2015 Peterbilt 389 Spark Plug Type & Gap

Quick reference

Why the right plug matters

For the 2015 Peterbilt 389, spark plugs are not applicable as this model is exclusively equipped with diesel engines. Diesel engines operate on a compression-ignition principle, meaning they ignite fuel solely through the heat generated by compressing air, unlike gasoline engines which rely on a spark from a spark plug.

While the information below is crucial for gasoline engines, it does not apply to your 2015 Peterbilt 389:

Spark plugs are vital components in gasoline engines, responsible for igniting the air-fuel mixture in the combustion chamber. Using the correct spark plug for your engine is paramount for optimal performance, fuel efficiency, and engine longevity. Factors like heat range, electrode material, and proper gapping all play significant roles.

Heat Range

The "heat range" of a spark plug refers to its ability to dissipate heat from the combustion chamber. A "hot" plug insulates heat, keeping its tip hotter, which helps burn off carbon deposits in engines that run hotter or at lower speeds. A "cold" plug dissipates heat more quickly to the cylinder head, ideal for high-performance or high-compression engines that run at higher temperatures and RPMs. An incorrect heat range can lead to pre-ignition (too hot) or fouling (too cold).

Electrode Metal: Iridium vs. Platinum vs. Copper

• Copper: These are the most basic and least expensive. Copper is an excellent electrical conductor, but its softness means the electrodes wear down more quickly, requiring more frequent replacement. They are often found in older vehicles or those with lower performance demands.

• Platinum: Platinum plugs offer better durability and a longer lifespan than copper. The platinum often coats the center electrode, providing a sharper, more consistent spark over time. They are a good balance of performance and longevity.

• Iridium: Considered the best in terms of performance and longevity, iridium is even harder and more durable than platinum. Iridium plugs typically have a very fine-wire center electrode, which concentrates the spark and reduces voltage requirements, leading to more efficient combustion and extended replacement intervals.

How to gap a spark plug

While not applicable to your diesel Peterbilt 389, here's how to gap a spark plug for gasoline engines in general:

Proper spark plug gap is crucial for efficient combustion. The gap is the distance between the center and ground electrodes. If the gap is too small, the spark may be weak; if it's too large, the ignition system may not be able to produce a consistent spark, leading to misfires.

1. Obtain a gap gauge: Use a wire-type or coin-style gap tool. Avoid using a feeler gauge, as it can damage fine-wire electrodes.
2. Measure the gap: Carefully slide the appropriate wire gauge (or the tapered edge of a coin gauge) between the electrodes. It should slide with slight resistance.
3. Adjust if necessary: If the gap is too small, gently bend the ground electrode outwards with the tool. If the gap is too large, gently tap the ground electrode on a hard surface to close it slightly. Never bend the center electrode.
4. Re-measure: Always re-measure after adjustment to ensure accuracy.

Replacement procedure tips

This section is not applicable to your 2015 Peterbilt 389 as it is a diesel vehicle and does not use spark plugs.

For gasoline engines, here are some general tips:

• Coil-on-plug removal: Many modern engines use a coil-on-plug (COP) design. To remove them, disconnect the electrical connector, then gently twist and pull upwards on the coil to free it from the spark plug well. Avoid pulling on the wires.
• Anti-seize debate: There's ongoing debate about whether to use anti-seize on spark plug threads. Most modern spark plugs use a special coating on the threads, and manufacturers often advise against using anti-seize, as it can alter the torque readings and potentially lead to over-tightening. Always check the plug manufacturer's recommendations.
• Dielectric grease on boots: Apply a small amount of dielectric grease inside the spark plug boot (the part that connects to the coil or wire). This helps seal out moisture, prevents corrosion, and makes future removal easier. Do not apply it to the tip of the spark plug.
• Torque specs: Always use a torque wrench to tighten spark plugs to the manufacturer's specified torque. Over-tightening can damage the spark plug and cylinder head threads, while under-tightening can lead to blow-outs and combustion leaks.

Known issues

As the 2015 Peterbilt 389 uses diesel engines, there are no "spark plug related" known issues like those found in gasoline engines. Common issues in gasoline engines, such as cracked porcelain on Ford 3-valve engines or broken plug shells, are not relevant to this vehicle.

Need plugs replaced? Find a shop on Repairius.