2018 Peterbilt 389 Spark Plug Type & Gap

Quick reference

Why the right plug matters

While the 2018 Peterbilt 389 is primarily equipped with diesel engines like the PACCAR MX-13 and Cummins X15, which do not utilize spark plugs, understanding why the right spark plug is crucial for gasoline engines is valuable knowledge for any technician.

For gasoline internal combustion engines, spark plugs are vital for igniting the air/fuel mixture. The "right" plug ensures optimal combustion, leading to efficient fuel economy, consistent power delivery, and reduced emissions. Key factors include the heat range and electrode materials.

Heat Range

The heat range refers to the spark plug's ability to dissipate heat from the combustion chamber. A plug that is too "hot" (dissipates heat slowly) can lead to pre-ignition and engine knocking, potentially causing severe engine damage. Conversely, a plug that is too "cold" (dissipates heat quickly) can foul with carbon deposits, leading to misfires and poor performance. The correct heat range is determined by the engine manufacturer based on engine design, compression ratio, and intended operating conditions.

Electrode Metal (Iridium vs. Platinum vs. Copper)

• Copper: These are the most basic and economical plugs. Copper has excellent conductivity but wears relatively quickly, requiring more frequent replacement intervals. They often feature a large copper core electrode.

• Platinum: Platinum plugs offer improved durability and a finer electrode compared to copper, leading to a more consistent spark and longer service life. They are a common upgrade from copper plugs due to their enhanced performance and extended replacement intervals.

• Iridium: Iridium is the hardest and most durable of these metals, allowing for extremely fine electrodes. This results in a very strong, focused spark and the longest service life of any spark plug type. Iridium plugs are standard in many modern engines, offering superior performance and the longest replacement intervals.

How to gap a spark plug

Again, while not applicable to your diesel-powered 2018 Peterbilt 389, knowing how to properly gap a spark plug is fundamental for gasoline engines. The gap is the distance between the center and ground electrodes. An incorrect gap can lead to misfires, reduced fuel economy, and poor engine performance.

1. Obtain the correct gap specification: Always refer to the engine manufacturer's specifications (or the spark plug manufacturer's recommendation for a specific engine) for the precise gap setting.

2. Use a gapping tool: A wire-type feeler gauge is the most accurate tool for gapping spark plugs, especially for those with fine-wire electrodes (platinum and iridium). Avoid using coin-style gapping tools on fine-wire plugs, as they can damage the delicate electrodes.

3. Insert the gauge: Carefully slide the correct thickness gauge wire between the center and ground electrodes. It should have a slight drag.

4. Adjust if necessary: If the gap is too small, use the gapping tool's adjustment tab to gently bend the ground electrode outward. If the gap is too large, gently tap the ground electrode on a hard, non-abrasive surface to close the gap. Never bend or pry on the center electrode.

5. Re-check the gap: Always re-check the gap after any adjustment to ensure it meets specifications.

Replacement procedure tips

For a 2018 Peterbilt 389 with its diesel engines, spark plug replacement procedures are not applicable. However, for technicians working on gasoline engines, here are some general tips:

• Coil-on-plug removal: Most modern gasoline engines use a coil-on-plug (COP) ignition system. To remove the spark plug, you'll first need to carefully disconnect the electrical connector from the ignition coil and then unbolt and remove the coil itself. Be gentle to avoid damaging the coil or wiring.

• Anti-seize debate: There's ongoing debate about using anti-seize compound on spark plug threads. Some spark plug manufacturers advise against it, stating that it can alter torque readings and potentially lead to over-tightening or issues with the plug's electrical grounding. Always follow the spark plug manufacturer's recommendations. If you do use it, apply a very thin coat and reduce the specified torque by roughly 10-20%.

• Dielectric grease on boots: Applying a small amount of dielectric grease inside the spark plug boot (the part that connects to the top of the spark plug) can help prevent sticking, improve electrical insulation, and make future removal easier. Be careful not to get grease on the electrode tips.

Known issues

Given that the 2018 Peterbilt 389 is equipped with diesel engines, common gasoline engine spark plug issues such as electrode wear, fouling, or specific design flaws (like the Ford 3-valve 2-piece spark plug breakage or GM 5.3 ionization-sensing coils) are not applicable.

Instead, focus on diesel-specific components like fuel injectors, glow plugs (if equipped), and emissions systems, as these are the relevant areas for maintenance and potential issues on a diesel Peterbilt 389.

Need plugs replaced? Find a shop on Repairius.