Kate is a research enthusiast with a Bachelor of Science degree from Sonoma State University. Kate has a passion for the automotive field.
Spark plugs fire a spark that ignites the gasoline in your vehicle to power the pistons that turn your vehicle axles. Over time spark plugs oxidize and wear out. Worn out spark plugs can cause many different problems such as decreased gas mileage, engine misfiring, computer fault codes (i.e., an illuminated check engine light), and poor vehicle performance. Also, you can often tell you have a bad spark plug because your vehicle will run rough when it is first started, but as it warms up, the engine will seem to "smooth out."
In order to maintain top performance of your vehicle, it is pertinent that you replace your spark plugs approximately every 100,000 miles.
Steps to Replacing Spark Plugs Yourself
- Safety First!
- Locate the Spark Plugs
- Clean the Area Around the Plugs
- Gap the Spark Plugs (if needed)
- Remove and Replace
Step 1: Safety First!
Before you perform this job, complete the following steps to ensure that you can do the work safely:
- Verify that the engine is turned off, the transmission is in park, and that the emergency brake is engaged.
- If you ran the vehicle recently, ensure that the engine has had enough time to cool down completely (at least a few hours since your last drive. Overnight is even better). This ensures that you will not get burned while working on your car.
- Open the car hood and locate the battery. Remove the positive power cable by loosening the bolt on the cable with a wrench. This is essential so that you are not shocked and you don't damage any electrical components.
Step 2: Locate the Spark Plugs
The next step is to locate the spark plugs and ensure you can reach each of the plugs. The easiest way to locate the spark plugs on any vehicle is to look for the thick individual wires, all the same color, leading from a single point under the hood to multiple points around the engine.
There will be the same number of wires as there are cylinders in your engine (for example, a vehicle with a "V-8" engine will have eight wires, a "V-6" will have six wires, and a four-cylinder will have four wires). These wires will generally be evenly spaced with large rubber boots on the ends.
The spark plugs will be at the end of each of these wires where the wires fan out to meet the engine. Typically, one end of these wires is connected to a distributor (the higher end), and the other end is connected to the spark plugs (the lower and usually the more difficult end to reach).
If you are still having trouble locating the spark plug wires, consult your vehicle manual for the exact location of each spark plug for reference.
Ensure that you can get a wrench in to reach near the ends of each of these wires, where the spark plugs are located. If necessary, remove any plastic pieces, air inlet ducts, or other components needed to allow you to reach the spark plugs.
Step 3: Clean the Area Around the Spark Plugs
Since the spark plugs are located directly on the side of the engine, you want to make sure the areas around each plug is free from grease and debris before replacing them. You don't want to accidentally knock any of that nasty stuff into the internals of the engine while replacing the plugs.
The easiest way to clean the area of both grease and debris is to use a can of brake cleaner. Spray the brake cleaner on the areas around each spark plug to clean it. Brake cleaner will dissolve grease and blast away debris. It will also quickly evaporate leaving the area dry and with no residue. Other options are to use compressed air or just a damp cloth (depending on how dirty the area is around each plug).
Step 4: Gap Your Spark Plugs (If Needed)
When you bought your spark plugs, they probably asked you if you want "pre-gaped" spark plugs or not. If you bought the pre-gaped plugs or were told that gaping was not necessary, then you can skip this step.
If you didn't, or you aren't sure, then you are going to have to "gap" the plugs before you replace them. What it means to "gap" a spark plug is to make sure that the small gap a the very tip of your new spark plugs is the correct size for your vehicle.
Either call your local auto parts store and ask or use an online lookup tool such as the BriskUSA Spark Plug Gap Lookup to find out what the correct size gap is for the spark plug on your specific car.
Once you know how big the spark plug gap should be, use a gapper tool to set the gap of each of the new plugs. This tool will make it very easy to check the size of the gap by sliding the wire of the correct size in the small gap at the end of the plug. If the gap is too small (it won't slide in), use the pry tool on this gapper tool to bend the small right-angled metal piece at the very end of the spark plug to make the gap bigger. If the gap is too big, bend the end in slightly closer and recheck. The wire should smoothly slide inside this gap without extra wiggle room.
Step 5: Remove and Replace
Next, locate the first spark plug and remove the spark plug wire from the plug itself. Make sure to remove the wire by pulling on the actual wire boot instead of the wire itself. The wire will just pop off with a little pulling by hand. If the wire has been on for a long time, it may take a little force and some slight wiggling to get it to come off.
Use a socket wrench with the appropriate size socket to remove the spark plug. The appropriate size socket to use is normally 13/6 inch or 5/8 inch deep type socket. An extension is often needed for spark plugs; as their location can be hard to reach. Turn the socket wrench counter-clockwise to loosen it.
Once the old plug has been removed, you can install the new plug. Afterward, tighten the plug using your hand clockwise making sure not to cross thread the plug. Then use a socket wrench to finish the job. Do not over tighten the spark plugs or engine misfire may occur, a little more than "snug" is fine. Put the spark plug wire back on the plug by pressing the boot down over the plug. Make sure you feel a solid "pop" to ensure a good connection.
Repeat the same process for the replacing the rest of the plugs (one plug per cylinder on the vehicle), being careful to pay attention to not mix up which wire goes to which plug. For this reason, I suggest you replace one plug completely before moving on to the next one.
Step 6: You're Finished!
Reinstall anything you removed under the hood to provide better access to the spark plugs and reconnect your battery's positive power cable.
That's it! Once your spark plugs have been replaced, your engine will respond with better horsepower, engine reaction time, better cold starting, and better gas mileage.
Hopefully, you noticed as you changed each plug the difference between the corroded, dirty plugs you took out of your car and the nice new ones you installed. Your vehicle will thank you! Your wallet will also thank you because you just saved yourself a lot of money by doing this job yourself!
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
Jo on July 16, 2020:
Um shouldn’t one remove the negative terminal first so one doesn’t accidentally electrocuted themselves?
pcdriverupdate from VA on March 20, 2012:
good practical knowledge that anyone can benefit from. thanks for sharing.