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.
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| 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) | Mopar dealer / Quadratec | — |
| Front crankshaft (timing cover) seal | RockAuto — National / Mopar | — |
| Timing cover gasket | RockAuto — Fel-Pro | — |
| RTV sealant (black) | Any auto parts store | — |
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.