Drain the lower radiator hose, flush with water, refill with fresh 50/50 coolant — the TJ 4.0L holds about 2 gallons. Do it every 2 years or 30,000 miles regardless of color.
The TJ's AMC 4.0L inline-six runs a conventional green-formula coolant system. Chrysler spec is green (silicate-based) coolant, which depletes inhibitor faster than OAT or HOAT formulas — 2 years or 30,000 miles is the real service interval even if the coolant still looks green. Old coolant turns acidic and starts attacking aluminum components: the water pump impeller, the thermostat housing, and the heater core are the first casualties.
The TJ doesn't have a petcock drain on the radiator — you drain by disconnecting the lower radiator hose at the radiator inlet. It's a 30-second job but messy. Have a wide drain pan and rags ready.
Capacity is approximately 7.5 quarts (1.875 gallons) for the 4.0L. The recovery bottle holds another 0.5–0.75 quarts. Two gallons of 50/50 premixed coolant covers it with a little left over.
Running a pre-flush product (BlueDevil, Prestone flush, or similar) is worth doing if the coolant hasn't been changed in more than 4 years or is visibly rusty. Add the flush product, run the engine for 10–15 minutes, drain, flush with clean water, and then refill.
| Part | Vendor | Est. price |
|---|---|---|
| Peak 50/50 premixed coolant (1 gallon) | Peak | ~$12 |
| Zerex Original Green concentrate (1 gallon) | Zerex | ~$18 |
| BlueDevil Radiator Flush (optional pre-flush) | BlueDevil | ~$8 |
| Thermostat — Stant 45352 (195°F) | Stant | ~$12 |
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.