Jaguar XJ40 Oil Leak
On the XJ40 most oil leaks start at the top and run down: the cam cover gasket at the rear right corner, or the cooler pipe O-rings under the oil filter. I'd clean the engine, find the highest wet point and fit the cheap seals before anyone mentions the rear main seal.
How bad is it · Garage, carefully
Drivable if the level holds; stop if it pours or smokes
A slow seep only needs the level watched. A cracked cam cover gasket can dump quarts onto the hot catalyst in a few miles.
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 cam cover gasket gone hard and cracked
The AJ6 sits tilted to the right, so oil pools at the rear right corner of the head above the gasket line. After years of heat the rubber shrinks and goes hard, then cracks or a section pulls away, and the leak can go from a seep to a stream in one drive. The oil runs onto the exhaust manifold and the catalyst below it.
Oil at the rear right of the cover and down the back of the engine; blue smoke from under the heat shield when warm, or at a standstill on a cold morning; a burning oil smell. A badly cracked gasket can lose a couple of quarts in a few miles. Oil dripping off the exhaust manifold is this gasket running down, not the manifold — a manifold leak gives smoke, not oil.
Reach up behind the head and feel along the rear corner of the cover. Lift the cover and look at the gasket: hard all round, with a split or a missing piece at the back, settles it. Loose cover screws give the same symptom on a smaller scale.
A new gasket with new half-moon seals and plug well seals. A torn gasket can be patched with oxygen-sensor-safe sealant just to get home. If the screws were simply loose, snugging them up may be all it needs.
A new cam cover gasket that leaks again
The cover seals on a rubber gasket and two rubber half-moons at the rear, not on clamping force. Non-genuine gaskets often don't fit the cover, and over-tightened screws split the gasket or distort the cover. Oil left in the blind screw holes locks the screw hydraulically, so it feels tight before it is home, or snaps.
A seep along the lower rear corner within days of the job; a new gasket that kept slipping out of the cover while fitting; a gasket split at the back; a leak at the half-moons or round the screw heads.
Look at the half-moons and the screw washers first. With the cover off, check it is flat on glass with feeler blades and that the faces have no burrs. Blow the screw holes clean and dry. If re-tightening doesn't stop a seep on a new gasket, take the cover off and look for swarf or grit in the gasket's channel and a flaw on its edge.
A genuine Jaguar gasket fitted dry, a dab of RTV on the half-moons only, new screw washers, and the screws taken down evenly just until the lip closes. The book figure is 10-12 Nm (7-9 lb ft); it is easy to misread in inch-pounds, and snug by hand with a good screwdriver is usually plenty. The kit is EAC2912, six EAC4049 plug well seals and two C30431 half-moons. A gasket that won't stay in the cover at the front bends goes in more easily softened in hot water or with a hair dryer, held with a smear of grease. Pattern gaskets harden early; a genuine one has stayed dry for 78,000 miles.
Oil in the spark plug wells from old plug well seals
Each plug tube is sealed to the cover by its own rubber ring. They compress and harden with age, and reusing them under a new main gasket leaves the wells leaking. Oil standing in the wells soaks the plug boots.
Oil on top of the cover round the plug boots, or wet plugs when you take them out; sometimes a misfire or a cold no-start once the boots are soaked.
Pull the plug leads and look down each well with a torch. Oil in one or more wells means the seals.
Fit all six plug well seals with the gasket, soak the oil out of the wells and clean the boots. Fix it soon: oil in the wells eventually kills plugs and boots.
The oil cooler pipe O-rings under the oil filter
Two cooler pipes plug into the adaptor block between the filter head and the engine block, each sealed by small O-rings. The rings harden with age and the leak can start suddenly, often the commonest sudden leak on these engines. The pipes run to a small separate oil cooler in front of the A/C condenser on earlier cars, and its hoses and unions weep too, so oil under the radiator can be the cooler rather than the engine.
A drip from the left side of the engine below the oil filter, running down onto the sump; a steady drip with the engine running. A drip that starts right after a steering rack change: the cooler pipes sit just above the rack and are easily disturbed.
Clean the area under the filter, run the engine and watch the joint where the two pipes go into the adaptor. Oil appearing there, rather than at the filter seal, points at the O-rings.
Take the filter off, undo the single nut on the clamping plate and pull the pipes down, then pick the old rings out; they often come out in pieces. They are 3x19 mm O-rings (CAC5118). Access is easier from above with the throttle body off, or from below without draining the sump. Take care not to cross-thread the nut in the aluminium.
The oil filter or the filter mount itself
The filter only needs to be hand tight; over-tightened, sealing on dirt, or fitted over the old seal stuck to the block, it can leak heavily. The filter mount bolts to the block and is sealed with a gasket on some years and sealant on others; refitted dry, it weeps.
A sudden fairly heavy leak around the filter, usually right after an oil change or after the mount has been disturbed.
Take the filter off: look for the old sealing ring on the block and for grit on the seat. If the mount has been off, look for oil at its joint to the block. Look also for a fine spray from the oil pressure sender beside the filter, and at the inner and outer gaskets of the filter adaptor, held by four bolts round the filter.
A fresh filter on a clean seat, tightened by hand. A mount refitted with RTV on surfaces cleaned with acetone, and the cooler O-rings changed while it is apart.
A blocked crankcase breather pushing oil out
The cam cover and crankcase are vented into the intake. If the breather pipe or the small heated connection at the intake elbow clogs, pressure builds and finds the weakest seal, so a new cam cover gasket starts leaking again.
A cam cover leak that comes back soon after a new gasket; oil forced out at the gasket or the rear head plate when the engine is revved; oil in the plug wells. Oil weeping out of the bottom of the throttle body after the car has stood cold for a week: oil vapour condenses in the intake and runs to the lowest point.
Pull the breather hose from the cover and check it and the connection on the intake are clear. With the filler vent covered, revving shows whether oil is being pushed out of a weak spot. Check the small water-heated breather connection on top of the throttle body — its hole is only about 1/8 in and chokes up; clear it with a fine screwdriver.
Clean the breather pipe and its connection on the intake, and renew the rubber hoses and seals in the breather system before fitting the new gasket.
The cover plate at the rear of the cylinder head
A plate on the back of the head blanks off the valve gear chamber; its top half covers oilways. Its sealant ages and oil weeps out of the seams, then runs down the back of the engine where it looks like a rear main seal.
Oil smoke rolling up from the back of the head after a drive; oil on the bell housing.
Make sure the oil is not the cam cover gasket running back first. Then clean the plate and watch its edges.
A fresh bead of sealant round the plate can be enough. To do it properly, cover EAC2911 with gasket EBC2568, easier with the cam cover off and RTV on both sides.
The oil gallery plug at the back of the block
The main oil gallery is closed at the rear by a push-in plug with an O-ring, held in by the gearbox adaptor plate. After decades the O-ring goes hard and oil leaks out at the engine to gearbox joint, exactly where a rear main seal would.
A drip at the joint between engine and gearbox, or from the back corner of the sump, just above the starter.
Rule out everything above it first: cam cover, rear head plate, breather. If the oil starts at the back of the block itself, the plug is a better bet than the crank seal.
With the gearbox, flex plate and adaptor plate off, pull the plug out with a bolt threaded into its centre, and fit a new O-ring. Gallery plug EAC 2494, O-ring EAC 2495. The rear main seal is worth doing while it is apart.
A seep that gets worse on thin synthetic oil
Thin synthetic oil finds its way past tired seals that a mineral 20W50 doesn't. It doesn't make a new leak, but it can turn a seep into drips on the drive.
A leak that grows after a change to synthetic, or that comes and goes with the oil used.
Note what oil went in at the last change and whether the leak started then.
Going back to a semi-synthetic or mineral 20W50 can stop a seep outright. It is a way to live with an old engine, not a repair.
The upper timing chain tensioner housing behind the distributor
The upper tensioner is hydraulic and fed with oil under pressure. Its housing is sealed by a gasket and an O-ring, and the small O-ring in its cover is easy to nick on assembly. A worn one can lose a lot of oil quickly.
Oil at the cast port next to the distributor, just below the cam cover line, dripping towards the power steering pump.
Wipe the area and watch the tensioner cover and housing with the engine running. Make sure it is engine oil and not steering fluid from the pump below.
Worth trying first: snug the cover bolts. Otherwise take the distributor out (mark the rotor and base first), lock the tensioner spring with an Allen key, and fit a new O-ring (EAC 6779 for the cover plug) and gasket. The notch must locate on the slipper tang when it goes back.
The head gasket weeping oil by the distributor
On earlier AJ6 engines the head gasket does not seal the oil feed to the camshafts well, at the front right corner near the distributor; the later gasket was changed to cure it. It is an oil weep, not a pressure loss, and often starts past 80,000 miles.
Oil misting or weeping from the head to block joint just behind the distributor, with no coolant loss and no change in running.
Rule out the upper tensioner housing right next to it first. If the oil clearly comes from the gasket line, it is the gasket. Re-torquing the head bolts doesn't cure it: on one engine they moved but the weep carried on.
Worth trying first: clean it and run a bead of RTV round the joint, or a stop-leak in the oil, which keeps some engines dry for years. The real cure is a current-type gasket set.
The oil filler tube seal on the side of the block
The filler tube is a push fit into a rubber seal in a housing bolted to the side of the block. The seal hardens and the housing joint weeps, and the oil runs down the block to the bell housing.
Oil on the side of the block and the bell housing with a dry cam cover; a wet patch round the base of the filler tube.
Clean the base of the filler tube and the housing, run the engine and watch both joints.
Renew the seal and reseal the housing to the block. Where a small drain tube from the housing is perished, it can be capped off.
The blanking plate after a self-levelling delete
Removing the self-levelling hydraulics leaves a plate over the pump drive on the timing cover. The cavity behind it isn't pressurised, but the shaft bushes are fed oil under pressure, so a plate fitted dry, or bolt holes that break into the timing cover, leak heavily.
A profuse leak behind where the hydraulic pump was, starting after the conversion.
Clean the plate and its bolts and watch which edge or bolt head wets first.
Refit the plate with a gasket and the sealant Jaguar specifies, and sealant on the bolts where their holes break through. If it still leaks, the auxiliary shaft seal behind it is next.
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.
Make sure it is engine oil
Dab the drip on white tissue. Brown or black is engine oil; red is gearbox fluid; green or yellowish is the mineral oil of the brakes, self-levelling and steering, which has its own causes.
freeCheck the level
Read the dipstick on level ground. An overfilled engine can push oil out at the back; take the excess out and see if the drip stops.
freeClean it and find where it starts
Wash the engine down with brake cleaner, run it, then drive and park over newspaper. Oil runs back and down, so look for the highest wet point, not the drip.
freeLook at the rear right of the cam cover and the plug wells
Feel along the back corner of the cover and the half-moons, and look down the plug wells with a torch. While there, pull the breather hose and check it is clear.
freeLook under the oil filter
Check the filter is hand tight on a clean seat, then watch the cooler pipe joint at the adaptor block with the engine running.
freeFit the cheap seals
Cooler pipe O-rings if the oil starts under the filter; a genuine cam cover gasket with half-moons and plug well seals if it starts at the cover. Clean the breather before the new gasket goes on.
two O-rings or a gasket kitCheck behind the distributor
Oil at the port next to the distributor is the upper tensioner housing first, the head gasket second.
an O-ring and gasketOnly then the back of the block
If the oil really starts at the engine to gearbox joint, the rear gallery plug O-ring is likelier than the rear main seal; both need the gearbox out.
gearbox offWhat it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJ40 Low Oil PressureIf the oil light or the gauge is the worry rather than the drip.
- Jaguar XJ40 Head Gasket Failure, Low CompressionIf the oil weep by the distributor comes with coolant loss.
- Jaguar XJ40 Power Steering Problems, Steering RackIf the fluid is green or red rather than engine oil.
- Jaguar VIN decoderConfirm your exact model year, engine and factory options before ordering parts — free.
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