2025 Volvo VNL Spark Plug Type & Gap Specs

As an ASE-certified master technician, I often get double-takes when someone asks about spark plugs for a heavy-duty Class 8 truck. Most 2025 Volvo VNLs are powered by the mighty Volvo D13 diesel engine, which is a compression-ignition powerplant—meaning it doesn’t have spark plugs at all.

However, with the industry's push toward alternative fuels, the 2025 Volvo VNL is available with the Cummins X15N natural gas (CNG/LNG) engine. Unlike diesel, natural gas requires a spark-ignition system to ignite the fuel-air mixture. If you are operating a CNG-powered VNL, spark plug maintenance is one of the most critical aspects of keeping your rig on the road and avoiding costly misfires under load.

Here is your comprehensive guide to spark plugs for the CNG-powered 2025 Volvo VNL.

Quick reference

(Note: Always verify the exact Cummins part number with your engine serial number, as natural gas engine plug revisions happen frequently).

Why the right plug matters

When dealing with a 15-liter natural gas engine hauling 80,000 pounds, the combustion chamber environment is incredibly harsh. Natural gas has a higher ignition temperature than gasoline, and it burns extremely hot and dry.

Because of this, you cannot use standard copper or platinum plugs. The OEM specifies heavy-duty iridium spark plugs. Iridium is one of the hardest metals on earth and can withstand the extreme heat and electrical demands of a heavy-duty CNG engine without the electrodes melting or eroding prematurely. Using the wrong heat range or electrode material will lead to pre-ignition (detonation), which can rapidly destroy a piston in an engine of this size.

How to gap a spark plug

Gapping a spark plug for a heavy-duty natural gas engine is a delicate process, despite the massive size of the engine itself. The gap is incredibly tight—usually around 0.015 inches (0.38 mm) to ensure the spark can jump the gap under immense cylinder pressures.

1. Use the right tool: Do not use a cheap coin-style gapper. Use a precision wire feeler gauge.
2. Never pry on the center electrode: The iridium tip is brittle and will snap if you apply pressure to it.
3. Bend the ground strap: Use a specialized bending tool to gently manipulate the ground strap until the wire gauge slides through with just a slight "drag."

Replacement procedure tips

Working on a heavy-duty coil-on-plug (COP) setup like the X15N requires a methodical approach:

• Clean the plug well: Before removing the ignition coils, use compressed air to blow out the deep spark plug wells. Debris falling into a 15-liter engine cylinder will cause catastrophic scoring.
• The anti-seize debate: Cummins generally advises against using anti-seize on the threads, as the plugs have a special anti-corrosive plating. Adding anti-seize acts as a lubricant, which can cause you to over-torque the plug and damage the cylinder head.
• Dielectric grease: Always apply a small bead of dielectric grease to the inside of the ignition coil boot. This prevents the high voltage from arcing to the cylinder head and keeps the boot from baking onto the plug's porcelain insulator.

Known issues

Heavy-duty natural gas engines are notorious for being incredibly hard on spark plugs. While you won't see the famous Ford 3-valve two-piece plug breakage here, you will face other challenges:

• Rapid Gap Growth: Because CNG burns so dry, electrode erosion happens much faster than in passenger vehicles. Even with iridium, you must stick to the strict 1,500-hour / 50,000-mile replacement intervals. Ignoring this will lead to widened gaps, resulting in coil failure and cylinder misfires.
• Ash Deposits: Engine oil consumption (which is normal in heavy-duty engines) can leave ash deposits on the spark plug insulator. In a CNG engine, these deposits can create a conductive path, causing the spark to short out down the side of the plug instead of jumping the gap.

Need plugs replaced? Find a shop on Repairius.