The OEM plastic reservoir cracks. Aluminum aftermarket units (or used Mopar) are the standard fix. Also add a real radiator cap, not the integrated overflow style.
The XJ overflow bottle is unpressurized — coolant expands into it through the radiator overflow neck and is drawn back via vacuum on cooldown. The plastic bottle ages and cracks at the seam, and the cap fits loosely after 20 years.
Replacement options:
A more important related upgrade: the 1991-2001 XJ uses a fully sealed system with the cap on the radiator. The cap pressure (16 psi) raises the boiling point. After 20 years, the cap spring is weak and won't hold rated pressure — boiling temp drops, system burps coolant when you don't want it to. A new Stant 16 psi cap is $8 and is the single cheapest cooling upgrade you can do.
The Renix and very early HO use a pressurized reservoir, not the radiator cap arrangement. Year-specific.
| Part | Vendor | Est. price |
|---|---|---|
| Crown OEM-style reservoir | Crown | ~$25 |
| Stant 16 psi radiator cap | Stant | ~$9 |
| Mishimoto aluminum reservoir | Mishimoto | ~$120 |
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.