The XJ's 4.0L inline-six runs hot by nature, and neglected coolant accelerates corrosion inside the block, water pump, and radiator. Flush and refill the system every two years or 30,000 miles with conventional green coolant — or every five years with long-life HOAT. Add a thermostat swap while you're in there: it's a two-bolt job that takes ten minutes and eliminates one of the most common causes of intermittent overheating.
The 4.0L cooling system holds roughly 10.5 quarts. That's enough coolant to do real damage if the chemistry is off — corroded passages restrict flow, a stuck-open thermostat kills fuel economy in cold weather, and a stuck-closed thermostat puts you on the side of the road. This job covers both: draining and flushing the system, then installing a fresh thermostat and gasket before refilling.
The XJ does not have a block drain plug. You'll drain through the radiator petcock — a plastic valve on the lower passenger-side corner of the radiator — or by pulling the lower radiator hose. The petcock works, but it's fragile and awkward to reach. Many owners find the lower hose faster and more complete. Either approach gets the job done; both are covered below.
A note on coolant: use green ethylene glycol (conventional or HOAT, which extends service life). Do not mix Dex-Cool (orange OAT) into a system that has run on green — the chemistry conflict causes gel deposits that plug the heater core. If the existing coolant color is unknown or you're buying the XJ used, flush with distilled water first until it runs clear, then refill fresh.
Why it works
Restores cooling efficiency and inhibitor protection — degraded coolant is acidic and corrodes the water pump impeller, radiator tubes, and freeze plugs over time.
A new thermostat is inexpensive insurance. Failed thermostats (stuck open or stuck closed) are a common cause of running too cold in winter or sudden overheating, and the housing is already off once you're doing a flush.
A complete flush removes scale and corrosion particles that restrict flow and accelerate wear on the water pump seal.
Trade-offs
The plastic radiator petcock is brittle on older XJs. If it snaps, you'll need to pull the lower hose and eventually replace the petcock — budget an extra $8 for a replacement if the XJ has high mileage.
Air pockets in the 4.0L system can cause the temperature gauge to read erratically after refilling. If the gauge spikes shortly after a flush, shut the engine off immediately and let it sit — the system likely has an air bubble that didn't purge. Refill, restart, and monitor until the bubble works out. Running even briefly on an air-locked system can damage the head gasket.
If the flush water came out rusty or heavily contaminated, a cooling system flush additive (such as Prestone Radiator Flush) run for one heat cycle will do a better job than plain distilled water. Add it to budget if the system looks neglected.
Tools required
10mm socket and ratchet
socket extension
slip-joint pliers
hose clamp pliers (optional)
drain pan (at least 3-gallon capacity)
funnel
garden hose
flat-blade screwdriver
floor jack and jack stands (optional, for better drain pan clearance)
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.