Jaguar XJ6 (X300) Timing Chain Rattle, Tensioner
On the X300 six a rattle from the front of the engine is usually the early upper chain tensioner, which lets the top chain slap until oil pressure builds. The revised tensioner is a two-bolt job from the top of the engine. If the rattle stays, the guide rails and dampers have worn and the timing cover has to come off.
How bad is it · Garage, carefully
Drivable, but don't let a growing rattle run on
A short start-up rattle on its own rarely does harm. A rattle that gets louder, or a scraping inside the timing cover, means worn or broken guides, and a chain that jumps or breaks can wreck the engine.
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.
The early upper chain tensioner
The AJ16's double-row upper chain is kept taut by a hydraulic tensioner on the right front of the head, just above the camshaft position sensor. The early type, with a flat plate and an adjusting screw, doesn't hold its position once the engine stops, so the chain slaps until oil pressure catches up. The revised one has an arrow cast into its face and a ratchet that holds it.
A light metallic rattle for a few seconds after a cold start, sometimes again around 1500–2000 rpm under load.
Look at the tensioner: an arrow cast in its face is the revised type, a screw is the early one. A mechanic's stethoscope tells the top of the timing cover from lower down.
Fit the revised upper tensioner with a new gasket and O-ring. It's two 10 mm bolts, but the cam cover and the cam position sensor come off first: set number 1 at TDC and mark the sensor before removing it. Wind the piston in until it locks, fit with the arrow pointing up, and don't let anything drop into the timing chest.
A tensioner jammed shut, even a revised one
The tensioner piston can stick fully retracted with its ratchet pressed down, so it puts no load on the chain at all. Deep score marks on the piston are the giveaway. A blocked check-ball or oil feed starves it the same way.
A loud, constant chain noise, not just at start-up, on an engine that already has the revised tensioner.
Take the tensioner out and look at the piston and the small check-ball valve.
Clean and de-burr the piston, lightly hone the bore, clear the check-ball and the oil passage, fit a new O-ring and refit; or fit a new tensioner.
Worn guide rails, dampers and slippers
The chains themselves rarely stretch, but the plastic faces of the guide rails and dampers wear through, and the lower tensioner blade can bend or crack. Late oil changes and the wrong oil speed it up.
A rattle or scrape around 1500–2000 rpm that a new upper tensioner didn't cure; or a flopping noise on a cold engine at idle that fades above about 1100 rpm.
With the cam cover off you can see the top guide; the rest needs the timing cover off.
Replace all the rails, dampers and slippers, and clean or renew the lower tensioner; the chains can usually stay. The timing cover comes off without lowering the sump. It has no gasket — it's sealed with a sealant — and a new crankshaft seal goes on with it.
A broken slipper bracket or a loose tensioner bolt
The alloy bracket carrying the upper slipper is held by a bolt that can work loose. Then the bracket breaks and the slipper flops about inside the cover. Lower tensioner bolts fitted without locking compound can back out the same way.
Chain noise at start-up and a scraping at around 2000 rpm, as if the chain is touching the case.
Only visible with the timing cover off.
Fit a new bracket — one from an XJ40 engine has been used — and use locking compound on every bolt behind the cover.
Old or wrong-grade oil
The upper tensioner is fed through a very small oil passage. Old or wrong-grade oil delays the pressure it needs, so the start-up rattle lasts longer.
A rattle that changed after the last oil change, or one that comes and goes with the weather.
Note how long the rattle lasts on a cold start, then change the oil and filter and time it again.
Worth trying before any parts: fresh oil and filter. Some engines quieten on 20W-50, others on 10W-40 — the grade that suits yours is the one to keep.
Not the chain: the power steering pump drive
The power steering pump is driven from the engine through a dog coupling with small metal ears. They rattle at around 2000 rpm and sound exactly like a timing chain.
A rattle or buzz near 2000 rpm that new tensioners and guides didn't touch.
Test with the pump drive taken out of the picture before opening the timing cover.
Worth trying: a few drops of silicone on the ears of the drive dog.
Not the chain: a bearing on the belt side
The water pump, alternator, idler pulleys and A/C compressor all run on bearings at the front of the engine. A dry bearing rattles right next to the timing cover.
A rattle that starts as a belt squeal stops, or a knock near the water pump on a hot day at idle.
With belts removed, run the engine for a moment: if the noise goes, it's on the belt side. A stethoscope on each pulley finds which.
Worth trying: replace the noisy pulley, water pump or bearing.
Not the chain: a loose exhaust heat shield
The heat shields around the front catalysts and downpipes crack or come loose. They buzz at certain revs, under load, much like chain noise.
A buzz around 1500–2000 rpm under load, or a rattle only when the engine rocks in a sharp turn.
Tap the shields with the engine cold and look for cracks near the downpipe joints.
Worth trying: weld the shield or wedge heat-proof material into the gap.
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.
Note when it rattles
A few seconds after a cold start, a band around 1500–2000 rpm, hot only, or all the time. Each points somewhere different.
freeChange the oil and filter
Then time the cold-start rattle again.
an oil changeFind where it comes from
Top of the timing cover, lower down, or a pulley. Run it briefly with the belts off to rule out the belt side and the steering pump drive.
a stethoscopeCheck the oil pressure with a gauge
In place of the oil pressure switch, cold and hot, before spending on the timing gear.
a gaugeIdentify the upper tensioner
Right front of the head above the cam sensor. Arrow cast in the face is the revised type; an adjusting screw is the early one.
freeFit the revised upper tensioner
Cam cover and cam sensor off, number 1 at TDC, sensor marked. A couple of hours.
a tensioner and gasketStill rattling: the guides
Timing cover off; replace all rails, dampers and slippers at once and check every bolt behind the cover.
a big labour billWhat it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJ6 (X300) Oil LeakIf oil weeps from the upper tensioner housing.
- Jaguar XJ6 (X300) Low Oil PressureIf the rattle comes with a flickering oil lamp.
- Jaguar XJ6 (X300) Power Steering ProblemsIf the noise follows the steering pump.
- 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.