The 4.0L uses a single-row timing chain with a nylon-faced cam sprocket that stretches and sheds teeth with age — this guide covers replacing the chain and both sprockets, and why there's no separate tensioner to chase.
A stretched timing chain throws off valve timing, which shows up as a rough idle, hard starting, lost low-end torque, and sometimes a rattle from the front of the engine on startup. The classic test comes straight from the factory manual: with the cover off, push the chain sideways — if it deflects more than half an inch, it's done. On a high-mileage XJ that has never had this addressed, it almost always is. Left long enough, the nylon teeth on the factory cam sprocket break off, the chain jumps a tooth, and timing goes far enough out that the engine runs badly or won't run at all.
There is no adjustable tensioner on the 4.0L. Unlike many overhead-cam engines, this pushrod inline-six runs the chain directly between the crank and cam sprockets with no spring-loaded tensioner to service. A plastic chain guide sits inside the cover to keep slack in check, and that's it. So "timing chain and tensioner" on this engine means the chain, both sprockets, and the guide — there's no tensioner part to buy. If a shop tries to sell you a tensioner for a 4.0L, they're working off the wrong engine family.
RENIX vs. HO — same job. The 1987–1990 RENIX 4.0L and the 1991–2001 HO 4.0L share the same bottom end and timing setup, so this procedure applies to both. The accessories bolted to the front of the engine differ, but the chain, sprockets, and timing marks are identical.
Buy the upgraded chain set. The factory single-row chain with the nylon-coated cam gear is the part that fails. A Cloyes True Roller set (9-3110) replaces it with all-steel sprockets and a stronger roller chain — it's the standard upgrade and costs little more than OEM. This is a job you do once per couple hundred thousand miles, so spend the extra few dollars and don't reuse the old sprockets.
Steps
Disconnect the negative battery terminal. Drain the cooling system into a clean pan if you plan to reuse the coolant.
Remove the radiator overflow bottle, fan, and fan shroud. The fan nut takes a large adjustable wrench (~36mm); a sharp hammer tap usually breaks it loose. Store the viscous fan clutch standing upright, never flat — laying it on its side can let fluid migrate and damage the clutch.
Remove the radiator (four 10mm bolts, two per side) and set it aside where it won't get bent.
Break the crankshaft bolt loose with a 19mm socket and a breaker bar before removing the belt — you need the engine to resist turning. With a manual transmission, put it in gear and set the parking brake. With an automatic, you'll need to hold the flywheel or use an impact.
Relieve the serpentine belt tension with a 1/2" breaker bar in the tensioner and slip the belt off. Remove the alternator and its bracket (15mm holds the alternator, 14mm holds the bracket) — the bracket blocks timing-cover bolts.
Remove the crankshaft bolt and its washer. Don't forget the washer — leave it on and the damper won't come off.
Pull the vibration damper (harmonic balancer) with a proper puller. The puller tip must seat in the crank's center recess, not against the threads — pulling against the threads will strip the crankshaft and turn a five-hour job into a catastrophe. Go slow and confirm the tip is seated before you load the puller.
Remove the timing cover bolts. They're a mix: mostly 10mm, two 16mm, and four 11mm oil-pan bolts along the bottom. Push each bolt through a piece of cardboard in its layout position so they go back exactly where they came from — lengths differ.
Slide the cover off. Watch for the plastic chain guide, which likes to fall out — keep track of it and inspect it for cracks or wear.
Rotate the crank until the timing marks on the crank and cam sprockets line up — the two marks face each other on a line through both sprocket centers. You may need to slip the damper back on temporarily to turn the engine. Do not turn the crank or cam again once the marks are aligned.
Remove the camshaft sprocket bolt and washer with a 16mm socket, then pull the cam sprocket and chain together as one unit so nothing rotates.
Remove the key from the crankshaft and slide the lower crank sprocket off. If it's stubborn, confirm the key is fully out before forcing it — a small gear puller helps. Replace both sprockets; don't reuse the old crank gear because it "looks fine."
Install the new crank sprocket and reinstall the key (a dab of RTV holds it in place). Hang the new chain on the cam sprocket and install the cam sprocket and chain together with the timing marks aligned. Verify there are 15 chain pins between the two timing marks — this is the factory check that timing is set correctly.
Torque the camshaft sprocket bolt to 50 ft-lb (68 Nm). Putting the transmission back in gear helps hold the engine while you torque.
Drive the old seal out of the timing cover and install the new one squarely, lip facing in, lightly oiled, with a thin film of RTV on the outer edge. Support the cover from behind while you tap it in so you don't distort it.
Scrape both gasket surfaces clean — cover and block. The XJ front cover is a known weep point, so apply RTV to both sides of the new gasket and run a bead across the sharp lower corners where the cover meets the oil pan.
Reinstall the cover, using the seal/crank as a pilot so the new seal slides over the crank snout without rolling the lip. Make sure the chain guide is seated. Torque the cover hardware: 1/4" cover-to-block bolts to 60 in-lb (7 Nm), 5/16" cover-to-block bolts to 192 in-lb (16 ft-lb / 22 Nm), and the 1/4" oil-pan-to-cover bolts to 84 in-lb (9.5 Nm).
Reinstall the vibration damper with a balancer installer that threads into the crank and presses the damper on. Never hammer the damper on — you'll damage the thrust bearing. Then install the crank bolt and washer and torque to 80 ft-lb (108 Nm).
Reassemble in reverse: alternator and bracket, belt, radiator, fan and shroud, overflow bottle. Refill coolant, reconnect the battery, and run the engine while watching for front-cover leaks and confirming a smooth idle.
Why it works
Restores correct valve timing — idle smooths out, low-end torque comes back, and hard-starting tied to a stretched chain goes away.
Eliminates the failure mode where the nylon cam-gear teeth break off and the chain jumps, which can leave you stranded.
The upgraded steel roller set is effectively a permanent fix for the life of the engine.
Cheap in parts — the chain set, seal, and gasket run well under what a shop charges for the same job.
Trade-offs
It's deep front-end disassembly: fan, radiator, belt, accessories, and damper all come off before you reach the chain. Plan a full day.
You need a harmonic balancer puller and installer. Renting or buying them adds cost, and the puller is where most people damage the crankshaft if they rush.
Getting the timing marks aligned and verifying the 15-pin count is unforgiving — a one-tooth error means pulling it back apart.
While you're this far in, the water pump, thermostat, and front seal are right there. If they're original, replace them now; doing them separately later means repeating most of this teardown.
Tools required
Floor jack and jack stands
1/2" breaker bar and socket set (10, 11, 13, 14, 15, 16, 19mm)
Harmonic balancer puller AND installer (do not skip the installer)
Inch-pound torque wrench and a foot-pound torque wrench (to 80 ft-lb)
Front crank seal installer (or a correctly sized socket/seal driver)
Gasket scraper and wire brush
Large adjustable wrench (~36mm) for the fan nut
RTV (black) and a drain pan
Parts
Part
Vendor
Est. price
Timing chain set — Cloyes True Roller (4.0L), includes both sprockets and chain
Summit Racing / RockAuto
—
Timing chain set — Mopar OEM single-row (alternative to Cloyes)
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.