Replacing the spark plugs are key part of a tune-up. Indeed, in modern vehicles with electronic fuel injection and electronic ignition systems, it may be the only procedure in a "tune-up." If your "CHECK ENGINE" light comes on, and an engine diagnosis indicates a "misfire", changing plugs is the first, and cheapest, place to start.
New spark plugs are pretty inexpensive, and unlike ones on older vehicles, can go many tens of thousands of miles before they should be replaced. Add to this the difficulty getting to spark plugs in today's engines, it is best to replace them, rather than clean and re-gap like in the old days.
Before removing sparks plugs, clean out any dirt or debris around the base of the plug that might fall into the combustion chamber and do damage to the engine. If available, use compressed air. If not, you can blow out the crud using a drinking straw or a small length of hose or tubing. Make sure to wear goggles to protect your eyes.
If the plugs have not been removed for a long time you might find one or two that are a bit tough to loosen. Allowing the engine to cool down often helps especially with aluminum cylinder heads. You might also soak the plug base with penetrating oil.
Using the correct size spark plug wrench, apply a steady pressure rather than jerky hammering to remove a stubborn plug. Be careful not to apply too much torque or too much impact force. You can shear off the top of the plug or strip the threads resulting in some real problems.
If the spark plug is especially difficult to screw out by hand after it is loose, carbon might have collected in the threads in the cylinder head. This condition is more common in older engines. The threads should be cleaned with a thread chaser before installing the new plug. The seat should be cleaned to insure good seat contact when you reinstall the plug.
Inspect each plug to detect any engine problems. They should have no more than a light gray or golden brown coating. Oil-soaked or sooty plugs are an indication of more serious mechanical problems such as a faulty fuel injection system or worn piston rings.
When replacing plugs you might consider precious metal spark plugs, for example platinum. While more expensive, they can go 50,000 miles or more before the next replacement. They are just the ticket for engines where a couple of the plugs are very difficult to get to.
Before installing new plugs, check the electrode gap to see if it is within specifications. The specs can be found in the owner's manual or on later models on the decal usually located on the underside of the hood. Use a wire-type spark plug gap-measuring tool.
If you use a bit of anti-seize compound, on the threads the plugs will be much easier to remove next time. It also ensures proper tightening and helps transfer heat between the engine and plug. If you have plugs with a sealing washer type gasket, make sure it is on the plug. Also make sure the old gasket did not get left in the spark plug port.
Install the new plug by hand until finger tight. They should screw in easily. If not, it might be cross-threaded which will definitely strip the threads in the cylinder head. Aluminum heads strip very easily.
Once the plug is finger tight, use a torque wrench to complete the job. Torque to the recommended amount according to the specifications.
It is good idea to remove and replace one plug at a time to preclude mixing up the spark plug wires. Otherwise mark the wires using tape unless it very obvious which wire goes to which plug.
