The 6.4L Hemi has 16 spark plugs — two per cylinder — and the factory interval runs out around 96,000 miles, though many owners running copper-tip plugs change them sooner. It is a straightforward job made fiddly by tight access to the rear plugs and the two-coil-per-bank ignition layout. Budget $60-$200 depending on whether any coils need replacing. The two things that bite people: cross-threading a plug into a hot aluminum head, and losing track of which of the 16 plugs you have done.
The Hemi's twin-plug design fires two plugs per cylinder for cleaner, more complete combustion and lower emissions. It also means twice the parts count and a bit more labor than a typical V8. Fresh plugs cure the most common 6.4L misfire codes and restore the smooth idle and throttle response that worn plugs erode. This is a high-value tune-up on a high-mileage truck.
The published service interval lands near 96,000 miles, but plug life depends on the tip material. Factory-style copper-core plugs wear faster and some owners replace them in the 30,000-50,000 mile range, especially if they tow or run the engine hard; longer-life platinum/iridium plugs hold the gap closer to the full interval. Use the OEM-specified plug and heat range, and confirm the gap against the placard — pre-gapped plugs can shift in shipping. Do not over-gap a Hemi; the coils are happiest at the factory gap.
Work on a cold engine to avoid galling the aluminum threads. Blow out each plug well with compressed air so debris does not fall into the cylinder when the plug comes out. Disconnect and remove the coil over each plug (label or work one at a time so you keep the firing components organized — there are eight coils feeding 16 plugs). Use a swivel and extensions to reach the rear cylinders, which are the tight ones against the firewall. Thread each new plug by hand first to avoid cross-threading, then torque to the service-manual value — a torque wrench matters here because an over-torqued plug damages the head and an under-torqued one blows out. Apply a thin film of anti-seize sparingly (or follow the plug maker's guidance) and dielectric grease on the coil boots.
Reconnect the coils, start the truck, and confirm a smooth idle with no misfire. If a misfire code (P0300 or a specific cylinder) appears or persists, the cause may be a tired coil rather than the plug — swap the suspect coil to a neighboring cylinder and re-scan to see if the misfire follows it. Pair this job with the OBD-II guide so you can read and clear codes as you verify.
Twin plugs mean eight coil packs feeding 16 plugs, and a rough-running 6.4L is as often a tired coil as a worn plug. Before condemning anything, pull the freshest codes: a P0301-P0308 names the exact misfiring cylinder, while a bare P0300 is a random/multiple misfire that points more at fuel, vacuum, or a batch of worn plugs. The field test is the swap — move the suspect coil to a neighboring cylinder, clear the code, and drive it: if the misfire follows the coil, replace the coil; if it stays put, the plug or that cylinder is the problem. A coil boot that is oil-soaked or arc-tracked (gray carbon lines up the side) will misfire under load even with a fresh plug, so inspect every boot and re-grease with dielectric. Read and clear codes with the OBD-II diagnostic guide.
Aluminum heads are unforgiving, so the numbers matter. Gap the 6.4L plugs to the placard spec (near 0.043 in) and verify each one — pre-gapped plugs drift in shipping and a dropped plug is a bent electrode. Torque to the service-manual value, roughly 13 ft-lb (about 156 in-lb) on a warm-to-cold head; a torque wrench is not optional here because an over-torqued plug cracks the porcelain or damages the threads and an under-torqued one can blow out under compression. On anti-seize, the plug makers are split: NGK ships plated shells and says none is needed, while many techs run a very thin film on the threads only — never on the electrode. If you use it, back the torque off about 10-15% because lubricated threads read higher clamp load at the same wrench value. Pair the tune-up with fresh oil from the oil-change guide so the whole ignition-and-lubrication side is fresh at once.
| Part | Vendor | Est. price |
|---|---|---|
| Spark plugs — set of 16 (OEM heat range, gapped to spec) | Mopar / NGK / Champion | ~$80 |
| Coil-on-plug (replace only if a coil tests bad) | Mopar / NGK | ~$45 |
| Dielectric grease + anti-seize | Permatex | ~$10 |
Written and maintained by an AZ wheeler and driveway wrencher. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.
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