While every part of your vehicle’s engine is important, spark plugs are often underestimated. They are critical components that can significantly impact performance and efficiency. Critical functions, such as power output, fuel economy and even engine temperature are affected by the condition of your plugs, so knowing how to replace spark plugs is essential to any DIY car owner. Here’s an overview of how to replace spark plugs for most vehicles.
Starting off, it’s good to know what makes one plug different from the next. The central electrode is normally made up of copper, nickel or chromium, but it can also be made of rare metals such as platinum or iridium, which are more durable and produce a more powerful spark. You should check your vehicle’s manufacturer specs, found in your owner’s manual, before upgrading to rare-metal plugs. The best spark plug to use is the one that matches the same specifications as the one that came with the engine from the factory. The plug needs to not just match the same physical shape, but also the same heat range as original. Unless you are an engine tuner, don’t stray beyond what your engine specification demands.
Removing Old Plugs
First, remove your old plugs. Work one on spark plug at a time to reduce the possibility of mixing up the ignition firing order. This is especially true when working on an older engine with spark plug wires and a remote coil. The biggest mistake you can make is pulling off all the spark plug wires at once.
Pull off the spark plug wire by grasping the wire’s boot — never pull by the wire itself. A pair of spark plug terminal pliers works well for this task. If your vehicle uses coil-on-plug ignition, be extra careful not to damage the coil at the top of the short plug wire as well. For coil-on-plug engines you may find it easier to disconnect the coil pack from the ignition wiring harness first, then twist the coil pack back and forth as you gently pull it off the spark plug.
Next, use a brush and a shop vacuum to remove any debris that could fall into the engine after you remove the plug. Do not use compressed air! A blast of compressed air aimed down a spark plug hole will eject whatever engine crud is inside right back at you. If it was been a while since your last spark plug change, you can spray a little lubricant down the spark plug holes to make things a little easier to remove.
If you have clearance use a spark plug socket and extension on your ratchet to loosen the plug. If things are tight you may have need to use a specialty socket to make things easier. Once it is loose, disconnect the ratchet and finish the job by hand. Stuff the plug hole with a clean shop rag to keep debris out. The last thing you want is anything to fall down into the engine.
Before you throw that old plug away, inspect it for signs of a deeper problem. A spark plug can tell you a lot about how the engine is running: Black soot on a plug could mean that the engine’s fuel mixture is too rich, while black moisture is a telltale sign of an oil leak. Blisters or burnt spots on the insulator and electrode are indicators that a fuel mixture is too lean, the engine is overheating, the gap was too wide or that you have a timing issue.
Set The Gap
Every engine has a unique specification for their spark plugs’ gaps. The gap is the space between the electrodes that the spark jumps across to ignite the gas inside the cylinder. Before installing your new plugs, check your manual for the correct gap and set it using a feeler gauge. Use the gauge’s hook to open the gap slightly if necessary; if it’s too wide already, gently press the electrode down on a non-marring surface until it meets specifications.
Installing New Plugs
When inserting the new spark plugs resist the temptation to use anti-seize on the threads. As long as the cylinder head threads are clean the plugs should come out just fine next time. Thread the spark plug by hand with the socket and extension; be especially careful not to cross-thread the plug. Take your time and go slow. Once its hand-tightened, use your ratchet to finish the job. Tighten the plugs to the manufacturer’s recommended torque specs, reinstall your plug wires in the correct order, and you’re all set.
Knowing how to replace spark plugs is simple and can save you money when doing a tune-up. Fresh plugs help your engine start and run more smoothly, and can even improve your fuel economy.
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Erich Reichert has been an editor and on-air personality in the radio control car hobby for 12 years. A certified car nut since birth, he has written for internationally published titles such as RC Car Action, RC Driver and Xtreme RC Cars, as well as Stuff Magazine, Road and Track and Super Street. He's covered everything from product reviews and tech articles to high-profile lifestyle pieces and celebrity interviews. Erich found his passion for writing after a successful career as an art director, working with brands such as Pepsico, NASCAR, MTV, Nintendo, WWE, Cannondale Bicycles and HBO. He's also a father, an avid hockey fan and an FIA race license holder who enjoys hiking, playing drums and movies.