Steps
- Expect spark-plug work to be tedious. If this is your first time, plan 3 or 4 hours after engine cools (or 1 hour on several days), allowing 15 minutes per plug. It is harder than changing air filters, fixing flat tires, or starting a mower.
- Consult your vehicle's manual. Look up where your spark plugs are, how many you have, the correct "gap", and the size socket needed to remove them. Also write down the vehicle's make, model, and year. The gap can also be found on the emissions label under the hood.
- Visit your local auto parts store. Find (either by looking up in the provided reference book or by asking an employee) the correct spark plugs for your vehicle. The store also has socket wrenches, plus spark-plug sockets (with gasket), and socket-extension rods or swivel-joints to reach recessed plugs.
- Find out (from the reference book, the packaging, or the employee) if these spark plugs need to be "gapped". Some modern plugs should not be gapped (but others can have different gaps, depending on use in either 6-cylinder or V-8 engines, etc.).
- Park vehicle, turn off the engine, and open the engine compartment, to cool for hours. (WARNING: After running a car for a long time, the spark plugs can be the hottest part of engine! While it can require several hours to cool enough, it can require several weeks to heal burnt skin.) Especially with aluminum-head engines, let cool to room temperature to reduce the probability of damaging the threads.
- Take (if needed) a wire-gauge spark plug gap tool and adjust the distance between the two electrodes. Between the electrodes is where a spark is made. One electrode will be an L-shaped piece of metal (hook), the other a metal prong centered directly across from it. Set the gap between the two electrodes, from .028-.060 inch, such as .035/.040 /.043 /.050, as in book (see Tips below).
- Collect tools & new plugs (perhaps in a tool-tray). Remember which direction the socket-wrench switches to reverse/unscrew: wrench might not be visible when working back plugs.
- Check fit of new plugs inside wrench-socket gasket. If new plugs stick to rubber gasket, consider removing gasket with screwdriver in square hole, to just use tape. Like taping screws to a screwdriver, the socket can be taped to spark plugs (not the threads) with scotch tape, for easy release once inside the engine. Otherwise, have pliers to pry the socket off new plugs once installed.
- Locate (with the help of your manual or a repair manual for your vehicle) the distributor spark-plug cables/wires. The number of wires will be equal to the number of spark plugs your engine has. Often these wires are red or black, and will be equally divided on opposite sides of the engine.
- Using masking tape, mark each of these wires for where they connect. Don't rely on memory: if interrupted, easy to forget, and engine can run rough with crossed plug wires. For 8 cylinders, deducing plug connections is almost impossible (120 choices for 5 wires) -- in that case you must contact an expert or study wiring guides.
- Remove each spark-plug cable, pulling the caps (to avoid breaking cable wires). Caps should come loose by very intense twisting/pulling (avoid jerking/hitting fingers).
- Using compressed air, pressurized engine cleaner, or a brush, clean all debris from around the plug.
- Using a spark-plug socket, remove each plug from the engine, and replace each with a new spark plug. Don't over tighten (usually just 1/16 turn, after finger-tight).
- Replace the spark-plug cables on the same plugs they originally came from, and remove the masking tape.
- Remove tools near engine (beware the moving belts), close your engine compartment, and start your vehicle.
Video
Tips
- Disconnecting only one wire at a time while changing that one spark plug while leaving the other spark plug wires in place helps to avoid mixing up the firing order.
- Using a spark plug socket (with internal gasket) instead of a conventional socket will help you to not drop a spark plug when removing or inserting them. (If dropped, the gap often changes, must be re-gapped/cleaned.)
- To ensure that the plugs are not over- or under-tightened, use a torque wrench and tighten them to your vehicle's specs. This information can be found in shop manuals or by calling the service department of your local dealership.
- Examine your old spark plugs when you remove them. They should look slightly burnt at the tip, maybe with some white spotting. If they are bent, black, or broken, you could have a bigger problem and should consult a mechanic.
- Diesel engines do not have spark plugs.
- These basic steps apply to replacing spark plugs in all engine types.
- Whether or not you service your own car, invest in a set of the dealer's shop manuals from the car maker. These are much more in-depth than the repair guides you find in the auto parts store.
- Put 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.
- Twist and pull only on the boot insulator portion and not the wire cable itself, lest it separates immediately necessitating the purchase of a whole new set of ignition wires. There are optional tools made just for this step.
- Use a small amount of dielectric silicone compound on the inside of the spark plug wire boot to ease removal in future maintenance
Warnings
- Spark plugs involve many dangers: hot engines not cooled for hours; dropping a tool inside engine/belts; forgetting which wire for which plug; setting too-wide gaps causing high-speed misfires; buying wrong plugs; dropping objects in open plug holes.
- Biggest problem is "hot engine": driving from the auto-parts store and not waiting hours, with open hood, for the engine-block to cool. Most people are too busy to wait, but spark plugs are about the last thing to cool. If desperate, use gloves to reach easiest plugs, while back plugs cool more. Remember engine heat also fries patience and boils tempers.
- Dropping tools: After decades of car design, you might expect a built-in work rack to hold tools near an engine: don't. Many engines seem designed for all to slide, break and kill. Use your own tool-tray if possible, but ALWAYS beware dropping tools in the engine with moving belts.
- Hard-to-reach plugs: Newer cars seem to have plugs that are difficult to reach, so pre-look for all plugs to see where to reach. Consider replacing hidden plugs second (while still having patience), before replacing easier plugs.
- Count your tools when done: It is too easy to overlook a wrench, socket, gap-gauge, or old plug perched inside the engine. Memory fades after replacing 6 or 8 plugs. Count 6: wrench, plug-socket, pliers, gap-gauge + perhaps 2 old plugs.
- Fuel-fouled plugs: If the engine has run without some plug firing (perhaps forgot to re-attach a plug cable), fuel collects there, flooding the plug. An engine might run almost a full minute to burn the collected fuel under that plug, and run smoothly again. Just remember: a "lot of fuel burns a lot of air" (more than just a few cycles of air).
- Wrong plugs: Triple-check spark plug model numbers. Contrary to obvious naming, plugs are often numbered with dull numbers, 45 & 46, or the same forgettable digits merely swapped: such as "5245" or "2425" (there's no memorable "1973" or "1776"; at least biscuits had "1869"). Write it down & triple-check before buying: or you might think "5224" was it, and then drive a hot engine to get other plugs.
- Falling inside: Make sure when replacing spark plugs that nothing falls into the hole the spark plug is threaded into. Use compressed air to blow away dirt and debris right before you remove an old plug. Fix: If dirt fell in, consider starting the car without that plug, allowing the piston to force air/dirt out in loud bursts.
