Replacing the water pump on the 4.0L is a half-day job that prevents being stranded with an overheating engine. The most common mistake: ordering the wrong pump. Get the year right before you buy.
The XJ 4.0L water pump runs belt-driven off the front of the timing cover. It lasts 80,000–130,000 miles in most cases, but the bearing and shaft seal wear out before the impeller. When the seal goes, coolant weeps from a small drain hole on the underside of the pump housing — look for a dried orange or green crust below the pump and along the front of the engine. If you hear a low grinding growl from the front of the engine that changes with RPM and isn't the alternator, the bearing is on its way out.
The part number trap. The 1987–1990 Renix and 1991–2001 HO 4.0L use different water pumps with impellers pitched in opposite directions. They bolt on identically. Ordering the wrong one means the pump spins but moves coolant the wrong way — the engine overheats quickly and the impeller cavitates. Gates 42137 for Renix, Gates 41141 for HO. Confirm your year before you buy anything.
While the coolant is drained, replace the thermostat (it's a $10 part already in your hands) and inspect the bypass hose — the short rubber hose running from the pump inlet to the thermostat housing. On high-mileage XJs it's often original and cracks the moment you disturb it. Have a Dorman 56398 on hand. The thermostat housing on many XJs is plastic and prone to hairline cracks; Dorman makes a direct-fit aluminum replacement if yours looks suspect.
Steps
Let the engine cool completely. Pressurized coolant burns — don't rush this.
Drain the coolant. Open the petcock at the bottom driver's side of the radiator into a catch pan. If the petcock is stuck (common on older XJs), crack the lower radiator hose clamp instead. Save the coolant for recycling or dispose of it — antifreeze is toxic to animals.
Remove the serpentine belt. Use a 15mm socket or breaker bar on the automatic tensioner and rotate it to release tension, then slip the belt off the water pump pulley. Take a phone photo of the belt routing before removal.
Remove the fan shroud. Two to four bolts or clips on the top hold it to the radiator. Lift it out — you may need to tilt it forward to clear the fan blades. Set it aside without bending the plastic.
Remove the fan clutch and fan assembly. Four bolts (typically 10mm) attach the fan clutch assembly to the face of the water pump pulley. Hold the pulley from spinning with a strap wrench or by sliding the belt back on temporarily and having a helper hold it. Remove all four bolts and lift the fan + clutch assembly out over the shroud area. Set aside on a flat surface — don't let it tip over, as the clutch fluid inside will distribute unevenly if left on its side for long.
Remove the water pump pulley. Four bolts (13mm) attach the pulley to the pump shaft hub. Remove them and pull the pulley straight off.
Disconnect the bypass hose. The short rubber hose connecting the pump inlet to the thermostat housing may have a spring clamp or screw clamp. Squeeze or loosen it and twist the hose off. If it tears — and it often does — that's confirmation the Dorman replacement was worth buying.
Replace the thermostat. Two bolts (13mm) hold the thermostat housing to the intake manifold. Remove the housing, pull the old thermostat, scrape off the old gasket material, and install the new thermostat and gasket. Torque the housing bolts to 17 ft-lb.
Remove the water pump. Six bolts secure the pump to the timing cover — note that they are two different lengths. Mark or photograph which bolt came from which hole before you pull them. Installing a longer bolt in a short-bolt hole will crack the aluminum timing cover. Remove all six bolts and break the pump free with a rubber mallet tap on the housing edge, or a careful pry at the corners — never pry against the timing cover gasket surface.
Clean the mating surface. Use a plastic scraper to remove all old gasket material from the timing cover face. The surface needs to be flat and clean. Don't use metal scrapers against the aluminum.
Prepare the new pump. Compare the new pump's impeller to the old one — the impeller blades should be pitched identically. If they look like mirror images, you have the wrong pump. Check the part number again before proceeding.
Install the new pump and gasket. Most quality pumps include a new gasket; if yours doesn't, use a thin, even bead of Permatex Ultra Black RTV on the pump flange. With a paper gasket, install it dry. Set the pump on the face, start all six bolts by hand — confirm each one threads in easily before applying torque. Torque to 17 ft-lb in a crisscross pattern.
Install the new bypass hose and clamp.
Reinstall the water pump pulley. Torque the four bolts to 20 ft-lb.
Reinstall the fan + clutch assembly. Snug the four bolts evenly — there's no critical torque spec here, but don't over-torque into the pulley threads.
Reinstall the fan shroud.
Route and reinstall the serpentine belt per your photo. Rotate the tensioner to clear, seat the belt on all pulleys, release the tensioner. Double-check the belt is fully seated in every groove.
Fill the cooling system. Use a 50/50 mix of HOAT (orange, Jeep-spec) coolant and distilled water — the XJ holds approximately 8–9 quarts. Add through the radiator fill neck with the cap off.
Bleed the air. The 4.0L traps air reliably. With the cap off and heater set to full hot, start the engine and let it idle. Squeeze the upper radiator hose repeatedly to push air through. Watch the coolant level — it will drop as the thermostat opens and coolant flows into the block. Add coolant as needed. When the thermostat opens (watch for the upper hose to go hot and coolant to start circulating visibly), continue running until the level stabilizes and no more large bubbles surface. Cap the radiator, run to operating temp, and check the level again when cold the next day. Top off as needed.
Check for leaks. Run the engine to temp and inspect around the pump, hose connections, and thermostat housing. A small drip on initial startup is common if you used RTV — it cures under heat. An active drip from the pump face means a bolt wasn't torqued evenly; re-torque the affected corner.
Why it works
Prevents roadside overheating — a failed water pump on the trail or highway is a major problem
Doing the thermostat and bypass hose at the same time costs almost nothing extra in labor
Gates and ACDelco pumps come with a new gasket included, keeping the job clean
Trade-offs
Wrong part number is a real risk — double-check year before ordering; Renix and HO pumps look identical
The bypass hose often tears on removal; budget $10 for the replacement
Cooling system air purging takes patience; rushing this step leads to mysterious overheating after the job
Tools required
Socket set (10mm, 13mm, 15mm)
Torque wrench
Serpentine belt tool or 15mm breaker bar (belt tensioner)
Drain pan (2-gallon minimum)
Plastic gasket scraper
Strap wrench or pulley holder (to hold pulley while loosening fan bolts)
Flathead screwdriver (hose clamps)
Parts
Part
Vendor
Est. price
Gates 41141 Water Pump (HO, 1991–2001)
RockAuto / Gates
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Gates 42137 Water Pump (Renix, 1987–1990 — reverse rotation)
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.