugs are the trusty soldiers that ignite the gas mixture in the engine cylinders, powering your car. When in good condition, they help keep the engine running cleanly and efficiently. When worn out or fouled, they put stress on several of the car's systems and can trigger problems that are very expensive to repair.

How difficult is replacing them? It depends on your engine's configuration, but in these days of computer-controlled engine functions, the spark plug change is one of the few traditional "tune-up" tasks just about any backyard mechanic can still do.

When should you change the plugs? Ideally, before your car has started losing performance. Refer to your car owner's manual for a service schedule.

Regular spark plugs with copper alloy electrodes will last anywhere from 6,000 to 30,000 miles (roughly 10,000 to 48,000 kilometers), depending on your engine and driving conditions. For example, ill-maintained engines (or older cars without electronic ignition systems) will need a change more often, as will cars using leaded fuel. (Leaded fuel leaves deposits on spark plugs, "fouling" them.)

Premium spark plugs can last 100,000 miles (160,000 kilometers) without being changed. However, numerous variables can shorten plug life, and many technicians advise changing even premium plugs more often--say, every 30,000 to 60,000 miles (48,000 to 97,000 kilometers), or whenever your manual says you should check the spark plug wires, distributor cap, and rotor for wear.

The engine and ignition system of every make and model of car are different, so get a service/repair manual for your particular vehicle (in addition to your owner's manual) before you start any work on it. This is especially advisable if you've never worked under a vehicle's hood before. These manuals are available in bookstores and auto supply stores, as well as at some auto dealerships and public libraries.

Note: The car's engine should be off and cool before you start changing the plugs.

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Get to know your ignition system

Turning your car's ignition key opens the circuit from the car's 12- or 14-volt battery to a part called the ignition coil (the car's alternator should then take over from the battery). The coil steps up the voltage to between 6,000 and 35,000 volts. This high-voltage charge travels to a part called the distributor, which sends the current at precisely timed intervals to each of the spark plugs. (Some newer cars don't have distributors, but use sensors to send voltage directly from the coil to the plugs.)

The plugs are screwed directly into the engine, one per cylinder, with their tips inside the cylinder. The charge races through the plug and jumps a gap between two electrodes at its tip, creating a hot spark that ignites the gas mixture in the cylinder. Bam! The piston is driven down the cylinder shaft, delivering the power that turns the wheels (among other things).

If a plug is damaged, clogged, or worn, the cylinder may not fire, or its firing may put stress on the rest of the ignition system. Either way, it's bad news.

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Gather tools and parts

Before heading off to the auto parts store, take a few minutes to locate your spark plugs and decide what kind of tools you need.

With the car's engine off and cool, open the hood and locate the engine block--it's that heavy-looking metal thing in the middle with all the wires and hoses coming out of it. Look for indentations along the engine's top or side with an insulated wire emerging from each. These wires are the plug wires, and they're attached at one end to the tops of the spark plugs and at the other end to either the distributor or (in distributorless cars) the ignition coils.

There should be as many spark plugs as your engine has cylinders--usually four, six, or eight. If you can see all of them and they look easily accessible, you'll probably just need a spark plug socket (which has a padded interior) and a ratchet wrench. If one or more of the plugs is out of sight or obscured by hoses or other parts, your job will be more difficult, and may require a ratchet wrench or socket with a universal joint (so you can approach the plug from any angle), as well as a ratchet extension to give you better access.

Take your owner's manual to the auto parts store; it has plug specifications listed in it. The store should also have indexes matching your car to compatible parts, and the salespeople should be able to point you to the necessary tools if you don't have them yet. Note: Don't use different plugs if your store doesn't have the type specified by your manual. Plugs are not created equal, and the wrong ones can damage your engine.

While you're at the store, pick up a feeler gauge if you don't already have one (more about this device later).

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Remove a spark plug

The plug wires are attached to the plug and distributor (or coil) with round or L-shaped rubber nipples called "boots." Grasp a boot close to the engine block and twist or pull it off the plug. (Don't pull on the wire, or you may damage it.) The ceramic insulation and metal tip of the spark plug's terminal should now be visible.

Brush or wipe away any dirt or oil from around the plug. (If there's more than a trace of oil, you may have a leak and should see your mechanic.) Fit the ratchet wrench (or its extension) to the socket, and fit the socket squarely over the head of the spark plug. Turn counterclockwise to loosen, and pull the plug out of the engine block when it's free.

Read the plug. The plug's tip (the part that was inside the cylinder) should be grayish or tan in color, and the edges of the center electrode may look a bit rounded from wear. Black, whitish, or gray deposits, corrosion, or physical damage anywhere on the plug's tip all indicate an engine problem. Label any damaged or crusted plugs with the cylinder location, and take them to your mechanic for diagnosis as soon as possible. (You can finish replacing the spark plugs first.)

Note: Each wire connects a specific plug to a specific terminal on the distributor or ignition coil. The connections are not interchangeable, so remove and replace the spark plugs one at a time so you don't get the connections confused.

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Install each new plug

New spark plugs are manufactured to have the proper gap between the two electrodes at their tips. However, the plug may have suffered some damage in transit, so always check the gap before you install a plug.

Refer to your owner's manual to find the right gap size, then insert the tip of a feeler gauge into the gap of the plug. The gauge should slide between the electrodes with light to moderate contact on either side. If the gap is too large, tap down on the side electrode (the one that bends at a right angle over the plug's tip) with a light hammer or wrench until it's the right size. If it's too small, bend the side up slightly with the flat tip of a screwdriver inserted into the gap. Tap and bend until the gap is right. This whole process of checking and adjusting is called "gapping" the plug.

A tight fit. If your engine block is made of aluminum, or if you had difficulty getting the plug out, your cylinder threads may need a bit of protection. Apply a little dab of antiseize compound to the threads of the new plug. Follow the instructions on the compound's container.

Insert the plug into its hole in the cylinder head and turn clockwise to tighten. When you can't turn it with your hand any longer, use the plug socket and ratchet wrench to continue tightening. Use the same degree of force to tighten it as you did to loosen it. Undertightening can hinder the cylinder's operation (and damage the engine), and overtightening can strip the threads on both plug and cylinder.

Note: A torque wrench, which can be used in place of a ratchet wrench, can measure precisely the pressure needed to tighten a plug. Your owner's manual should specify the torque needed for plugs in your engine.

Fit the boot of the plug wire firmly over the head of the spark plug, and move on to the next plug.

Start 'er up. When you've replaced every plug and wire in turn, start your engine and rev it a bit. If it starts rough or you can hear "misses" or gaps in the smooth firing of the cylinders, there's something wrong. Turn off the engine (let it cool if you ran it enough to heat it). Check that the plugs are all tight enough and that the wire boots are well-seated. Make sure you didn't connect any of the wires to the wrong plugs (this shouldn't happen if you replaced the plugs one at a time). If you think you may have mixed up the wires, refer to the wiring diagram in your service/repair manual.

If the engine sounds good, sit back a minute and let it purr. You've just done your car an important service in exchange for all it does for you.

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Troubleshoot other parts (optional)

Is your car running rough, losing fuel efficiency or power, or hesitating when you accelerate? If you're changing your plugs because you've noticed one of these performance problems (rather than as part of scheduled maintenance), there are a few other things you should do now. Note: Use your car's service/repair manual to perform all these tests and checks, but don't embark on any task unless you feel you understand how to do it safely and effectively.

Compression test. This measures the pressure of the air and gas mixture in the cylinder during the combustion cycle. If the compression is below given parameters, there may be a valve or seal leak in the cylinder or the engine head. These problems require expert attention. To perform a compression test, you need: a compression gauge, a helper, and your car's service/repair manual.

Wire tests. Examine the plug wires. If they're cracked, brittle, or soaked with oil, replace them, following the directions on the new plug wires' packaging and in your service/repair manual. Test their resistance as well, even if they appear to be intact. To do this, you'll need a device called an ohmmeter and your car owner's manual and service/repair manual. Follow the directions in your service/repair manual to test the wires.

Inspect distributor. If your car has a distributor, follow the directions in your service/repair manual to remove the distributor cap and inspect both the cap and the rotor beneath it. If these show signs of damage, including hairline cracks, burned or eroded terminals, or corrosion, they should be replaced. Consult your service/repair manual for further guidance.

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