2019 Volvo VNL Spark Plug Type & Gap Specs

As an ASE-certified master technician, I get this question occasionally, and it almost always comes with a catch. The 2019 Volvo VNL is a heavy-duty Class 8 truck. The vast majority of these haulers are powered by diesel engines—specifically the Volvo D13 or Cummins X15—which use high compression rather than spark plugs to ignite the fuel.

However, if your fleet is running on alternative fuels, your 2019 Volvo VNL might be equipped with the Cummins ISX12N (Natural Gas) engine. Unlike its diesel brethren, this CNG/LNG powerplant relies on a spark-ignited combustion system. Natural gas burns incredibly hot and dry, making spark plug maintenance a highly critical part of keeping your rig out of the shop and on the highway.

Here are the precise spark plug specifications for the CNG-powered 2019 Volvo VNL.

Quick reference

Note: For diesel D13 and X15 engines, there are no spark plugs. Diagnostic issues related to misfires on these engines will point toward fuel injectors or compression.

Why the right plug matters

When dealing with a natural gas heavy-duty engine, you cannot simply grab a standard copper core plug. CNG engines operate with significantly higher combustion temperatures than gasoline engines. Because of this, the heat range and electrode metal of your spark plugs are paramount.

The Cummins ISX12N requires premium iridium spark plugs. Iridium is one of the hardest metals on earth with an extremely high melting point. While platinum is great for passenger cars, iridium is mandatory to withstand the harsh, dry environment of a heavy-duty natural gas combustion chamber. Using the wrong heat range or a lesser metal like copper will lead to rapid electrode erosion, severe misfires, and potentially catastrophic engine damage due to pre-ignition.

How to gap a spark plug

Even if your premium NGK iridium plugs come pre-gapped in the box, shipping and handling can bump the ground strap out of spec.

1. Use the right gauge tool: Avoid cheap coin-style gauges that can chip the brittle iridium center electrode. Use a precision wire feeler gauge.
2. Check the gap: Slide the 0.015 in (0.38 mm) wire between the center and ground electrode. It should pass through with a slight "drag."
3. Adjusting the gap: If the gap is too tight or too loose, use a dedicated spark plug gapping tool to gently bend the ground strap. Never pry against the center iridium tip—it will snap. Apply a light bend to the strap, re-check, and repeat until the drag is perfect.

Replacement procedure tips

Replacing plugs on the Cummins ISX12N requires careful attention to detail. This engine uses a coil-on-plug extension boot setup that is subjected to extreme under-hood temperatures.

• Coil and boot inspection: When removing the coil boots, twist them gently before pulling to break the heat seal. Inspect the long extension boots for carbon tracking (faint lightning-bolt shaped gray lines). If you see tracking, replace the boot, or the new spark plug will misfire immediately.
• Dielectric grease: Apply a small bead of dielectric grease to the inside of the spark plug boot before reinstallation. This prevents moisture intrusion, stops high-voltage leaks, and makes the next removal much easier.
• The anti-seize debate: Cummins generally advises against using anti-seize on spark plug threads unless explicitly stated, as the NGK plugs feature a special trivalent metal plating that prevents galling. Using anti-seize can act as a lubricant, leading you to accidentally over-torque the plug and damage the cylinder head. Install them completely dry and torque precisely to 28 lb-ft.

Known issues

Unlike the notorious Ford 3-valve engines known for two-piece spark plugs breaking off in the cylinder head, the Cummins ISX12N does not suffer from snapping plug shells. However, there is a known issue regarding accelerated plug wear and ignition coil degradation.

Because CNG burns so dry, the electrical resistance in the cylinder is very high. This puts immense strain on the ignition coils. If you stretch your spark plug intervals past the recommended 1,500 hours or 45,000 miles, the gap will widen significantly. The ignition coil will then have to generate a much hotter spark to jump that widened gap, which causes the coil to overheat and burn out prematurely. Sticking strictly to the OEM replacement interval will save you from buying expensive heavy-duty ignition coils down the road.

Need plugs replaced? Find a shop on Repairius.