A worn fan clutch is the most common reason an XJ runs cool on the highway but overheats at idle or in traffic — and it's an hour-long fix with hand tools.
The fan clutch is a thermal-viscous coupling bolted between the water pump pulley and the engine fan. When underhood air heats up, silicone fluid inside the clutch thickens and locks the fan to the pulley, pulling air through the radiator at low speed. After 100,000–150,000 miles that fluid breaks down and the clutch slips even when it's hot. The symptom is specific: the temp gauge stays normal at highway speed (air is moving on its own) but climbs in stop-and-go traffic or at idle, when the fan is the only thing pulling air. In Phoenix heat this shows up sooner than the mileage suggests.
Test it before you buy anything. With the engine cold and off, spin the fan by hand — it should turn one to three revolutions and stop. With the engine at full operating temperature and shut off, the fan should be locked or very hard to turn, and it should stop almost immediately when the engine cuts. If the fan spins freely for several seconds after shutdown at temp, or you see an oily silicone film weeping from the front of the clutch, it's done. A loud roar that builds with RPM and never lets up points to the opposite failure — a clutch stuck engaged — which kills 1–2 MPG and is worth replacing too.
**Buy for your year.** The 1987–1990 Renix 4.0L runs a reverse-rotation water pump; confirm the listing matches your engine before ordering. The Hayden 2787 is a severe-duty unit that engages harder and cools better in hot climates at a small fuel cost. The Four Seasons 36711 is a quieter OE-equivalent. Either bolts on the same way.
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| Part | Vendor | Est. price |
|---|---|---|
| Hayden 2787 Severe-Duty Fan Clutch | RockAuto / Hayden / Amazon | — |
| Four Seasons 36711 Fan Clutch (OE-equivalent) | RockAuto / Four Seasons / Amazon | — |
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.