Spark Plug Replacement Interval by Engine Type

Spark plugs ignite the air-fuel mixture in each cylinder thousands of times a minute. Modern iridium and platinum plugs last far longer than the copper plugs of older cars, so many engines now go 60,000 to 120,000 miles between changes. But the right interval depends on your specific engine and plug, and worn plugs cause misfires, poor fuel economy and, eventually, catalytic converter damage. Here's how to find your interval and tell when plugs are done.

The plug material at the electrode tip is the biggest factor in how long a plug lasts.

These are typical ranges, not your car's spec. Check your owner's manual; the scheduled interval is what your engine was designed and emissions-certified around.

Replace plugs with the type the manufacturer specifies. Installing copper plugs in an engine designed for iridium means you'll be changing them far sooner, and a mismatched heat range can cause problems.

The engine design changes how hard plugs work, and some engine types call for shorter intervals than the plug material alone would suggest.

Naturally aspirated port-injected engines

The easiest life for a plug. Many such engines with iridium plugs are specified around 100,000 to 120,000 miles.

Turbocharged and direct-injected engines

Higher cylinder pressures and temperatures wear electrodes faster and demand more voltage to fire. Many turbo GDI engines specify noticeably shorter intervals, often somewhere in the 40,000 to 60,000 mile range, and some performance engines shorter still. If you've added a tune that raises boost, plugs wear even faster and may need a colder heat range.

V6 and V8 engines with hard-to-reach rear plugs

The interval itself isn't shorter, but labor is much higher on some transverse V6 engines where the rear bank sits under the intake manifold. Doing the job on time matters because a seized plug in an aluminum head is an expensive problem.

Hybrids

The engine runs fewer hours per mile, but plugs are still on a mileage or time interval. Follow the manual.

Diesel engines

Diesels don't have spark plugs. They have glow plugs to help cold starts, which are replaced as they fail rather than on a schedule.

Rotary engines

Rotaries use multiple plugs per rotor and much shorter intervals. Follow the maker's schedule closely.

• Misfires: stumbling or shaking, especially under acceleration or load. Often logged as P0300 or cylinder-specific codes like P0301 and P0302.
• Rough idle: the car shakes at a stop.
• Hard starting, especially in cold or damp weather.
• Hesitation or lack of power when accelerating.
• Worse fuel economy than usual. Track it with the MPG calculator using fill-ups.
• Check engine light, or a flashing check engine light (severe misfire).

These symptoms aren't unique to plugs. Ignition coils, fuel injectors, vacuum leaks and low compression cause similar problems. If the plugs aren't near their interval, diagnose before replacing.

When you pull plugs, their condition tells you about the engine.

On many four-cylinder engines with coil-on-plug ignition, this is a 30- to 60-minute job with basic tools. On some V6 and V8 engines it can take hours and require removing the intake.

Tools

• Ratchet, extensions, and a spark plug socket with a rubber insert or magnet
• Torque wrench (inch-pounds and foot-pounds)
• Gap tool (feeler or wire style) for checking gap
• Compressed air or a vacuum to clean around plug wells
• Dielectric grease for coil boots
• The correct plugs, part number from the manual or a parts catalog

Steps

1. 1Work on a cold engine. Removing plugs from a hot aluminum head risks damaging the threads.
2. 2Disconnect the battery negative if the service procedure recommends it.
3. 3Remove engine covers, then unplug and remove the ignition coils (or plug wires, pulled by the boot, not the wire). Do one cylinder at a time so nothing gets mixed up.
4. 4Blow out the plug well before removing the plug so debris doesn't fall into the cylinder.
5. 5Loosen and remove the old plug. If it feels very tight, stop. Some engines are prone to seized plugs; a little penetrating oil and patience, or a shop, is cheaper than a stripped thread.
6. 6Check the gap on the new plug. Most iridium and platinum plugs come pre-gapped; don't bend fine-wire electrodes. Only check and adjust carefully if the maker allows it.
7. 7Start the new plug by hand to avoid cross-threading. A short length of rubber hose over the plug works well as a starter.
8. 8Torque to specification. Use the figure in the service manual or on the plug packaging. Many 14 mm tapered- or gasket-seat plugs in aluminum heads fall in a relatively low range, but values differ by thread and seat type. Overtightening is a leading cause of stripped threads.
9. 9Apply a small amount of dielectric grease inside the coil boot and reinstall the coil.
10. 10Repeat for each cylinder, reconnect the battery, start the engine and check for smooth running.

Replace coils too?

Not automatically. Ignition coils don't have a fixed interval. But if one has failed, or the coil boots are cracked or oil-soaked (common with leaking valve cover gaskets), replace those parts while you're there.

Plug prices vary from inexpensive copper to more costly iridium, and labor varies hugely by engine layout (typical estimate, varies by vehicle and region). A four-cylinder with coils on top is often a low-labor job; some V6s that require intake removal can be several times more. Get the labor time for your engine when comparing quotes, and add the job to your plan with the maintenance cost calculator.