Jaguar XJ40 Front Suspension Knock, Worn Bushes
On the XJ40 a front knock is nearly always rubber: the damper top and bottom bushes and the anti-roll bar bushes go first, then the wishbone and subframe bushes on high-mileage cars. I'd bounce the front, look at the damper tops and the anti-roll bar clamps, and only then put the car on a lift and lever the wishbone bushes.
How bad is it · Garage, carefully
Drivable, but find the knock soon
Most front knocks are worn rubber and get worse slowly. A damper top nut that has let go, a broken drop link or a worn ball joint, though, affects how the car steers and brakes.
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 damper bushes or a loose damper top
On the XJ40 the front knock is most often the rubber at either end of the damper. The foam top bushes perish and split, the lower bush in the wishbone wears oval, and the damper rod then hammers against its washers over every bump.
A clunk or rattle from one front corner over small bumps and rough ground, sometimes with that corner sitting a little low.
Bounce the front wing by hand and watch the damper top in the engine bay for movement. With the wheel off, light showing through the lower bush means it has had it.
Renew the top bushes and cup washers, and the lower bush separately since new dampers only come with the top ones. Tighten the top nut until the washers seat on the spacer tube; stay with genuine foam or Metalastic bushes rather than hard cheap ones.
Anti-roll bar bush out of its clamp or a broken drop link
The front anti-roll bar sits in rubber bushes held by U-shaped clamps on the subframe. A bush can work out of its clamp, leaving the bar knocking metal on metal, and a worn drop link gives a loud boing.
A heavy clunk on every small bump that may appear overnight, or a boing over humps and in hard turns, often louder turning one way.
Look at both U-clamps under the front for a missing or displaced bush and a gap around the bar, then shake each drop link and look for a broken end. Check the clamp brackets have not rusted away from the crossbeam.
Refit or renew the bushes, or fit a new link (CAC9827). Tighten the bar fittings with the car on its wheels so the rubber is not pre-twisted.
Lower wishbone and front subframe bushes
The lower wishbones and the front subframe both hang on bonded rubber bushes. Once they move, the knock comes over speed bumps and under braking, and the car starts to tramline.
Clicks and clunks over speed bumps and when braking hard, a knock felt through the footwell, and the car following ruts in the road.
On a lift, lever each bush with a long bar against a block. A bush that moves easily on the car is worn even if it looks fine; the subframe mounts sit near the exhaust downpipe.
Renew the lower wishbone and subframe bushes on both sides together, then have the alignment checked. New dampers often make worn bushes easier to hear rather than causing the knock.
A damper broken inside
A damper can lose a valve inside and still feel firm. It then clicks or clunks over corrugations even with good bushes.
The knock stays after the bushes are renewed and seems to come from one front corner from the moment you bought the car.
Don't trust a corner-bounce test, a broken damper can pass it. If the bushes are sound, the damper itself is the next suspect.
Fit a new pair of front dampers; the knock goes with the faulty unit.
Upper wishbone pivot bushes
The upper wishbone pivots on bushes held by one long bolt. Early cars have split two-piece bushes that wear quickly; later one-piece ones (CBC5523) press in and last much longer.
Clunks over potholes from one front corner, play felt when the wheel is rocked sideways, or an MOT note about the top wishbone bushes.
Put a pry bar against the upper arm at its pivots and watch for movement.
Renew the pivot bushes on both sides; the ball joint can stay connected. Seized old bushes come out by drilling the rubber and cutting the outer tube; press new ones in square, without heat or a hammer.
Front wheel bearing play
The front hub bearings are set with a nut to a small end float, not a preload. They lose adjustment and dry out, giving play at the wheel and a groan or roar in bends.
The wheel rocks when shaken top and bottom, or a groan that gets louder in a long sweeping bend as the bearing is loaded.
Jack the corner and rock the wheel at 12 and 6 o'clock; a dial gauge on the hub should read about 0.001-0.003 in of end float.
Repack with grease and set the nut with the wheel on: seat it while spinning, back off, then tighten until the rocking just goes. Re-check after a few hundred miles; hubs too hot to touch mean it is too tight.
Ball joints and track rod ends
Speed bumps load the wheels outward and wear the ball joints and track rod ends faster than miles alone would.
A clunk from one front corner that a renewed damper bush did not cure, sometimes with vague steering.
With the wheel off the ground, rock it at 3 and 9 o'clock while someone watches the track rod end, then lever the upright against the upper and lower joints.
Worth trying new track rod ends or the worn ball joint, with new Nyloc nuts.
Perished radiator support pads
The radiator sits on rubber pads at the bottom and is held by bushes at the top. When they perish the radiator bounces, and the body carries the noise so it sounds like the suspension.
A clang or boing from the front over bumps that the suspension work has not cured.
Grab the top of the radiator and see whether it moves in its mounts.
Worth trying new radiator support pads and top bushes, or packing around the top rods as a stopgap.
Engine or gearbox mount
A torn engine mount or a worn gearbox mount lets the drivetrain shift, which knocks through the body like a suspension part.
A clunk just as the car moves off or changes load, rather than as the body rises and falls over a bump.
Note exactly when the clunk comes: on take-off points to a mount, on bumps to a bush or damper.
Worth trying a new engine or gearbox mount if the clunk follows the throttle.
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.
Bounce the front and listen
Push each front wing down hard a few times with the bonnet open. Watch the damper top for movement and listen for which corner the knock comes from.
freeLook at the anti-roll bar
Under the front, check both U-clamps still hold their bushes and the bar is not touching metal, then check the drop links and brackets.
freeCheck the damper tops
From the engine bay, look for split or flattened foam bushes and a nut that is not down on its spacer tube. If the knock started with new dampers, check the washer order first.
freeShake each front wheel
Wheel off the ground: rocking top to bottom points to bearing end float or a ball joint, side to side to a track rod end.
freeLever the bushes on a lift
With a long bar, lever the lower damper bush, both wishbones at their pivots and the subframe mounts. Anything that moves easily is worn.
free with a liftSuspect the damper itself
If every bush is sound and the knock stays in one corner, the damper may be broken inside even though it passes a bounce test.
a pair of dampersLook beyond the suspension
Check the radiator for movement on its pads, and the engine and gearbox mounts if the clunk comes on take-off rather than on bumps.
freeWhat it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJ40 Rear Suspension ProblemsIf the knock comes from the back of the car, or the rear sits low.
- Jaguar XJ40 Power Steering Problems, Steering RackIf the clunk comes through the steering wheel rather than over bumps.
- Jaguar XJ40 Steering Wheel ShakeIf the steering wheel shimmies at speed or wobbles under braking.
- 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.