How to Change Spark Plugs

On some engines this is a twenty-minute job with a socket. On others it means removing half the intake manifold. Find out which you have before you buy anything.

• Time: 30 minutes to several hours, entirely depending on the engine
• Difficulty: intermediate
• Shop: $100–$250. Parts: $20–$70. Saves: $80–$180

First: is yours an easy one?

Open the hood and look at the top of the engine. Take the plastic cover off if there is one.

• A row of four coils sitting on top, each held by one bolt — that is a straightforward four-cylinder. Go ahead.
• A V-shaped engine where you can see three coils on the front bank and nothing on the back — the rear three are under the intake manifold. That is a much bigger job. On many V6 engines it is the single most common reason people give up halfway.
• Thick rubber wires running to the plugs — an older design, usually easy, and replace the wires at the same time.

Search your year, make, model and engine plus “spark plug replacement” and watch two minutes of a video before deciding. This is the job where five minutes of research saves an entire ruined Saturday.

What you need

• The exact plugs your manual specifies. Not “the equivalent.” Heat range and reach matter, and the wrong plug can damage an engine.
• A spark plug socket — usually 5/8″ or 13/16″, with a rubber insert that grips the plug.
• A socket extension and ideally a swivel joint.
• A torque wrench. More important here than on most jobs — see below.
• Dielectric grease for the coil boots, and anti-seize only if the plug manufacturer says so — many modern plugs come pre-coated and adding more causes overtightening.

The job

1. Disconnect the negative battery terminal.
2. Remove the engine cover if there is one, and photograph everything before you unplug it.
3. Work on one cylinder at a time, start to finish. That way nothing can go back in the wrong place.
4. Unplug the coil connector, squeezing the tab rather than pulling on the wires. Remove the single bolt holding the coil and pull the coil straight up with a slight twist.
5. Blow or brush the dirt out of the plug well before the plug comes out. Anything sitting down there falls straight into the cylinder otherwise.
6. Break the plug loose, then unscrew it slowly. If it feels gritty or seizes, stop, screw it back in a little, and work it out gradually.
7. Look at the old plug. Light tan or grey is a healthy engine. Black and sooty means it is running rich. Oily means oil is getting into that cylinder. White and blistered means it is running too hot. Worth a photo if anything looks odd.
8. Check the new plug’s gap against the manual figure if the packaging does not say pre-gapped. Most modern plugs are, and iridium and platinum plugs should not be adjusted — the tip is fragile.
9. Start the new plug by hand — drop it into the socket, lower it in, and turn it by hand several turns. If it does not turn freely by hand, stop and start again. Cross-threading a cylinder head is the disaster on this job.
10. Torque it to the manual’s figure, typically 15 to 25 lb-ft. Too loose and it blows out; too tight and it cracks the insulator or strips the head.
11. Smear dielectric grease inside the coil boot, refit the coil, its bolt, and the connector until it clicks.
12. Next cylinder. Repeat until done, reconnect the battery, and start it.

Expect a slightly rough idle for the first minute while the computer relearns, and a possible brief check engine light after the battery disconnect.

When are they actually due?

Copper plugs, around 30,000 miles. Platinum, 60,000 to 100,000. Iridium, often 100,000 or more. Your manual has the number, and modern plugs genuinely do last that long — being told at 40,000 miles that your iridium plugs are due is worth questioning.

Symptoms of worn plugs: rough idle, hesitation under acceleration, harder starting, worse fuel economy, or a misfire code such as P0301. Note that a misfire code names the cylinder, not the cause — see reading a check engine code.

The mistakes that catch people

• Working on a warm engine. Stripped aluminium threads.
• Cross-threading. Always start by hand.
• Guessing at torque. This is the job that justifies buying the wrench.
• Dirt falling into the cylinder because the well was not cleaned first.
• Dropping a plug. If a new plug hits the floor, put it aside — the ceramic can crack invisibly.
• Starting the job on a V6 without checking the back three.

Information only, not professional advice, and no substitute for your vehicle’s service manual. Work on your own vehicle is at your own risk. See the disclaimer.