2014 GMC Sierra 2500HD Spark Plug Type, Gap & Torque

As an ASE-certified master technician, I’ve seen my fair share of heavy-duty trucks roll into the bay running rough, misfiring, or gulping fuel simply because they were overdue for a spark plug change. For the 2014 GMC Sierra 2500HD, spark plug maintenance is straightforward but absolutely critical for keeping that workhorse on the road.

Keep in mind that the 2014 Sierra 2500HD was offered with two main engines: the 6.6L Duramax diesel (which uses glow plugs, not spark plugs) and the 6.0L Vortec V8 gas engine. This guide focuses entirely on the 6.0L V8.

Quick reference

Why the right plug matters

Throwing the cheapest copper plugs into a modern heavy-duty truck is a recipe for disaster. The 6.0L Vortec was engineered specifically for Iridium spark plugs.

Iridium is one of the hardest metals on earth, allowing for a fine-wire center electrode that requires less voltage to jump the gap. This provides a highly concentrated spark, improving combustion efficiency and yielding a 100,000-mile service life. Standard copper or platinum plugs will erode much faster under the intense combustion pressures of a heavy-duty truck hauling a load. Furthermore, heat range is critical. If you use a plug that is too "hot," it can cause pre-ignition and catastrophic engine damage. Always stick to the OEM heat range specified by the ACDelco or NGK part numbers above.

How to gap a spark plug

While modern iridium spark plugs come pre-gapped from the factory at 0.040 inches, you should always verify the gap before installation. Shipping and handling can occasionally bump the ground strap, closing the gap.

To check the gap, use a wire-style gap gauge or a feeler gauge. Do not use a coin-style ramp gauge on iridium plugs. The fine-wire center electrode is fragile and can easily snap off if pried against. If the gap needs adjustment, use the bending tool on your gauge to apply a light bend to the ground strap only. Never apply pressure to the center electrode.

Replacement procedure tips

The 2014 GMC Sierra 2500HD 6.0L utilizes a Coil-Near-Plug setup, meaning each spark plug has its own ignition coil mounted on the valve cover, connected by a short spark plug wire.

1. Accessing the plugs: The driver's side is wide open, but the passenger side, specifically cylinder #8 (closest to the firewall), can be tight due to the HVAC box and dipstick tubes. A swivel socket and a few different extension lengths are your best friends here.
2. The anti-seize debate: The factory ACDelco plugs feature a special nickel-plated coating designed to prevent seizing in the aluminum cylinder heads. Do not use anti-seize. Adding liquid thread lubricant alters the friction coefficient, meaning your torque wrench will over-tighten the plug, potentially stripping the threads out of the expensive aluminum head.
3. Dielectric grease: Always apply a small dab of dielectric grease to the inside of the spark plug wire boots. This seals out moisture and prevents the rubber boot from vulcanizing to the ceramic shell of the spark plug, making your next tune-up much easier.

Known issues

While you won't face the infamous Ford 3-valve 2-piece plug breakage issue on this GM 6.0L, there are a couple of things to watch out for:

• Cracked Porcelain: Because of the tight clearance on the passenger side, DIYers often put a side-load on their socket while trying to loosen or tighten the plugs. This easily cracks the porcelain shell, leading to immediate misfires. Always ensure your socket is perfectly straight.
• Corroded Plug Wires: The heat shields around the spark plug boots can trap moisture and road salt. When removing the old wires, the metal terminals inside often rust and rip out, staying stuck on the old spark plug. It is highly recommended to replace the short spark plug wires at the same time you replace the plugs.

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