Overheats at idle but cools on highway: fan. Highway only: thermostat or restriction. All conditions: water pump, head gasket, or low coolant.
XJ overheating has a finite list of causes; work through them in this order. First check coolant level and condition - low coolant or rusty/contaminated coolant comes first. Verify the radiator cap holds 16 psi (a weak cap drops boil point and lets coolant push into the overflow). If overheating happens only at idle and in traffic but cools on the highway, the issue is fan-related: failed mechanical fan clutch (test with the engine off and warm - clutch should resist hand-spinning significantly; if it spins freely it is shot, Mopar 52027823) or failed electric auxiliary fan (test by jumpering it to battery 12V - should run; if not, motor or relay). If overheating happens primarily at highway speeds, suspect a clogged radiator (internal scale or external bug-coverage), thermostat stuck closed, or collapsed lower hose. If overheating happens under all conditions, suspect failing water pump (visible weep hole drip, noisy bearing) or head gasket/cracked head (see 0331 entry). Run-the-diagnostic: pressure test + thermostat removal test + IR temp gun on the radiator inlet vs outlet (should differ by 15-30 F when working).
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.