2015 GMC Sierra 1500: Spark Plug Type & Gap

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Quick Reference: 2015 Sierra 1500 Spark Plugs

Why The Right Plug Matters

Choosing the correct spark plug is critical for the performance, efficiency, and longevity of your Sierra's EcoTec3 engine. These modern, direct-injected engines are precisely calibrated, and the spark plug is a key component in a complex system. Here’s what matters:

• Electrode Material: The 2015 Sierra engines were specified with Iridium-tipped fine-wire spark plugs from the factory. Compared to older copper or even platinum plugs, iridium offers a much higher melting point and superior wear resistance. This allows for an extremely long service life—up to 100,000 miles—and maintains a precise, consistent spark for optimal combustion. Using a cheaper copper plug would drastically reduce the service interval and could compromise performance.
• Heat Range: A spark plug's heat range determines how quickly it dissipates heat from the firing tip. A plug that is too "hot" (dissipates heat too slowly) can lead to pre-ignition or detonation, which is damaging to the engine. A plug that is too "cold" (dissipates heat too quickly) can accumulate carbon deposits, leading to fouling and misfires. You should always use the exact heat range specified by the manufacturer.

How to Gap a Spark Plug

Properly gapping a spark plug ensures the spark has the perfect distance to jump, creating the most effective flame kernel to ignite the air-fuel mixture. For the specified ACDelco iridium plugs, this step requires finesse.

These plugs come pre-gapped from the factory to approximately 0.040 inches. You should always verify the gap with a wire-style or feeler gauge tool before installation, but be extremely gentle. The iridium fine-wire center electrode is brittle and can easily be broken.

If an adjustment is needed, never pry against the center electrode. Instead, use the appropriate slot on a gapping tool to gently bend the ground strap (the J-shaped electrode) toward or away from the center electrode until the gauge indicates the correct 0.040" gap. A light touch is all that is needed.

Replacement Procedure Tips

The 2015 GMC Sierra utilizes a modern coil-on-plug (COP) ignition system, which simplifies the replacement process as there are no distributor caps, rotors, or spark plug wires to manage.

1. Coil-on-Plug Removal: Each spark plug has its own ignition coil mounted directly on top of it. To access the plug, you first unplug the electrical connector by depressing its locking tab. Then, remove the single 10mm bolt securing the coil to the valve cover. The coil can then be gently twisted and pulled straight up to remove it.

2. The Anti-Seize Debate: Many technicians traditionally apply anti-seize compound to the threads of spark plugs to prevent them from seizing in the aluminum cylinder heads. However, modern plug manufacturers, including NGK and ACDelco, often advise against this. The specified plugs feature a special trivalent or nickel plating that acts as its own anti-seize and release agent. Applying additional lubricant can alter the torque reading, causing you to inadvertently over-tighten the plug, which can stretch the plug’s shell or damage the threads in the cylinder head. The best practice is to install these specific plugs clean and dry.

3. Dielectric Grease: While you should skip anti-seize on the threads, applying a small amount of dielectric grease to the inside of the spark plug boot before reinstalling the ignition coil is highly recommended. This grease prevents the boot from sticking to the plug's porcelain, seals out moisture, and prevents voltage leaks that can cause misfires.

Known Issues

While some engine families are infamous for difficult spark plug jobs (like the Ford 3-valve Triton engines and their two-piece plugs), the GM EcoTec3 family of engines in the 2015 Sierra is not known for any widespread, chronic issues regarding spark plug replacement. The procedure is straightforward.

The most important considerations are to work on a cool engine, blow out any debris from the spark plug wells before removing the old plugs, and use a quality torque wrench. Over-torquing is the biggest risk, potentially damaging the plug or the cylinder head. Always use a proper spark plug socket with a rubber insert to protect the porcelain insulator and to help retrieve the old plug from the well.

Need plugs replaced? Find a shop on Repairius. ""