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.
Steps
Let the engine cool completely. The fan and shroud sit inches from a hot radiator, and you'll be reaching past both.
Unclip and lift out the fan shroud. The plastic shroud is held to the radiator by two to four clips or bolts along the top. Free it and slide it back toward the engine so it rests over the fan — on the XJ you can usually leave it loose in place rather than fully removing it, which buys clearance to reach the clutch bolts.
Hold the pulley and break the four clutch nuts loose. Four nuts (13mm) hold the fan-and-clutch assembly to the water pump pulley flange. The pulley wants to spin as you turn them. Stop it with a strap wrench around the pulley, or leave the serpentine belt on and have a helper hold the crank. Crack all four loose before removing any.
Remove the four nuts and lift the fan-and-clutch assembly straight up and out, past the shroud. Keep it upright on the bench — laying a clutch on its side lets the silicone fluid pool to one edge.
Move the fan blade to the new clutch. Four bolts hold the plastic fan to the back of the old clutch. Swap the blade onto the new clutch in the same orientation and snug those bolts evenly.
Set the new assembly onto the pulley studs and start all four nuts by hand. Tighten them evenly in a cross pattern to roughly 18–20 ft-lb — firm, not gorilla-tight; the flange is thin. Confirm the fan sits centered and clears the shroud all the way around by spinning it once by hand.
Reseat and clip the shroud back to the radiator. Make sure no wiring or hose is resting against the fan.
Start the engine and let it reach operating temperature. Listen for the clutch engaging as it warms — a rise in fan roar — and watch the temp gauge through a full warm-up and a few minutes at idle.
Why it works
Fixes the most common XJ idle-and-traffic overheat for under $100 and an hour of work.
Hand tools only — no special puller or fan-clutch wrench needed on the bolt-on XJ design.
A severe-duty clutch noticeably improves low-speed cooling in hot climates.
Trade-offs
A slipping clutch can mimic a failing water pump or a clogged radiator. Verify the symptom (cool on the highway, hot at idle) before spending — you probably don't need anything more than the clutch, but rule out a low coolant level and a tired pump first.
A severe-duty clutch is slightly louder and costs 1–2 MPG over a stock unit.
If you're already chasing chronic overheating, a mechanical clutch is a band-aid for a marginal cooling system; an electric fan conversion is the bigger fix, not this.
Tools required
Socket and wrench set (13mm for the clutch-to-pulley nuts)
Strap wrench or a helper to hold the pulley from spinning
Flathead screwdriver (fan shroud clips)
Flashlight
Parts
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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.