Jaguar XJ6 (X300) Suspension Knock, Rear Shocks
On the X300 a knock over bumps is most often the front shock top bushes, and next the rear shocks with their foam top mounts and spring isolators. I'd look at the front shock tops under the bonnet and bounce each corner before buying anything, then check the anti-roll bar and the rear A-frame bushes.
How bad is it · Garage, carefully
Usually fine to drive gently, but find it soon
Most of these knocks are worn rubber and only get louder. A snapped rear shock rod or a separated subframe mount upsets braking and handling — if a corner starts bouncing or the car clunks hard under braking, keep the speed down.
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.
Worn front shock top bushes
The front shock is clamped to the body through two small rubber or foam bushes. Once they crumble, the shock top hits the body panel over every bump. The knock travels, so it can seem to come from the steering column, under the seat or even the back of the car.
A hollow knock or squeak over bumps and potholes, worse on one side; the 10 mm top nut may spin with the shock rod. The clunk can seem to come from behind the instruments.
Open the bonnet and look at each shock top: a gap between the top washer and the body panel, or a bush squeezing out, gives it away. One side needs the coolant expansion tank lifted, the other the power steering reservoir moved. With the front jacked up, the metal top washer can show about 1/8 in of play against the body. If the nut spins with the rod, take nearly all the weight off the wheel so the rod is under tension, and hold it by the flats on its top.
New top bushes (MMD2144AA) on both sides, with the washers and spacer tube refitted exactly as they came off. Tighten the nut down onto the spacer tube with the lower arm on a jack; a squirt of grease only quietens a worn bush for a while. Tighten until the upper and lower plates clamp the ferrule, so the rod can't touch the body; tightening a bush that has already failed only squashes it.
Tired rear shocks, top mounts and spring isolators
The rear shock top sits in foam, and the spring sits on a foam isolator. The foam doesn't like the heat from the exhaust and crumbles away, so the shock top knocks and the corner drops.
A clunk from the back on every bump, one rear corner lower than the other, or the car wandering on rutted roads.
Measure the ride height at both rear arches. Rock the bumper corner: one return is fine, a second bounce means the shock is done. The shocks are sealed, so a dry-looking one can still be finished. An MOT pass proves nothing here: the shocks aren't flagged even with the rear clunking.
Rear shocks in pairs with new top foams, new upper spring isolators (MMD3565AA; CCC6875 or MJA2595AC on the XJR) and the lower spring seat CCC7350 if it's missing. If the rear still sits low, the springs have sagged — shocks don't lift the car.
A front shock at the end of its life
Inside each front shock is a bump stop. If the dust cover has come off, water rots it and the shock bottoms out with a clunk. Worn valving makes the ride harsh rather than soft.
A clunk only when the suspension is fully compressed — you can make it by pushing the corner down hard; or a harsh, crashing ride over potholes that's fine on the motorway.
Push each front corner down and let go. No give at all means the shock has frozen; more than one rebound means it's worn. A knock on compression points to the shock, one on extension to the ball joints.
New front shocks as a pair; genuine-spec Bilsteins bring the original ride back. Renew the top bushes at the same time.
Worn rear A-frame and subframe bushes
The whole rear suspension hangs from the body on big rubber bushes just behind the rear doors. When the rubber parts from its sleeve, the subframe touches its bracket and every bump or change of drive comes through the floor.
A clunk from the back over bumps or when you go from drive to reverse, a sideways shimmy in corners, sometimes a grind on acceleration that sounds like the differential. A shudder under braking that feels like warped front discs can come from here.
Look at the bushes behind the rear doors: cracked rubber or the alloy centre sitting off-centre is wear. Lever between the bracket and the subframe and watch for movement. A video of the rear cradle under braking shows it: perished bushes let it shift fore and aft.
New bushes both sides, fitted the right way round. They need a press and even pressure on the outer ring; a machine shop can press them if you take the parts in. New cross-member bushes with a pair of rear shocks can cure a braking shudder with nothing touched at the front.
Anti-roll bar bushes and a loose bracket
The front anti-roll bar sits in two rubber bushes held by brackets. A bracket screw that has backed off a fraction, or a bush worn oval, lets the bar knock as the suspension loads up.
A dull thud from low at the front, often felt through the pedal, as the car straightens after a turn; a light tap when you turn the wheel on jacks. A worn bar bush can sound as if it's at the shock, or even at the back of the car.
With the car on stands, try every bracket screw on the bar and look for a gap round the bar in each bush. You may see no movement and still have the knock. A bush can look pliable and uncracked and still be worn where the bar turns in it.
Tighten the bracket screws; renew the bar bushes if they've gone oval. Four are needed.
A snapped rear shock rod
The rear shock needs a swivelling spherical bearing at the bottom. Shocks made with a rigid lower bush, often with hard top rubbers, crack at the base of the rod — usually on the passenger side.
A sudden heavy rattle or bouncing at one rear corner, worse over potholes, the shock loose inside the spring, poor braking at that corner. It happens with aftermarket rear shocks too: the rod breaks near its top or the bottom eye lets go.
Bounce the rear corners in turn; one that keeps bouncing is suspect. Check what is fitted: the original rear shocks are Bilsteins with a spherical lower bearing.
Drive home gently and fit rear shocks with the spherical lower bearing, in pairs. Refit the dished top washer concave side up, as the factory did.
Something that isn't suspension at all
A lot of things bolted to the body knock like a suspension part. Loose radiator mounts, a loose engine mount or a coolant tank without its grommet are all easy to mistake for a shock.
A clunk at engine start and shut-down rather than over bumps; a knock from the bonnet area; a clunk under the seat; a rattle from the back with the boot full. A light click over uneven surfaces is often a wheel centre cap or the handbrake cable; a rear clunk can even be a loose tyre.
At the front: the radiator top isolators, the expansion tank grommet and the engine mounts. Underneath: the gearbox crossmember bracket and the exhaust hangers. At the back: the spare wheel bolt, the bumper mountings and anything loose in the boot. Tap each centre cap with a fingertip. Look at the handbrake cable where it splits at the Y near the rear axle, and the rubbers in the rear silencer brackets. Brake while driving over the same patch: if the click changes, it's the pads or dust shields — weak anti-rattle clips on aftermarket pads do it.
Snug the engine mount down, renew the radiator top isolators, refit the tank grommet, or secure the spare wheel. A loose radiator can rub through the A/C pipe beside it, so look there too.
Worn front lower shock bushes
The bottom of each front shock bolts through a bush pressed into the lower arm. It's small for the load it carries and wears quickly, especially if the bolt was tightened with the wheel hanging.
A clunk at full compression or over sharp bumps that stays after new top bushes. They fail quickly behind hard aftermarket top mounts or stiff shocks on soft springs.
On stands, lever the lower shock eye with a bar and watch for movement in the bush. Keep a jack under the lower arm — the shock limits its travel.
Worth trying: genuine Metalastik bushes, pressed out and in with a long bolt, washers and sockets without removing the spring. Tighten the bolt only with the corner jacked up to normal ride height. Without the factory tool, the old bush can be destroyed in place — rubber cut away, centre collar out, outer shell sawn and knocked free — and the new one pulled home on a length of threaded rod. Shocks with soft original top mounts make the new bush last.
Worn front wishbone bushes
The front upper and lower wishbones pivot on rubber bushes. Once the rubber is gone, the arm moves up and down and fore and aft, and the front clunks over bumps.
A front clunk over bumps, often on both sides; a lower bush visibly torn with rubber squeezing out; an MOT advisory on the lower inner bushes.
With the front on stands, lever each arm at its pivots with a bar and watch for movement; check the ball joints at the same time.
Worth trying: new bushes, then a front alignment — new bushes change the geometry even with the bolts marked. Tighten the pivots with the suspension loaded at ride height; tightened hanging, the bush twists past its limit over a bump and tears within a year. The top bushes can be done without a spring compressor, the bottom ones can't, and the rack has to be dropped to slide the lower bolt out; a machine shop will press them for a few dollars.
A dry or seized rear lower shock bearing
The rear shock's lower bearing has a rubber boot, so it looks like a bush. When it dries out or seizes it creaks and clunks over bumps. Other rear bushes can dry out and squeak the same way.
A creak or squeak from the back over bumps or railway lines, made by bouncing the rear.
Bounce the rear and listen at each corner. The bearing is hard to judge with the shock fitted.
Worth trying: penetrating oil and working the bearing free, and lubricating the rear bushes you can reach, before buying shocks.
Front subframe mounts
The engine and front suspension sit on a subframe held by bonded rubber Vee mounts above the springs. If the rubber parts from the steel, the subframe drops under braking until it hits a stop. It's more of a V12 problem, the heaviest car.
A clunk from the front under hard braking, or when you stop and release the brake; sometimes worse when cold.
Look at the Vee mounts above the front springs for rubber separated from the metal; only small safety straps then hold the subframe.
Worth trying: new Vee mounts, and the rear crossmember bushes checked at the same time.
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 the corner
Push each corner down hard and let go. A noise on compression points to the shock and its bushes. Remember the sound travels: a front shock top can sound like it's at the back.
freeLook at the front shock tops
Bonnet up: a gap between the top washer and the body panel, a crumbled bush or a nut turning with the rod is your answer.
freeBounce test each shock
One rebound is good; more than one is worn; no give at all is a frozen shock.
freeCheck the cheap things underneath
On stands: anti-roll bar brackets and bushes, front lower shock bushes, the rear A-frame bushes behind the rear doors, exhaust hangers, the spare wheel bolt and the boot. Lever the wishbones at their pivots too.
freeRenew the front top bushes
Cheap and quick, and they're the usual cause. Refit the washers and spacer tube as they came off.
a set of bushesRear shocks and isolators
Fit them in pairs, with new top foams and spring isolators, and only shocks with a spherical lower bearing. Soak the bolts in penetrating oil for days first; the lower shock bolts often have to be cut out.
parts plus about four hoursWhat it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJ6 (X300) Air Conditioning ProblemsIf the A/C stopped cooling after a front clunk — a loose radiator can rub through the A/C pipe.
- Jaguar XJ6 (X300) Rough Idle or High IdleIf the engine shakes or stumbles at idle rather than knocking over bumps.
- Jaguar XJ6 (X300) Vibration at Speed, Steering Wheel ShakeIf the car shakes at speed rather than knocking over bumps.
- Jaguar XJ6 (X300) Wheel Bearing NoiseIf the rear wheel rocks at 12 and 6 or there's a hum at speed.
- 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.