2017 Peterbilt 389 Spark Plug Type & Gap Guide

As an ASE-certified master technician, I often get questions about heavy-duty truck maintenance. When discussing a Class 8 behemoth like the 2017 Peterbilt 389, the first thing we have to clarify is the engine type. Most 2017 Peterbilt 389s are equipped with diesel engines (like the Cummins ISX15, Cummins X15, or PACCAR MX-13), which use compression ignition and do not have spark plugs.

However, Peterbilt also offered natural gas (CNG/LNG) engine options through Cummins Westport, specifically the Cummins ISX12 G. Unlike diesel engines, these natural gas engines rely on spark-ignited combustion and absolutely require spark plugs. If you are running a CNG/LNG 2017 Peterbilt 389, this guide will cover the exact spark plug specs, gaps, and torque requirements for your rig.

Quick reference

Note: Natural gas spark plugs have much shorter maintenance intervals than modern passenger cars due to the demanding nature of CNG combustion.

Why the right plug matters

For the Cummins ISX12 G natural gas engine, using the correct spark plug is non-negotiable. Natural gas requires a significantly higher ignition voltage to create a spark than gasoline. Because of this, Cummins/Westport strictly specifies NGK Iridium spark plugs.

Iridium is one of the hardest metals on earth, which allows the electrode to be machined to a very fine wire. This concentrates the electrical charge, reducing the voltage required to jump the gap while resisting the extreme combustion chamber temperatures of a heavy-duty CNG engine. Using standard copper or even platinum plugs in this application will result in rapid electrode erosion, misfires, and catastrophic power loss. Additionally, running the correct heat range prevents pre-ignition, which can severely damage commercial engines.

How to gap a spark plug

Even though premium iridium plugs for the ISX12 G often come pre-gapped, shipping and handling can bump the electrodes. Always verify the gap is exactly 0.015 inches (0.38 mm) before installation.

1. Use the right tool: Do not use a cheap coin-style gauge on fine-wire iridium plugs, as sliding it can easily snap the brittle iridium tip. Use a wire-style feeler gauge.
2. Measure gently: Pass the 0.015" 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 wide, use a dedicated spark plug gapping tool to bend the ground strap. Never pry against the delicate iridium center electrode. Lightly bend the ground strap, recheck with your wire gauge, and repeat until perfect.

Replacement procedure tips

Replacing plugs on the 12-liter Cummins ISX12 G involves a few heavy-duty specific quirks compared to a standard passenger car:

• Coil-on-Plug (COP) Extensions: This engine uses coil-on-plug ignition with long extension boots that reach down into the deep valve cover wells. Always inspect these boots for cracking, carbon tracking, or oil saturation. A compromised boot will allow the high voltage to arc to the cylinder head instead of the plug.
• Dielectric Grease: Apply a small dab of dielectric grease to the inside tip of the coil boot. This prevents moisture intrusion, stops high-voltage arcing, and ensures the boot won't bake onto the ceramic porcelain of the spark plug.
• The Anti-Seize Debate: Cummins generally recommends installing spark plugs dry into the cylinder head, relying on the crush washer to seal and secure the plug. If you must use anti-seize due to fleet practices, use it extremely sparingly on the upper threads only, and reduce your torque value by about 10-15% to prevent over-tightening and stripping the cylinder head threads.

Known issues

The biggest "known issue" with CNG spark plugs in heavy-duty trucks is simply the interval. Fleet operators used to million-mile diesel rebuilds are often caught off guard by the 45,000-mile (or 1,000 to 1,500-hour) spark plug replacement interval on the ISX12 G.

Pushing the plugs past this interval leads to widened gaps from electrode wear. When the gap widens, the ignition coils have to work much harder to push voltage across the gap. This elevated resistance will prematurely burn out the ignition coils, turning a standard spark plug service into a highly expensive coil replacement job.

Need plugs replaced? Find a shop on Repairius.