2023 GMC Sierra 2500HD Spark Plug Type & Gap

Quick reference

Why the right plug matters

Choosing the correct spark plug for your 2023 GMC Sierra 2500HD is crucial for optimal engine performance, fuel efficiency, and longevity. The spark plug's primary job is to ignite the air-fuel mixture in the combustion chamber, and several factors determine its effectiveness:

• Heat Range: This refers to the plug's ability to dissipate heat. A "hot" plug retains more heat, while a "cold" plug dissipates it quickly. An incorrect heat range can lead to pre-ignition (too hot) or fouling (too cold). For the 6.6L L8T engine, the specified heat range is carefully selected for its operating conditions.
• Electrode Metal:
  • Iridium: These are standard for your Sierra 2500HD's 6.6L L8T engine. Iridium is incredibly durable, allowing for a finer wire center electrode that produces a stronger spark. This leads to longer service intervals and improved ignition performance.
  • Platinum: While not OEM for your vehicle, platinum plugs offer good durability but typically do not match the lifespan or spark quality of iridium.
  • Copper: These plugs have excellent conductivity but are much softer, leading to faster wear and shorter replacement intervals. They are generally not recommended for modern engines like the L8T.

Using the recommended iridium spark plugs ensures your engine runs as the manufacturer intended, providing consistent performance and meeting emissions standards.

How to gap a spark plug

While most iridium spark plugs come pre-gapped from the factory, it's always a good idea to double-check and adjust if necessary, especially when using aftermarket plugs. The specified gap for your 2023 GMC Sierra 2500HD 6.6L L8T is 0.040 inches (1.0 mm).

1. Obtain a Gapping Tool: Use a wire-type spark plug gap gauge. Avoid coin-style gauges on iridium plugs, as they can damage the fine center electrode.
2. Measure the Gap: Carefully slide the appropriate wire gauge between the center and ground electrodes. There should be a slight drag.
3. Adjust (if needed): If the gap is too small, gently pry the ground electrode slightly away from the center electrode. If the gap is too large, gently tap the ground electrode on a hard, non-abrasive surface to close it. Be extremely careful not to touch or damage the iridium tip.

Replacement procedure tips

Replacing spark plugs on your 2023 GMC Sierra 2500HD 6.6L L8T engine requires a methodical approach. Always ensure the engine is cool before starting any work.

1. Coil-on-Plug Removal: Your Sierra uses a coil-on-plug (COP) ignition system. Each spark plug has its own ignition coil directly mounted on top. To access the spark plugs, you'll need to:
   • Disconnect the electrical connector from each coil.
   • Remove the bolt securing the coil to the valve cover (typically 7mm or 10mm).
   • Gently pull the coil and boot assembly straight up to remove it. Inspect the coil boots for cracks or tears.
2. Anti-Seize Debate: There is ongoing debate regarding the use of anti-seize on spark plug threads. Most modern spark plugs, especially those with nickel-plated shells (like the OEM ACDelco plugs for your Sierra), do not require anti-seize. In fact, applying it can alter the torque specification, leading to overtightening and potential damage to the cylinder head threads or the spark plug itself. Follow the manufacturer's recommendation, which for the L8T engine usually means installing dry.
3. Dielectric Grease on Boots: A small amount of dielectric grease applied inside the spark plug boot (where it contacts the ceramic insulator of the spark plug) can aid in future removal and prevent moisture intrusion. Do not apply it to the electrical connector pins.
4. Torque Specification: Tightening spark plugs to the correct torque (11 lb-ft / 15 Nm for the 6.6L L8T) is critical. Overtightening can strip cylinder head threads or crack the spark plug's ceramic insulator, while undertightening can lead to combustion gas leaks and misfires. Use a good quality torque wrench.

Known issues

Fortunately, the 2023 GMC Sierra 2500HD with the 6.6L L8T engine does not have widespread, commonly known spark plug-related issues akin to the Ford 3-valve 2-piece plug breakage or specific GM ionization-sensing coil problems. The L8T is a relatively robust engine, and sticking to the recommended maintenance schedule and genuine OEM or equivalent aftermarket parts should ensure hassle-free operation.

Need plugs replaced? Find a shop on Repairius.