Cars with petrol or LPG engines run on controlled bursts of energy managed in part by spark plugs. These elements carry electrical current from the ignition system by igniting the fuel. Spark plugs are an essential part of any functioning combustion engine; like any mechanical part, they are subject to wear, but they are also relatively simple parts to check and repair, if you have the right tools and some mechanical knowledge. Read on to learn more.
Steps
Part 1 of 2: Disassemble the Old Candles
Step 1. Locate the spark plugs in the engine compartment of your car (see the use and maintenance manual)
When you open the hood of the car, you should see a set of 4-8 cables leading to different points in the engine compartment. The spark plugs are located on the engine at the end of these cables, protected by a sheath.
- On a 4-cylinder engine, the spark plugs are mounted in a row above or to one side of the engine.
- In 6-cylinder engines, you can find them on the top or side of the engine head. In the V6 or V8 models they are distributed evenly on each side of the engine.
- Some cars are equipped with a crankcase that you have to remove in order to see the wires connected to the spark plugs; following the cables you will reach the candles themselves. You must always read the use and maintenance manual to find the spark plug housing, know the number, check the distance of the electrodes and know the size of the socket wrench you need to remove them. You also need to label the various cables, so you know how to mate them to the cylinders and not confuse them when it comes time to install the new spark plugs. At this point it is good practice to check for any damage or cracks on the sheaths and cables to evaluate a possible replacement.
Step 2. Wait for the engine to cool before removing the spark plugs
If you have been driving a car for a while, the spark plugs, engine and exhaust manifold will be very hot. Unscrew the candles only when each component is cool enough to touch. In the meantime, gather the tools; to change the car spark plugs you will need:
- A socket wrench with ratchet;
- An extension for the socket wrench;
- A candle socket, typically an accessory included in almost any key set of this type;
- A feeler gauge, available in all auto parts stores.
Step 3. Remove the first candle
Disconnect the cable from the engine by grasping it as close to the base as possible and gently moving it to reach the spark plug below. Don't yank it to detach it from the spark plug or you'll end up with a bigger problem and ruined the cable. Insert the socket wrench with the extension and slowly and carefully unscrew the spark plug from its housing.
- When you want to check the spark plugs to see if they need to be replaced, just remove one and check the gap between the electrodes. If the contacts seem burned to you, reinsert the spark plug, screwing it in with the correct tightening and go to the auto parts store to purchase new ones before proceeding with the work. You have to take them apart one at a time, respecting a certain order; the spark plugs ignite following a certain sequence and if you cross the cables by connecting them to the wrong spark plug you would cause malfunctions to the engine which may not even start or be damaged.
- Remember: if you disassemble more than one candle at a time, label the wires with a small piece of masking tape so you know how to connect them in the future. Use a numerical criterion and assign the same value to the corresponding candle.
Step 4. Measure the distance between the electrodes
This is a very specific value that can range from 0.71mm to 1.52mm, with a minimum of play depending on the specific spark plug set in your car. Currently, almost all candles are pre-calibrated before sale, based on the model number and their application; however, it is always worth checking them before assembly. Refer to the specifications given in the user and maintenance manual to know the optimal electrode gap. Always use a feeler gauge for this measurement.
- If the spark plug is still in good condition and is an adjustable model, but the gap is larger than it should, then you can try to calibrate it by tapping it on a wooden surface after inserting the feeler gauge between the electrodes. Continue in this way until the distance reaches the desired value; alternatively, buy some new candles. On average they should be replaced every 20,000 km or as indicated in the use and maintenance manual. These are not very expensive spare parts and you should change them regularly to prevent engine problems and ensure good ignition.
- If you have decided to change the spark plugs yourself, then buy good quality tools and materials, such as a precision feeler gauge. In practice it is a metal ring that allows you to check that the distance between the electrodes is correct to guarantee ignition. The same goes for spare parts: rely only on original and quality products, spending a few euros more allows you to feel comfortable.
Step 5. Check the spark plugs for wear
It is completely normal that they are somehow dirty, even if they work properly. However, they should be replaced when they are white, show limescale deposits around the electrodes, or when there are obvious burn marks or electrode pieces missing. If the candle is covered with a thick layer of soot, you need to change it.
If the spark plugs are blackened, bent or broken, then you have a mechanical problem with the engine and you should take the car to an authorized repair shop or dealer without further delay
Part 2 of 2: Fit the New Spark Plugs
Step 1. Buy the correct replacement spark plugs
You can read the maintenance manual or the catalog that you can find at the auto parts shop to find out which type of spark plug is suitable for your car model, based on the year of production. There are literally hundreds of different combinations of candles of varying sizes, prices are between 2 and 15 euros and the material could be platinum, iridium, yttrium and so on. Candles made of precious metals are generally more expensive, but the coating is more resistant to wear. If you have any doubts about the choice, ask the shop assistant or the dealer for advice to buy original spare parts.
- As a general rule you should buy the same type of spark plugs that are already on the engine. Never switch to a lower quality or cheaper product and don't think about improving something that is already working great. The automaker has chosen a certain type of spark plug for a good reason, so simplify your work and buy identical spark plugs whenever possible, making sure they are the right ones! Always consult the maintenance manual or ask the dealer for information.
- You can usually buy adjustable spark plugs on which you can change the distance between the electrodes. In this case you must check them periodically and make the necessary adjustments. The most important thing to check is that the gap between the electrodes is within the specifications for your vehicle. If you do this yourself, you are sure of the results. For this reason, take the new candles out of the packaging and do a quick gap check.
Step 2. Before inserting the new spark plugs, clean the threads
You should take advantage of these maintenance jobs to check for wear on the cables and clean their terminals. Get a wire brush or use compressed air to clean the connections and make sure the spark plug housing is free of debris. If necessary, change the cables.
Step 3. Insert the new spark plugs and tighten them with the socket wrench
You can use a specific spark plug wrench to unscrew and screw them back into the engine. Rotate them just a little, just 1/8 of a turn past the hand tightening. Never screw them too hard, as you can strip the thread of the cylinder head resulting in time-consuming and expensive repairs. Remember to connect the cables exactly as they were in the beginning and remove the tape you used to label them.
Step 4. Lubricate the spark plugs before fitting them
Put a small amount of anti-seize paste on the threads of each in case you need to screw them to an aluminum motor. This product avoids the reaction between different metals. You can also use a small amount of dielectric silicone compound inside the protective sheath, where it connects to the cable; this will make it easier for you to unplug the cable at the next opportunity. Once inserted in its housing, always rotate the spark plug in the opposite direction to the tightening one until you have perfectly aligned the two threads; this precaution avoids screwing in the spark plug badly and damaging both the engine and the spark plug itself.
Advice
- New car models have spark plugs mounted in hard-to-reach places; for this reason it tries to identify them all to understand how to disassemble them. Start replacing the "hidden" ones before changing the ones that are easier to access.
- To make sure the spark plugs aren't too tight or loose, use a torque wrench and tighten them to your vehicle's specifications. You can find the torque value in the maintenance manual or by asking your local dealership workshop.
- Use a spark plug socket wrench (with an internal gasket or a magnet) instead of a regular one to prevent them from falling out during the replacement process. If a spark plug falls, the gap between the electrodes may change; at this point you should calibrate and clean the piece again or replace it completely!
- Diesel engines do not have spark plugs.
- When replacing the spark plugs, make sure nothing falls into the receptacles they are screwed into. Spray compressed air to blow away any residue or dirt before you unscrew the old spark plug. If any dust falls into the housing, start the engine without inserting the spark plug and let the piston force the air - and therefore also the dirt - out with powerful bangs. In this case, move away from the engine to avoid getting hit in the eyes and keep children away from the work area.
- It is rare to have to calibrate the electrode gap on brand new spark plugs, but it is always worth checking. This way you avoid having to check the same candle twice just for distraction.
- Twist and pull only the insulating body of the cable and not the cable itself, to prevent it from breaking and then forcing you to buy a new set of jump leads. There are specific tools for this operation (although they are not essential).
- Regardless of whether or not you service your own car, it is worth buying a technical manual at the dealership, at a car enthusiast's gathering, on eBay or at a flea market. These are more detailed and specific guides than the common use and maintenance manual that comes with the machine and are worth all the money you spend.
- If the engine was used without a few spark plugs, then fuel has accumulated in these empty housings, subsequently flooding the spark plug. The engine must run for at least one full minute to burn the gasoline that has accumulated under the spark plug and will eventually run smoothly again. But remember that a "large amount of fuel burns a lot of air" (more than a few cycles of air).
- Check the model number of the candles very carefully. Instead of being labeled with a clear denomination, they are often labeled with somewhat opaque numbers, such as 45 & 46, or with a series of easily forgotten or mistaken numbers, such as "5245" or "HY-2425" and etc. Write down the numbers and / or letters on a sheet of paper and always check the code before purchasing; even a trivial mistake could cost you a waste of time, work and you would not be refunded.
- If you don't have a specific socket wrench for spark plugs, then you can loosen them with a regular socket wrench and then use the insulating sleeve terminal of the cable to grab and pull them out of the engine head. Insert the new candles by placing them first in the terminal of the sheath and initially screw them by hand; finally, the tightening with the socket wrench ends.
Warnings
- Wait as long as necessary for the engine to cool before replacing the spark plugs. These elements reach very high temperatures and the engine block could burn you.
- Keep children away from the work area and always wear safety glasses.
