Stock is 195F. Some swap to 180F for lower trail temps, but the PCM expects 195F and a colder stat hurts fuel economy and emissions.
Stock XJ thermostat is 195F (some early years 192F). The Renix and HO PCMs are tuned around that operating temp — colder coolant means colder intake air (slightly), richer mixture (the PCM thinks engine is still warming up), worse MPG, and at extremes a check engine light.
The 180F argument: at full lockup on the highway in summer, peak coolant temp creeps to 220-230F on a hot day. A 180F stat doesn't change peak temp at all (cooling system capacity sets that), only the lower bound. So a 180F stat opens earlier, runs cooler at light load, but does nothing for the actual overheating condition.
The right answer: stay at 195F. If you have an overheating problem, fix the cause (see overheating sequence entry) rather than masking it with a colder thermostat. Use a quality stat — Stant SuperStat (PN 45359) or Mopar OE. Cheap thermostats stick.
The install includes new gasket and a careful bleed of trapped air via the heater hose or the bleed bolt on top of the water pump (some years). Park nose up and fill slowly.
| Part | Vendor | Est. price |
|---|---|---|
| Stant SuperStat 195F | Stant | ~$18 |
| Mopar OE thermostat | Mopar | ~$28 |
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.