Replacing worn spark plugs can do wonders for your engine’s performance and economy, and you can handle it yourself with just a few common tools.
Recommended spark plug replacement intervals vary from as little as 30,000 to as much as every 100,000 miles (and everywhere in between), depending on car manufacturer, spark plug metal types, and spark plug design.
Follow your owner’s manual recommendations. It’s fine to upgrade, but never to downgrade from the manufacturer’s factory requirements. You’ll be sacrificing performance and possibly damaging sister components or the engine itself.
Spark plugs fire constantly for a given mileage and run the risk of wearing down, yielding less performance over time. On some platforms, replacing spark plugs can be difficult.
Your biggest challenges will be accessibility and somewhat sticking in both instances (removing the spark plug wires or ignition coils/boots and unbolting the actual plugs).
Sometimes spark plugs are difficult to get to, and the upper plenum/intake manifold must be removed in order to access them. A new plenum gasket will be required, of course. Check the Repair Guide for your particular vehicle and engine.
Why Replace Spark Plugs?
Reason No. 1 for replacement is this: Your car has gone the spark plug replacement distance listed by the manufacturer. This information should be in your owner’s manual.
On most cars today, the manufacturer specifies replacing plugs at around 100,000 miles, but it could be less for your car, and the owner’s manual should always be your guide.
That said, this is remarkable longevity considering that not so long ago, plugs were being replaced as often as every 12,000 to 15,000 miles.
Reason No. 2: Your car isn’t behaving, and after some diagnostic homework, the spark plugs appear to be to blame. Rough running, misfiring, and/or difficulty starting, as well as an OBD II diagnostic code, can all point to new spark plugs.
In general, any spark plug replacement job will be for all the spark plugs. Some engines will have one plug per cylinder, but there are cars on the road today running two spark plugs per cylinder.
If you don’t know this, your dealer or parts store guy/gal can tell you how many you need.
How Hard Is It to Replace Spark Plugs?
For the trained mechanic, plugs on any car can be replaced. For the newcomer to the work, however, replacement may seem impossible. Here’s why.
With most engine layouts today, two things can loom:
- The top of the engine and the plugs may be hidden beneath one or more covers, heat shield(s), a multitude of hoses, tied-down bundles of wiring, and the under-hood support structure.
- If the powerplant is cross-mounted, and particularly a V6, the rear cylinder head may lurk against the firewall, making the plugs virtually impossible to see.
So, you have to replace the plugs after moving or removing anything that gets in the way. And you may have to consider the need to reach down into a tight space between the engine and the firewall to get at the plugs hidden there.
It is these steps that can be so daunting. Not insurmountable, but worth considering. You have to ask yourself, “Is the opportunity of the challenge?”
How To Change Spark Plugs?
#1. Gather The Necessary Tools.
Changing a spark plug will require at the very least:
- A special socket that is deep enough to accept the plug.
- An interior rubber gasket that will protect the tip of the plug.
- A spark-plug gapping tool to check the spacing between the electrode and the ground strap to see if it matches your vehicle’s specifications.
The most common socket sizes for spark plugs are 5/8 inch and 13/16 inch.
#2. Make Sure The Engine Is Cold.
Of course, a hot engine can burn your skin. To make matters worse, changing a spark plug with the head warm can result in damaging the threads in which the plug is placed.
#3. Disconnect The Battery.
When working on any aspect of the electrical system in an engine, it is always a good idea to disconnect the battery terminals in order to prevent any chance of shorting out.
If you do not know how this is done, read the article How to Disconnect a Car Battery?
#4. Remove Spark-Plug Wires Or Coil-On-Plug Connectors.
Older vehicles have spark-plug wires running from them to the ignition system. Many newer vehicles have a coil-on-plug design that uses a separate ignition coil for each individual spark plug.
Whatever the configuration of your vehicle, you will need to remove them before tackling the task of replacing your spark plugs.
Spark-plug wires are removed easily with a gentle straight-up pulling motion while holding on to the boot that slips over the plug itself. Be ready to label them so you can put them back in the same order.
Coil-on-plug connectors often have a locking tab you may have to depress with a screwdriver in order to release before pulling the connectors off—be sure to label them for reinstallation.
#5. Remove Spark Plug With the Socket.
Fitting the spark plug socket over the prominent plug, gently twist counter-clockwise to unscrew it from the engine.
Too much power can split the plug, and pieces will fall into the engine. Take care that nothing falls into the spark plug hole when you lift it out.
#6. Check The Plug.
The condition of the spark plug you are removing will tell a lot about how well your engine is working and help to diagnose faults.
#7. Install a New Spark Plug.
Thread a new plug into the cavity where you took the old one from, being careful not to cross-thread or force in the plug.
Screw it in until finger tight, then use your socket to give it another half-turn. With most brands of spark plugs, rubbing a little anti-seize lubricant on the threads can help ensure that the spark plugs will be easy to remove the next time.
#8. Reattach Wires/Coil-On-Plug Connectors/Battery.
Wrap up your installation by rimming the vehicle’s spark-plug wires or coil-on-plug connectors in order, and in doing that, your last step is simply making sure the battery is connected, and all your tools are gathered.
The engine is free and clear to start. Just the way you started the job, starting the engine after the plug replacement is a good way to validate your work.
FAQs.
What are the symptoms of a bad spark plug?
If the engine doesn’t catch when you’re starting right up, that might be your first heads-up. This may mean that the engine hesitates or isn’t pulling quite the same when you accelerate. Rough idling with engine misfires is another sure sign, along with a tendency towards more fuel being required.
If you experience any or all of these things, it’s definitely time to have your spark plugs changed.
What’s the replacement cost for spark plugs?
A spark plug usually costs less than $10 each. The cost depends on life expectancy and what the electrode is made of. The most expensive, platinum- or iridium-tipped plugs using laser technology, could run as much as $25 a plug.
The cost for a set of four plugs could run from $16 to $100, depending again on the type of plug you need and how many cylinders your engine has. You might want to figure in that some specialty engines use two spark plugs per cylinder.
How often do I need to change my spark plugs?
In the past, experts recommended changing spark plugs every 30,000 miles or so. With today’s extended-life spark plugs, the life expectancy can go as high as 100,000 miles.
Checking and replacing the spark plugs is usually part of your vehicle’s routine service interval. Rarely do you need to make an appointment just for the spark plugs. Consult your Owner’s Manual for more details.
Do I need to replace my spark plug wires?
Usually, yes. You should replace the spark plug wires when you get new spark plugs. With standard spark plugs, that might be every 30,000-40,000 miles or so. If your vehicle uses extended-life spark plugs, you could go as long as 100,000 miles before changing the spark plug wires.
It’s still a good idea to regularly inspect the wires for any frays, cracks, and even bites (yes, bites!), since some wires are made from a soy-based material that’s a favorite snack for rodents.