Drain the radiator and block, flush with distilled water until the runoff is clear, then refill with HOAT (G-05) coolant at 50/50. The TJ 4.0L holds about 2.6 gallons dry; plan for 1.5 gallons of new coolant on a drain-and-fill.
The TJ 4.0L cooling system is robust, but coolant breaks down chemically every 5 years regardless of mileage. Once the corrosion inhibitors are spent, the inside of your radiator, heater core, and block start eating themselves. A fresh flush is cheap insurance and the most common cause of TJ overheating is not a bad water pump — it is a radiator core silted shut from a decade of skipped flushes.
Full dry capacity is 10.5 quarts (about 2.6 gallons). A standard drain through the radiator petcock recovers around 1.5 gallons, which is what you replace on a routine service. If the existing coolant is brown, rusty, or smells off, you need a full flush — drain the radiator, drain the block (8mm hex plug on the passenger side of the engine, above the starter), and then run distilled water through the system until clear.
Coolant choice matters. The factory spec for 1997–2001 TJs is the green ethylene glycol coolant; 2002+ shipped with HOAT (Mopar's pink). Either is fine for older Jeeps as long as you fully flush before switching. The safe, modern answer for any TJ is Zerex G-05 — it is the OEM HOAT formula sold under the Valvoline label and is compatible with iron blocks and brass/copper radiators. Stay away from universal "works with everything" coolants; they tend to gel when mixed with leftover residue.
A few things to inspect while the system is empty: pressure-test the radiator cap (16 psi is spec), squeeze the upper and lower hoses for soft spots and cracks at the bend, and look at the back of the water pump weep hole for crusty buildup. If the thermostat is original, this is the moment to swap it for a $14 Mopar 195°F replacement and stop guessing about why the temp gauge wanders.
Refilling needs a bleed. The TJ has a high spot in the heater core circuit that traps air. With the cap off, start the engine, set the heater on full hot, and let the system burp for 10–15 minutes. Top off as the level drops. Drive a heat cycle, let it cool overnight, and check the level again before you call it done.
| Part | Vendor | Est. price |
|---|---|---|
| Zerex G-05 HOAT concentrate (1 gal) | Valvoline | ~$20 |
| Distilled water (2 gallons) | any grocery | ~$4 |
| Mopar 195°F thermostat (optional refresh) | Mopar | ~$14 |
| Radiator cap, 16 psi (optional) | Stant | ~$8 |
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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