Every gas engine uses spark plugs to fire the air-fuel mixture as the end-point in the ignition system. A spark plug screws into the cylinder head on each cylinder and has a pair of contacts on its end. The ignition system coil sends a high-voltage spark across the contacts at the appropriate time, which ignites the air-fuel mixture in the combustion chamber.
The size of the gap between the contacts is critical, as that determines how much energy is required for current to spark across the gap. Each time a spark plug fires, there is a tiny amount of wear on the contacts, which causes the distance across the gap to increase. The metal used in the contacts makes the biggest difference in how much wear there is. Over time, the contacts can also build up carbon deposits or become fouled with oil or fuel. Either way, spark plugs do wear out, and they have a scheduled replacement interval, which can be found in the owner’s manual.
Replacing a spark plug is usually pretty simple. The spark plug wire, or coil, is removed from the plug, then it is unscrewed from its position on the engine. The gap on a new plug is checked, and then it’s screwed into place.
Costs of Spark Plug Replacement
The cost of replacing spark plugs depends largely on how many spark plugs there are.
For a four cylinder engine, on average the cost to replace the spark plugs is about £120. For a six cylinder engine, the average cost is about £155. For an eight cylinder engine, the average cost is about £190.
There are some exceptions, of course:
- On a 2010 Dodge Charger with a 5.7, for instance, there are two spark plugs per cylinder, for a total of 16 plugs, and access is restricted to four of them. The spark plugs are standard, but labour is more expensive, and replacing plugs costs about £240 in most places.
- For a 2007 Maxima with a 3.5-liter V6 engine, the labour time to replace the spark plugs is about 3.4 hours. On the right side of the engine (which is up against the firewall), the intake plenum lays over the plugs and has to be removed before they can be accessed. With gaskets and labour, the job comes to about £360 most places.
- For an example, on the cheaper side, to replace the standard spark plugs on a 2003 Honda Civic, the cost usually comes to around £75.
Different Kinds of Spark Plugs
The main improvement to spark plugs over the years has been the use of more durable metals in the contacts. A standard spark plug uses an ordinary nickel alloy steel for its contacts and is typically good for 30,000 to 50,000 miles. Newer plugs use platinum inserts on one or both of the contacts, or iridium, or ruthenium, and can be good for more than 100,000 miles. The main advantage of the newer types is that they last longer; there is usually no difference in engine performance or miles per gallon.
In many cases, a vehicle that specifies standard spark plugs can be upgraded to platinum or other kinds of spark plugs, and it will usually work just fine. But it’s also common enough for non-OE spark plugs to have small differences in shape, or in the value of the internal resistor, or in the heat range they are engineered for, all of which can cause problems. There are many vehicles that are quite finicky about spark plugs and that don’t reliably run well unless they have the specific OE spark plugs. For that reason, many shops are reluctant to install spark plugs that aren’t OE.
What Else Might be Recommended
The most common thing is spark plug wires. Most vehicles are coil-on-plug nowadays and don’t use spark plug wires, but if spark plug wires are used, it’s common to replace them with the spark plugs. They can be fairly delicate and easily damaged when disconnected from the plugs, so it’s not a bad idea.
If the vehicle uses a cap and rotor, those are usually replaced on the same schedule as the spark plugs, and then they’re all replaced together as a “tune-up,” usually with a bundled price. Very few vehicles use a cap and rotor these days, though.
It’s also very common for spark plugs to be done at a service interval where other things need to be done as well, such as intake system cleaning, transmission service, and so forth. At a dealership, there is often a bundled-cost service, while other service facilities will separate out the costs. For the most part, there isn’t a cost savings one way over the other, and the most important thing is just to stick to a regular maintenance schedule regardless of who does the work.
What can go Wrong
Probably the most common problem that comes with spark plug replacement is shipping damage or a dropped plug that causes the gap to close up. If the gap isn’t correct, then the spark won’t be adequate, or won’t happen at all, causing a misfire. This is the reason to check the gap on each plug before installation.
The body of each spark plug has a metal part carrying the threads and the hex that it is screwed into, and then a ceramic insulation around its positive connection to the coil. If a plug is dropped, the ceramic insulation can crack, and then the voltage spike can follow the crack instead of bridging the gap in the combustion chamber. This also causes a misfire.
The threads in the cylinder head can strip, which is pretty much a worst-case scenario. Sometimes the thread can be cleaned up or repaired in place, other times the cylinder head may need to be removed for repair. The two practises that help avoid that are always starting the threads by hand and torqueing the spark plug to the correct specification. A spark plug too tight can damage the threads, while a spark plug too loose can blow out, taking the threads with it.
One controversial topic is whether to use anti-seize on the spark plug threads. Some mechanics always do, in order to avoid the plug seizing in place. But anti-seize is abrasive and causes a little bit of wear on the threads when tightened down. It also interferes with torqueing the spark plugs correctly, as the specs all assume dry threads.
There’s a wide variety of different service intervals depending on the engine and the kind of plug used; the range is from every 30,000 miles to every 120,000 miles. Consulting the owner’s manual is the best approach.
Possibly, but it could also get worse. Many engines are very particular about what plugs they require, and can have misfires or run rough if they don’t have the recommended plugs. For that reason, most shops will prefer to install only OE plugs or plugs as close to OE as possible.
On most engines, yes, it’s a pretty straightforward job with few tools needed. There are exceptions, of course, but a general rule is that if you can see where the spark plugs go, you can probably replace them.