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Replacing Spark Plugs

By R&D
Published on July 5th, 2024

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With today's vehicles boasting "100,000 miles between tune-ups", spark plug replacement as a routine maintenance practice is often overlooked or considered unnecessary. Truth is, regular spark plug replacement on older vehicles is still a necessity, and even newer vehicles will benefit from a periodic inspection. So if your skills in this area are a little rusty, now is as good a time as any to brush up on them.

The spark plug is responsible for generating the spark that ignites the air/fuel mixture in the cylinder during the compression cycle, so the expanding gases generated by the exploding fuel apply maximum force (power) against the piston. The spark plug uses the high-voltage electricity generated by the ignition coil to generate a spark of sufficient strength to ignite the air/fuel mixture. Voltage generated by the spark plug can range from 40,000 to 100,000 volts.

Normal wear, as well as fouling, burning, or other premature damage, can affect the spark plug's ability to generate a sufficient spark. This can result in engine performance problems (such as "misfire), poor fuel consumption, and excessive vehicle emissions, and may result in damage to other engine components or systems.

When to Replace

The style of plug, type of ignition system and your personal driving habits can all affect the spark plug replacement interval. While some newer vehicles equipped with platinum-tipped spark plugs can go as far as 100,000 miles between replacements, a good “rule of thumb” for most vehicles is to replace at 30,000 miles. Check the owner’s manual or service manual for your vehicle for the manufacturer’s recommendations.

Additionally, trouble in other vehicle systems (defective intake valve stem seals, excessively rich air/fuel mixture, or a defective oxygen sensor) can result in “fouling”, which can cause misfire or other “ignition-related” symptoms. In these cases, a “quick check” of the spark plugs is an easy an inexpensive starting point.

Tools and Materials

BE SURE you have the necessary tools and materials “on hand” before you begin. Depending on your specific situation, you’ll need some or all of the following:

Replacement spark plugs
Spark plug wrench and spark plug socket (with extension, if necessary)
Spark plug gap gauge (a "tapered" gauge is preferable to the "wire" type)
Penetrating oil
Shop cloth
Compressed air

Removal, Inspection and Installation

BE SURE to note the routing of each plug wire before disconnecting, and BE SURE to re-route the wires accordingly when reconnecting. A misrouted plug wire can pick up interference from other vehicle electrical systems, and result in ignition problems.

NOTE: Remove, inspect and replace ONLY ONE spark plug at a time to avoid confusion when reconnecting plug wires. If you decide to disconnect ALL plug wires at the same time, use tape to label each wire with its proper location.

Removal

Unscrewing a nut to replace a car spark plug

Allow the engine to cool to ambient temperature. Working on a “hot” engine can make the plugs difficult to remove, and can result in serious burns.
Use a rag (and compressed air, if available) to clean grease and oil from the area around each spark plug. This will help prevent debris from falling into the cylinders when removing and replacing the spark plugs.
Grasp the plug wire at the base of the rubber boot and twist slightly while pulling to disconnect the plug wire from the spark plug.

NEVER pull directly on the plug wire. This can damage the plug wire and result in ignition problems down the road.
Use a ratchet and spark plug socket to unscrew and remove the spark plug.

DON’T force a “stuck” plug. If a plug is difficult to loosen, apply penetrating oil to the plug, wait a few minutes, for the oil to “do it’s stuff”, then try loosening again.
Inspect the cylinder chamber for dirt or debris, and use a rag to clean if necessary.

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Inspection

A quick inspection of your spark plugs can tell you a lot about how your engine is running. Check your plugs for the following CONDITIONS. If any of the SYMPTOMS are noted, you should investigate further, using the POSSIBLE CAUSES listed as a guide.

Condition	Symptom	Possible Cause
Turbulence Burning	Worn insulator on one side of the plug	● Normal turbulence in the combustion chamber ● Overheating (if plug also shows premature wear)
Overheating	White or gray blistering of the insulator., excessive gap wear.	● Improper spark plug (too "hot") ● Over-advanced ignition timing ● Detonation (spark knock) ● Cooling system malfunction ● Excessively lean air/fuel mixture ● Low-octane fuel ● Stuck (closed) heat-riser valve
Glazing	Shiny yellow glaze over the insulator. When sufficiently heated, the glaze acts as a conductor, allowing current flow through the glaze that "shorts out" the plug.	Frequent acceleration to "wide open throttle" following prolonged periods of "low-speed" operation.
Cold Fouling	Dry, fluffy, black carbon deposits on the tip of the plug.	● Excessively rich air/fuel mixture ● An ignition fault that causes the spark plug not to fire ● Sticking valves (when only one or two plugs show evidence of cold fouling) To prolong plug life, BE SURE to correct the cause of the problem before reinstalling or replacing the plugs.
Wet Fouling	Excessive oil contamination on the tip of the plug	● Worn valve guide or valve guide seals ● Defective transmission vacuum modulator (automatic transmissions only) ● Worn rings (high-mileage engines) ● Excessive cylinder wear (high-mileage engines) To prolong plug life, BE SURE to correct the cause of the problem before replacing the plugs.
Splash Fouling	Excessive carbon deposits on the tip of the plug	Generally occurs immediately following an overdue tune-up. Deposits accumulated in the combustion chamber (due to misfiring) loosen during high-speed driving, and stick to the hot insulator and electrode surfaces of the plug. In some cases, these deposits can "bridge" the gap between the electrodes and stop the plug from sparking.
Pre-ignition Damage	Melted electrodes or chipped electrode tips	Excessive engine temperature due to: ● Over-advanced ignition timing ● Burned head gasket ● Low octane fuel ● Loose plug ● Using plugs of the improper heat range To prolong plug life, BE SURE to correct the cause of the problem before replacing the plugs.

Reuse or Replace?

Plugs that show evidence of turbulence burning can be re-used, so long as there is no evidence of premature wear.
Plugs that show evidence of cold fouling or splash fouling can be used again, so long as no other damage is present. Clean the electrodes thoroughly and re-gap the plug before re-using.

NOTE: If you normally drive in "slow and go" conditions, and cold fouling is present, you may be able to prolong plug life by switching to "hotter" spark plugs.
DO NOT re-use plugs that show evidence of overheating, glazing, wet fouling, or pre-ignition damage.

Installation

NOTE: Refer to the owner’s manual or service manual for your vehicle for the proper spark plug gap specifications. BE SURE to gap each plug properly, both NEW plugs and any plugs you plan to REUSE.

Installing a new spark plugs into the engine

Use a spark plug gap gauge to gap the new plug. Use the tool to bend the side electrode as needed to obtain the proper gap. When the plug is gapped properly, you’ll feel a small amount of resistance as you pull the gap gauge between the electrodes.

NEVER tap the electrode on a bench or other surface to “close” the gap.
Clean the seat of the new plug. BE SURE the spark plug gasket (if applicable) is in place before installing. If you plan to reuse an OLD plug, BE SURE to install a NEW gasket.
Hand-thread the spark plug into the cylinder head just until snug. If you feel any resistance while tightening, remove the plug and check for dirt, debris or thread damage. Remove any dirt or debris with a rag.

DO NOT install the spark plug if thread damage is observed. Professional assistance may be needed to avoid further damage to the cylinder head.
Tighten the spark plug firmly, but do not OVER-TIGHTEN or damage to the cylinder head may occur. If you have a torque wrench on hand, refer to the owner’s manual or service manual for your vehicle for the proper torque specifications.
Hold the plug wire at the base of the rubber boot and twist slightly while pushing to connect the plug wire to the spark plug.