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.
Steps
Let the engine cool completely before opening anything. Coolant under pressure is hot enough to cause serious burns. If the XJ was driven, wait at least two hours.
Spread a tarp or cardboard under the front of the vehicle. Coolant is toxic to animals — clean up any spills before they dry on the driveway.
Raise the front of the vehicle on jack stands if drain pan clearance is tight. The lower hose is more manageable to access from above, but some drain pans won't fit under an unladed XJ.
Place a drain pan (minimum 3-gallon capacity) under the lower passenger-side corner of the radiator.
Petcock method: Locate the plastic petcock valve on the lower passenger corner of the radiator. Turn it counterclockwise by hand — do not use a wrench or pliers on the plastic body or it will snap. Coolant will drain slowly. If it won't budge after firm hand pressure, use the lower hose method instead.
Lower hose method: Loosen the clamp on the lower radiator hose where it connects to the radiator inlet using pliers or a hose clamp tool. Slide the clamp back and pull the hose off the fitting. Direct the hose into your drain pan. This drains faster than the petcock and gets more of the coolant out.
Open the radiator cap (engine cold) to vent the system and speed draining. The cap is on the radiator neck — the overflow bottle cap doesn't vent the main system.
Once flow slows to a drip, flush the system: reattach the lower hose (or close the petcock) and fill the radiator with plain distilled water. Do not use tap water for flushing — the minerals leave scale deposits. Fill to the neck.
Start the engine, turn the heater to max heat and high fan, and let the engine warm up until the temperature gauge reaches its normal operating range (about 210°F). This opens the thermostat and circulates water through the heater core, pushing contaminated coolant through the entire system.
Let the engine idle for 10 minutes after the gauge stabilizes, then shut it off and let it cool fully. Drain the flush water the same way you drained the original coolant. If the flush water came out dark or rusty, repeat the distilled-water flush before proceeding.
Thermostat access: The thermostat housing sits at the front of the engine where the upper radiator hose meets the intake manifold. Two 10mm bolts hold the housing to the manifold. You may need a short extension to clear the upper hose bracket.
Remove the upper radiator hose from the thermostat housing by loosening its clamp and pulling it back. Some coolant will drain out — have a rag ready.
Remove both 10mm housing bolts. Keep track of which bolt goes where if they're different lengths. The housing lifts straight up; the thermostat sits in a pocket in the intake manifold below it.
Pull the old thermostat out of the pocket. Note the orientation — the thermostat installs with the copper sensing element (the pellet end) facing down into the engine, and the spring facing up.
Scrape any old gasket material off the housing and manifold mating surfaces using a flat scraper. Do not gouge the aluminum. Both surfaces must be clean and flat or the new gasket won't seal.
Drop the new thermostat into the manifold pocket with the pellet end down. Place the new gasket on the manifold surface — do not reuse the old gasket.
Set the housing over the thermostat and start both bolts by hand. Torque them to 16 ft-lbs (22 Nm) using a torque wrench. Do not overtighten — the housing is cast aluminum and the bolts will pull through. Alternate between bolts in 2-3 passes rather than fully torquing one side first.
Reconnect the upper radiator hose to the housing and secure the clamp. Make sure the clamp is positioned past the raised bead on the fitting.
Reconnect the lower radiator hose (if removed) and tighten its clamp until snug. The clamp should compress the hose visibly but not cut into it.
Prepare your coolant. For a 10.5-quart system, use a 50/50 mix: roughly 5 quarts of concentrated green coolant plus 5 quarts of distilled water — or use pre-mixed 50/50 directly. Avoid tap water. The 50/50 ratio provides freeze protection to about -34°F and boil-over protection to about 265°F.
Pour coolant slowly into the radiator neck until it reaches the base of the filler neck. Fill the overflow reservoir to the "cold" line. Do not overfill — the system will push excess into the reservoir as it heats up.
Leave the radiator cap off and start the engine. Watch the coolant level in the neck as the engine warms up. Bubbles will appear as air purges from the system — this is normal for the first 5-10 minutes.
Once the engine reaches operating temperature, coolant will begin flowing visibly in the radiator neck as the thermostat opens. Top off as needed. Continue idling until the heater blows hot air, confirming the thermostat has opened and the heater core is circulating.
With the engine at operating temp, inspect both radiator hose connections and the thermostat housing for leaks. Wet spots or drips mean a hose clamp needs tightening or the housing gasket didn't seal.
Shut the engine off, let it cool for 30 minutes, then recheck the coolant level in the radiator neck and the overflow reservoir. Top off if the level dropped. Install the radiator cap — press down and turn clockwise until it clicks into the locked position.
Dispose of old coolant at a recycling center or auto parts store. Most accept used coolant at no charge.
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.