Open the petcock at the bottom of the radiator, drain into a pan, refill with a 50/50 mix of distilled water and full-strength antifreeze, run with the heater on max to bleed air, and top off — total system capacity is about 10 quarts.
YJ cooling systems are old enough that the original coolant has been in the truck longer than most readers have owned the truck. Antifreeze breaks down chemically over time — the corrosion inhibitors deplete first, then the dielectric properties — and used coolant becomes mildly acidic, which eats heater cores, head gaskets, and water pumps from the inside. A coolant flush every three years or 36,000 miles is the cheapest preventative you can do.
The system holds **10 quarts (2.5 gallons)** on a 4.0L and slightly less on a 4.2L 258 — figure two gallons of pre-mixed 50/50 plus an extra gallon for top-off. The factory specified the green ethylene glycol — Prestone, Peak, or Zerex are all fine. Avoid mixing colors (orange Dex-Cool, gold, etc.) because the chemistries fight each other and form a gel. Always use distilled water if you mix your own — tap water deposits minerals on the inside of the head and radiator over time.
The thermostat is a 195°F unit and lives under the housing at the front of the head. While you have the system drained, swap it. Replace the housing gasket at the same time — they cost $6 and are the most common leak point after a thermostat job. Torque the housing bolts to **17 ft-lbs**; over-torquing cracks the cast housing. If the original housing is corroded or has cracks around the bolt holes, replace it now with a Mopar or Crown aluminum housing. Plastic aftermarket housings warp under heat and are not worth the savings.
Bleeding air is the step most people get wrong. Park the truck nose-up on a hill or ramps so the radiator cap is the highest point in the system. Refill with the cap off, start the engine, set the heater to maximum heat with the fan on low, and let it warm up until the upper radiator hose is hot. The thermostat opens, the air burps out, the coolant level drops — top it off. Squeeze the upper hose a few times to help. Cap on, drive around the block, let it cool overnight, recheck level in the overflow bottle in the morning.
| Part | Vendor | Est. price |
|---|---|---|
| Mopar 195°F thermostat | Mopar | ~$18 |
| Stant 195°F thermostat | Stant | ~$12 |
| Thermostat housing gasket | Felpro | ~$6 |
| Prestone 50/50 antifreeze, 1 gal | Prestone | ~$16 |
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.