Collapsed engine mounts let the 4.0L twist under load — which cracks exhaust manifolds, walks the fan into the radiator, and puts a clunk in the driveline that owners chase everywhere except the right place.
The XJ engine mount is a rubber-and-steel sandwich between a bracket on the block and a bracket on the unibody frame rail. It carries torque reaction, not much weight — so when the rubber separates from the steel, the engine stays put at idle and moves under throttle. On a 25-year-old XJ, both mounts are usually cooked whether or not you can see the damage.
The failure matters more than most owners assume. A sagging or torn mount lets the block rotate a fraction of an inch under load, and that rotation lands on whatever else is bolted between the engine and the body: the exhaust manifold (the number one hidden cause of the classic 4.0L manifold crack), the fan shroud, the radiator, and the driveshaft angles. Replace the mounts before you replace a manifold.
Look for these before you buy parts:
A single hard clunk on throttle tip-in or when shifting from reverse to drive
Vibration through the floor that changes with engine load, not road speed
Fan blade witness marks on the shroud, or coolant loss traced to fan-to-radiator contact
Exhaust ticking on cold start that quiets as the engine warms — a cracked manifold, often mount-driven
Visible rubber separation, cracking, or the bracket riding down against the frame stand
Pry test. With the engine off, put a pry bar between the block bracket and the frame bracket and apply moderate pressure. More than about 1/4 inch of lift, or any daylight opening between rubber and steel, means the mount is finished. Do both sides — one mount usually fails first, then the survivor takes the abuse and follows.
Rust check. In salt states, inspect the frame-side stand where the mount bolts through. A rotted stand is a fabrication repair, not a bolt-in one, and it needs to be addressed before the new mount goes in.
OE-style rubber (Crown 52040267, Anchor, Westar). Around $20 to $35 per side. Correct choice for a daily-driven XJ on stock or mild power. Same failure mode as the originals, but you get another decade.
Polyurethane "bomb proof" mounts. Strong, and they do not tear. They also pass engine vibration straight into the unibody, which you feel in the seat and the steering wheel at idle. Worth it on a dedicated trail rig, punishing on a commuter.
Torsion-style heavy duty (M.O.R.E. JM600). A steel bomb-proof housing around a Clevite OE-style rubber bushing. Strength close to poly with daily-drivable vibration. The compromise pick if you wheel the Jeep and drive it to work.
For a stock-power 4.0L that sees moderate trails, the OE-style rubber mount is enough. Heavy-duty mounts pay off when you add torque — a stroker, a high-numeric regear, or 35s — or after a mount has already let go on the trail.
Steps
Park on level ground, chock the wheels, set the parking brake, and soak every mount fastener with penetrating oil. Give it 15 minutes.
Disconnect the battery negative terminal, or at minimum unplug the under-hood light.
Remove the air box cover, filter, and the air box itself (13mm) to open up access on the driver side.
Place a floor jack under the oil pan with a wide block of wood spanning the pan rail. Raise until the jack takes light pressure — enough to unload the mounts, not enough to lift the engine. Never work under a Jeep supported by the engine jack alone.
Driver side: remove the nut from the main through bolt (18mm). Loosen but do not remove the two 15mm fasteners holding the mount to the frame stand. A universal-joint socket earns its keep here.
Note how the frame-side hardware works before you fight it: the outboard bolt threads into a nut welded to the frame, so no wrench is needed underneath. The inboard stud is welded to the mount itself — back off the nut from below.
Drive the main through bolt out with a hammer or back it out with a ratchet, then pull the old mount.
Passenger side: on many 1987–1995 XJs the oil filter adapter is clocked at 12 o'clock and blocks the through bolt. Drain the oil, pull the filter, loosen the adapter and rotate it to 9 o'clock. Plan the job as an oil change and it costs you nothing extra.
Repeat steps 5 through 7 on the passenger side.
Install both new mounts loose. Start the frame-side hardware first, then the main through bolt. Raise or lower the engine with the floor jack in small increments to line the bracket up with the mount, or persuade it with the pry bar.
With both sides in place and the engine settled, torque the frame-side bracket bolts and nuts to 30 ft-lb and each main through-bolt nut to 48 ft-lb.
Re-clock and tighten the oil filter adapter, install a new filter, refill the oil, reinstall the air box, and reconnect the battery.
Start the engine, let it idle, and watch the fan-to-shroud clearance. Blip the throttle and confirm the engine no longer walks toward the radiator.
While the jack is out, inspect the transmission mount and crossbrace. A fresh pair of engine mounts against a collapsed trans mount moves the clunk rather than removing it.
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Why it works
Removes a common source of driveline clunk that owners often misdiagnose as U-joints or motor mounts' downstream victims
Eliminates a leading cause of repeat exhaust manifold cracking on the 4.0L
Protects the radiator and fan shroud from contact damage under load
Under $70 in parts for the OE-style route, with no special tools
Trade-offs
Passenger-side access on early XJs requires draining oil and re-clocking the filter adapter
Rusted frame-side hardware can turn a three-hour job into a weekend with a cutoff wheel
Polyurethane mounts trade ride comfort for strength — pick deliberately
Requires supporting the engine on a jack; this is not a job to rush or to do on uneven ground
Tools required
Floor jack and a block of wood
Jack stands and wheel chocks
{'Sockets': '13mm, 15mm, 18mm'}
Universal-joint socket adapter
Long and short extensions
Torque wrench
Pry bar
Hammer
Penetrating oil
Oil drain pan and filter wrench (passenger side, early XJs)
Parts
Part
Vendor
Est. price
Engine mount, 4.0L (one per side) — Crown Automotive
Quadratec / RockAuto / Morris 4x4
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M.O.R.E. Bomb Proof Torsion motor mounts (heavy-duty pair)
Mountain Off-Road / Quadratec
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Engine oil and filter (passenger-side access on early XJs)
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.