The 6.4L Power Wagon takes roughly 12 quarts of Mopar purple OAT coolant (MS.90032), and the system is good for about 10 years or 150,000 miles from the factory fill. A drain-and-fill runs you $60 in fluid; the parts that actually fail on this truck are the plastic degas bottle and the radiator end-tanks, not the coolant itself. Do a full service before a hot-climate trail trip, because a Power Wagon at 6,900 lb crawling in low range builds heat fast.
The 6.4L Hemi uses a mechanical fan clutch and a single crossflow radiator with plastic end-tanks. It is a robust system, but heavy work — towing, sustained low-range crawling, or desert running in summer — pushes it harder than the duty cycle a half-ton sees. Knowing the real capacity and coolant spec keeps you from the two mistakes that cause trouble here: topping off with the wrong (orange HOAT) coolant, and chasing a "phantom" overheat that is really a brittle degas bottle leaking under pressure.
Use Mopar OAT coolant, purple, meeting MS.90032. Do not mix it with the older orange HOAT (MS-9769) that earlier Hemi trucks used — mixing organic-acid and hybrid coolants drops the corrosion protection and can gel. Total system capacity on the 6.4L is about 12 quarts; a basic radiator drain-and-fill recovers roughly 6-7 quarts, so a true flush takes two or three fill-burp cycles to fully exchange the fluid. Buy premixed 50/50 or cut concentrate with distilled water — never tap water.
Let the engine sit overnight so it is stone cold. Open the radiator cap (on the degas bottle), then open the lower drain or pull the lower radiator hose into your pan. Once drained, reconnect, fill the degas bottle to the cold line, and run the engine with the heater on full hot. The thermostat opens around operating temperature and the level will drop — keep topping the bottle. The 6.4L is prone to trapping air, so watch for the level to stabilize and the heater to blow hot before you call it done. A spill-free funnel makes the burp far cleaner.
The degas (overflow) bottle goes brittle and cracks at the seams, usually around 80-120k miles; a hairline crack vents pressure and mimics an overheat. Replace it on sight if it is yellowed or weeping. Check the radiator end-tanks where the plastic crimps to the aluminum core — seepage there means the radiator is on its way out. Squeeze the upper and lower hoses cold; if they feel rock-hard or mushy, replace the set. Finally, verify the fan clutch engages — a lazy clutch is a common cause of overheating only at low speed or while towing.
For the broader fluid-service picture on this truck — engine oil, axle, transfer case, and transmission — pair this with the maintenance schedule before a big trip.
Before you tear into the cooling system, confirm the gauge is telling the truth. A cracked degas bottle or a failing cap vents pressure and drops the boiling point, so the truck runs hot only under load — pressure-test the cap and bottle to about 16 psi and watch for a steady hold. A genuine overheat climbs steadily and does not recover with airflow: suspect a thermostat stuck closed, a lazy fan clutch, or an airbound system from a sloppy burp. An infrared thermometer swept across the radiator face shows cold spots where the core is clogged. Pull codes with the OBD-II guide — a P0128 (coolant below regulating temp) or P0217 (engine over-temp) tells you which way the fault runs before you spend a dollar.
Stock cooling handles stock duty, but a heavily loaded truck crawling in summer benefits from a few targeted changes. A severe-duty fan clutch engages sooner and moves more air at low speed — exactly when a crawling Power Wagon struggles and highway airflow is gone. Extending the axle vent tubes does not cool the engine, but it keeps water out of the diffs on the same trips that stress cooling, so do both before a desert or mud run. For trucks that tow near the 10,000-lb-plus range, an auxiliary transmission cooler offloads heat the radiator would otherwise absorb. Keep fresh coolant on schedule from the full fluid-service list so corrosion never narrows the passages in the first place.
| Part | Vendor | Est. price |
|---|---|---|
| Mopar OAT Coolant (purple, MS.90032) — 50/50 premix | Mopar | ~$25 |
| Engine coolant recovery (degas) bottle | Mopar | ~$90 |
| Upper + lower radiator hose set | Gates / Mopar | ~$70 |
| OEM-spec replacement radiator (plastic end-tank) | Mopar / Spectra | ~$350 |
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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