Jaguar XJ8 (X308) Timing Chain Tensioner Problems
On the X308 the weak point is the upper (secondary) chain tensioners: the early plastic-bodied ones crack with age and can let the exhaust-cam chain jump, often with no warning noise at all. I'd first find out which tensioners the engine has — metal-bodied ones went in from engine number 0108130000, August 2001 — and if they're plastic, replace them before anything else happens.
How bad is it · Garage, carefully
Get it looked at now — a failed tensioner can wreck the engine
This V8 is an interference engine: if the exhaust-cam chain jumps two teeth or breaks, valves meet pistons and the head has to come off. A cracked tensioner rarely announces itself, so a rattle on a car with plastic tensioners means booking the work, not waiting for the next service.
Which one applies to yours depends on your symptoms, your model year and what has already been done to it.
Likely causes, most common first
Read the check cost before the odds: a free check that rules a cause out beats a likely guess that costs money.
Cracked plastic upper (secondary) tensioners
Each bank has a short upper chain from the inlet cam to the exhaust cam, kept tight by an oil-fed tensioner under the cam cover. The first- and second-generation bodies are plastic and split lengthways with age and heat; the shoe then breaks away, the chain runs on bare metal, and a loose piece can jam at the exhaust sprocket. Heat and stretched oil-change intervals age the plastic faster, so after any overheat I would bring the job forward.
Often nothing. Sometimes a second or two of rattle from the front on a cold start; the later spring-assisted second-generation type tends to stay quiet right up to failure.
Take the cam covers off: a plastic body means early parts, a metal body means late. Look for cracks at the notch at the front, but don't trust a clean look — the sprung side faces downward and can't be seen in place. Bulletin 303-30 of December 1998 brought in the revised plastic type (NCA2017AF for A bank, NCA2017BE for B bank) — it is not the metal one.
Fit the third-generation metal-bodied tensioners (C2A1511 and C2A1512) with new, shorter bolts and new cam cover gaskets and plug seals.
Cracked primary tensioners and split guide rails behind the timing cover
The two lower (primary) chains run from the crankshaft to the cams behind the front cover, against plastic-faced guides and two more tensioners. The guides crack and break up, and the chain starts to flap. These rarely wreck the engine but do make most of the noise.
A clack at operating temperature that grows over months, or a rattle that lasts until oil pressure builds. On an XJR it can be mistaken for supercharger noise.
With a cam cover off, press the lower chain with a long screwdriver and feel for slack. A proper look means the front cover off — sensible on any car past about 100,000 miles.
Replace all four chains, tensioners and guides with the latest parts, and the water pump and thermostat while the front is apart.
The chain has already jumped a tooth or broken
When a tensioner gives way, the upper chain can skip on the exhaust sprocket. One tooth usually means rough running with no damage; two teeth or a broken chain and the exhaust valves are hit.
Misfire on one bank — cylinders 5-8, for example — rough idle, uneven pick-up, often with no warning lamp. Or no start: the starter stops dead with a thunk, or spins freely and nothing fires.
Stop cranking. Lock the crank and see whether both cam flats sit level; turn the engine by hand — if it only goes backwards, a valve is open against a piston. Exhaust valve clearances tell you whether valves are bent without lifting the head.
One tooth: re-time by the factory procedure with cam bars and fit new tensioners. A broken chain: chain, guide and tensioner if the valves survived, otherwise the head comes off.
Worn upper chains after new tensioners
Upper chains wear at the pins and stretch, and a chain that has run on bare metal is roughened. New tensioners on old chains can still tick, and wear new shoes faster.
A tick at idle or a cold rattle that stays after the tensioners have been done.
Look at how far the new tensioner has had to extend, and whether any guide has lost its plastic.
Worth doing: replace the upper chains when the tensioners have failed or the mileage is past about 100,000, and recheck the shoes after about 25,000 miles.
Cam timing set wrong during a tensioner job
The X308's cams are not keyed to their sprockets; the phase is set by clamping them against the locking tools. A new tensioner changes the chain path, so counting teeth is not enough — the cams have to be set exactly.
Lumpy running, a new tapping or the restricted performance warning straight after the chains or tensioners were done.
Go back over every step of the job, cam timing first. On a 4.0, make sure the VVT units and solenoids were not disturbed and the primary tensioner was primed.
Worth doing: re-time with the crank and both cams locked, rather than by marks on the chain.
The order to check them in
The point of the order is money: each step either rules a cause out or makes the next one worth doing. Green is free, amber costs a little, red is the one to arrive at last.
Find out which tensioners it has
Read the 10-digit engine number, not the VIN or the build date: 0108130000 or later has metal tensioners and the quieter primary chains. Earlier engines keep plastic ones unless invoices show otherwise. Some late 4.0 engines are reported to have left the factory with the second-generation type, so if in doubt, look.
freeListen on a cold start
After a night standing, bonnet open, listen at the front of the cam covers for a rasp like a serrated knife drawn over an edge. A stethoscope helps; a steady tick on top is usually the valvetrain.
freeIf it misfires or won't start, stop cranking
Check the cam flats and turn the engine by hand before anything else — a few more turns of the starter can bend valves that are still straight.
freeLook under the cam covers
Plastic or metal body, cracks, missing shoe material. If they're plastic, replace them while you're in there — they're cheap.
cam cover gasketsFit metal upper tensioners
The cable-tie method saves the timing tools, but only on an engine that hasn't jumped. Rags in the chain tunnel, cam caps released and refitted evenly in sequence, shorter bolts for the metal type.
about 3–6 hoursPast about 100,000 miles, do the whole front
All four chains, tensioners and guides, plus water pump and thermostat. Cam and crank locking tools; hold the damper, not the flywheel. On an early engine, a blow-by test under 30 L/min first rules out worn bores.
£700–1,100 at an independentWhat it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJ8 (X308) Won't Start, No Crank or Car DeadIf it just clicks and won't turn over, with no rattle beforehand, start with the electrics there.
- Jaguar XJ8 (X308) Cranks But Won't StartIf it spins over normally but won't fire after standing or a short run, rule out flooding and the fuel pumps there.
- Jaguar XJ8 (X308) Restricted PerformanceIf Restricted Performance comes up without a clunk or misfire on one bank, the throttle body and its plugs come first.
- Jaguar XJ8 (X308) MisfireIf the engine misfires but vacuum, compression and the cold-start sound are all normal, start with the coils there.
- Jaguar XJ8 (X308) OverheatingIf the engine is running hot — the water pump is worth doing while the front is apart.
- Jaguar XJ8 (X308) Wheel Bearing NoiseIf the noise follows road speed rather than engine revs, look at the wheel bearings.
- Jaguar VIN decoderConfirm your exact model year, engine and factory options before ordering parts — free.
Ratings, fuse numbers and torque figures vary by model year and market. Read the legend on your own car before pulling anything, and use the figures in the manual for your car, not the ones in a video.
Check your VIN: exactly which car you have and which bulletins apply to it.