2022 Freightliner M2 106 Spark Plug Type & Gap Guide

As an ASE-certified master technician, I often get questions about medium-duty truck maintenance. When it comes to the 2022 Freightliner M2 106, there is a crucial distinction to make right out of the gate: the vast majority of these trucks are equipped with diesel engines (like the Cummins B6.7, L9, or Detroit DD5/DD8) which do not have spark plugs. However, Freightliner offers this chassis with the PSI (Power Solutions International) 8.8L V8 gasoline and LPG (propane) engine options.

If you are running the PSI 8.8L gas or propane engine in your M2 106, this guide is for you. Here is everything you need to know about spark plug specifications, gaps, and replacement procedures for this heavy-duty V8.

Quick reference

(Note: Always verify your specific engine serial number with a Freightliner or PSI dealer, as industrial engine specifications can vary based on the fuel type—gasoline vs. liquid propane.)

Why the right plug matters

In a heavy-duty application like a Freightliner chassis moving thousands of pounds, engine load is constantly high. This makes the heat range of your spark plug critical. The heat range determines how fast the spark plug pulls heat away from its tip and transfers it to the cylinder head. If you use a plug that is too hot, you risk pre-ignition and catastrophic piston damage. If it's too cold, the plug will foul with carbon deposits.

Electrode material is equally important. The PSI 8.8L operates under extreme conditions, especially when fueled by LPG, which burns hotter and drier than gasoline.

• Iridium and Platinum plugs are highly recommended here. They withstand the high heat of propane combustion without the rapid electrode degradation seen in standard Copper plugs.

How to gap a spark plug

Even if you buy "pre-gapped" iridium plugs, they can be bumped during shipping. Always check the gap before installation.

1. Use the right tool: Avoid cheap coin-style gappers, which can snap the fragile iridium center electrode. Use a wire-type gauge or feeler gauge.
2. Check the gap: Gently slide the 0.035" (0.89mm) wire between the ground strap and the center electrode. It should slide through with a slight "drag."
3. Adjusting (if necessary): If the gap is too tight, use the bending tool on your gauge to lightly pry the ground strap up. Never pry against the delicate center electrode!

Replacement procedure tips

The PSI 8.8L utilizes a modern Coil-On-Plug (COP) ignition system. When servicing this engine in the M2 106 chassis, accessibility can be tight near the firewall (doghouse area).

• Coil-on-plug removal: Twist the coil boot slightly before pulling straight up. This breaks the seal and prevents the rubber boot from tearing and remaining stuck on the old spark plug.
• The anti-seize debate: Modern NGK and ACDelco plugs feature a trivalent metal plating that prevents seizing in the aluminum cylinder heads. Do not use anti-seize on these plugs; doing so lubricates the threads and can lead to over-torquing, which stretches the plug body and damages the head.
• Dielectric grease: Apply a small pea-sized dab of dielectric grease to the inside of the new coil boots. This prevents moisture intrusion (vital for commercial trucks) and makes future removal much easier.

Known issues

While the PSI 8.8L is a robust, heavy-duty engine, you should be aware of a few ignition-related quirks:

• High-heat boot degradation: Because the M2 106 engine bay is tightly packaged and the V8 generates massive heat under load, the coil boots nearest the firewall are prone to drying out and cracking. Inspect them closely during plug changes; if they look brittle, replace the coil boots to prevent misfires under load.
• LPG spark blowout: Propane engines require a stronger spark to ignite the air/fuel mixture. If you attempt to widen the spark plug gap beyond the 0.035" specification in an attempt to get a "better burn," you will likely experience spark blowout under heavy acceleration. Stick to the factory spec!

Need plugs replaced? Find a shop on Repairius.