The ZJ 4.0L cooling system holds about 12.7 quarts; the Magnum V8s hold about 14 quarts. Plan on flushing with distilled water twice, then refilling with a 50/50 mix of distilled water and extended-life antifreeze. Bleed the air through the bleed plug on the thermostat housing — skip that step and you'll overheat within 10 miles.
A ZJ that's never had a coolant flush will run hot in Phoenix summer, especially with the AC on, and the heater core will weep onto the passenger floor mat before it finally gives up. The 4.0L straight-six and the Magnum V8s both share an aluminum radiator on a plastic tank — these tanks crack with age, and old acidic coolant accelerates the failure. Flushing every 50,000 miles is the cheapest insurance you can buy against a $250 radiator.
The original Mopar HOAT (red) coolant was good for 5 years or 100,000 miles. Modern Mopar extended-life OAT coolant is fully backward-compatible with the ZJ if the system is fully flushed first. Don't mix coolant chemistries — flush twice with distilled water until what comes out is clear, then refill with a single coolant type. Prestone Cor-Guard, Peak Long Life, and Mopar's current OAT spec all work.
The radiator drain is a plastic petcock on the passenger-side bottom of the radiator. Most are old, brittle, and snap off when you turn them with pliers. Have a $25 replacement petcock on hand before you start. The block drain is a 1/4" NPT plug on the driver's side of the block — pull that too if you want a full flush, but it's optional and clears most of the residual coolant.
| Part | Vendor | Est. price |
|---|---|---|
| Mopar 5-year/100K extended-life antifreeze (OAT), 1 gal | any auto parts store; Prestone Cor-Guard or equivalent is fine | ~$18 |
| Distilled water, 4 gal | grocery store | ~$5 |
| Thermostat (while you're in there) | Mopar / Stant | ~$12 |
| Thermostat gasket | Felpro | ~$5 |
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.