Jaguar XJ40 Head Gasket Failure, Low Compression
On the XJ40 a suspected head gasket is one of two things: the gasket letting coolant into a cylinder, or a burnt exhaust valve on one cylinder. I'd start with the coolant level, the plugs and a hot compression test, and test the coolant for combustion gas before taking the head off.
How bad is it · Garage, carefully
Short trips only, with an eye on the temperature gauge
A leaking gasket gets worse with every overheat and can fill a cylinder with coolant, which locks the engine on the starter. A burnt valve won't strand you at once but keeps getting worse.
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 head gasket letting coolant into a cylinder
On the AJ6 six the gasket weakens with age and heat cycles, usually somewhere past 100,000 miles. Once a sealing track gives way, combustion gas is forced into the water jacket and coolant is drawn into the cylinder as the engine cools. The cooling system runs over-pressure, the expansion tank seeps or spits, and the engine overheats at speed.
Coolant level falling with no outside leak; the expansion tank weeping at its seam or the cap; a steam-washed clean plug or piston top on one cylinder; white steam from the exhaust; a sweet or fuel-like smell; mayonnaise under the filler cap. Mayonnaise at the oil filler alone means little on a car used for short trips: that is condensation, and a long run and an oil change clear it.
With the engine cold, take the tank cap off, start it and watch for gas bubbles rising. A chemical block tester on the coolant changes colour if combustion gas is present, and catches small leaks that a compression test misses. After a hot compression test, look for steam coming out of a plug hole. Before taking the head off, look at the thin hose from the back of the water pump along the block to the metal water rail: it cooks in the block heat and loses coolant just the same. A gasket leaking outward from a water passage can pass both the compression and the gas test, because the coolant side runs at only about 15 psi.
Head off, faces cleaned and checked flat with a straight edge, a new gasket set with new head bolts, and the valve clearances reset while it is apart. Fresh coolant afterwards, then re-check every hose clamp that was disturbed.
A burnt exhaust valve
An exhaust valve that no longer closes fully can't shed its heat into the seat, and its edge burns away. Lean running from air leaks, tight valve clearances, cheap fuel driven hard and a diet of short trips all help it along. That cylinder loses compression while the coolant and oil stay clean.
A sudden miss or shaking idle, often right after a long motorway run; one cylinder reading from near zero to about 30 psi while the others are healthy; no coolant loss, no smoke, no blow-by.
Compression test first. Then a leak-down test with that piston at TDC on compression: air escaping into the exhaust points at the valve. A vacuum gauge needle that dips regularly as the bad valve works points the same way. Measure and note the valve clearances before the head comes off: exhaust valves gone tight tell you why it burnt.
Head off and to a machine shop. Replacing just the one burnt valve can be enough; on a high-mileage head it pays to do all of them, set the clearances with new shims and fit stem seals. Then look for the air leak or lean condition that cooked it. If a valve has a piece broken out of it, have every valve checked for fatigue cracks; if it looks melted instead, find the lean cylinder or air leak first, or the new valve burns too.
Corrosion of the head face round the water passages
Old or neglected coolant eats into the aluminium round the water passages, most often at the rear of the head. A new gasket laid over pitted metal leaks again, and the gasket can act as a one-way valve, pushing coolant into a cylinder when a hot engine is switched off.
Steam or white smoke on a hot restart with no bubbles in the tank; a gasket that fails again soon after being changed; pitting visible round the rear passages once the head is off.
With the head off, look closely at both faces around every water port, then lay a straight edge across the head with a feeler gauge. By the factory figure a head may be skimmed by no more than 0.010 in (0.25 mm).
Have the eroded spots fill-welded and the face skimmed flat; light pitting can be filled with an epoxy filler. Change the coolant regularly afterwards.
Something refitted wrong after the head was off
A head job disturbs a lot: the distributor, the cam timing, vacuum hoses, two earth eyelets on the inlet manifold. A new fault straight after the job is usually something not put back, not the new gasket. If the cam caps are only slackened instead of the cams being removed properly, the valve shims slide off their seats and the valves stay open.
A misfire, a no-start or no spark, or no compression on any cylinder, starting the first time the engine is turned after the head went back on.
Go over every vacuum hose and the two earth eyelets on the front top inlet manifold bolt. With No. 1 at TDC on its firing stroke, both valves closed, the cam timing tool should fit and the rotor point at the No. 1 lead; a rotor at the opposite post means it is 360 degrees out. Zero compression everywhere means valves held open: check the shims are in their pockets and the camshafts not swapped. A cracked water-heated restrictor on top of the inlet manifold, overtightened on refitting, draws coolant into every cylinder.
Reconnecting the hose or the earths, or moving the distributor one spline, has cured it outright. Shims that slipped are reset with the cams off; cams swapped or turned with the caps tight mean bent valves and the head off again.
A low reading that comes from the test, not the engine
A compression gauge only tells the truth if the test is done properly. A leaking adaptor, a cold engine or bores washed dry by flooding all pull the numbers down without anything being wrong with the head.
Readings that change from one test to the next; all cylinders low together after the engine has been flooded; one weak cylinder on a car that otherwise had no coolant loss.
Test hot, with all six plugs out and the throttle held wide open, and make sure the adaptor seals properly in the plug hole. Figures around 180-200 psi are healthy; the manual's dry minimum is 160 psi and evenness matters more than the number. A teaspoon of oil in a low cylinder that lifts the reading points at the rings, not the head.
Retest before pulling anything apart. If a single weak cylinder comes back normal, a blocked injector is worth cleaning; it can make that cylinder run as badly as a burnt valve.
Rainwater in the No. 6 plug well
No. 6 has the lowest plug well. Rain running along the underside of the bonnet drips into it, shorts the plug and looks exactly like coolant from the gasket.
Rough running, liquid standing in the No. 6 plug well that comes back after a short drive, coolant level steady, the car parked nose-down outside.
Dry the well, park the car nose-up for a few days and look again; keep watching the coolant level.
No repair: dry it out and park nose-up, or clear the scuttle drains.
The gasket blown between two neighbouring cylinders
The ring of gasket between two bores can burn through, so each cylinder leaks into the next. It often follows a moment of coolant loss, such as a hose bursting at speed, a few weeks earlier.
Popping and loss of power with two adjacent cylinders, often 4 and 5 or 5 and 6, reading less than half the rest or zero; steam from the exhaust at a standstill after a fast run, sometimes with no overheating at all. At worst a cloud of steam from the exhaust, then the starter stops dead against water standing in a cylinder.
A compression test: two neighbours both low or at zero, with the others normal, is the pattern. Two low cylinders that are not side by side point at valves instead. If the starter locks solid, take all the plugs out before cranking again: water doesn't compress.
New gasket set and new head bolts, with the head checked for flatness before it goes back.
An oil weep at the front right corner by the distributor
Early AJ6 gaskets have an oil passage seal at the front right corner that tends to weep, and the later gasket was changed to cure it. The upper timing chain tensioner behind the distributor runs under full oil pressure and its O-rings leak in the same place.
Oil running down from the head joint just behind the distributor, with no coolant loss and no change in how the engine runs.
Take the distributor off and look at the upper tensioner: if its O-rings are hard or broken, that is the cheaper answer. If the oil comes clearly from the gasket line itself, it is the gasket.
Worth trying first: new O-rings on the upper tensioner. If it really is the gasket, a current-type gasket set cures it; it rarely causes any other harm meanwhile.
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.
Watch the coolant
Note the level cold over a few days and look for wet hoses, a wet passenger footwell and drips. Cap off, engine running from cold: steady bubbles in the tank mean combustion gas in the coolant.
freeRead the plugs
Pull all six. One plug noticeably cleaner than the rest, or coolant in a plug hole, tells you which cylinder to look at.
freeCompression test, hot
All plugs out, throttle wide open, adaptor sealed. Look for one low cylinder (valve) or two low neighbours (gasket between them). Add a little oil to a low one and retest: a big rise points at the rings.
a gauge to buy or borrowTest the coolant for combustion gas
The chemical tester catches a small gasket leak that the gauge can't see. A positive result settles it for the head gasket.
a block tester kitLeak-down test on the suspect cylinder
Piston at TDC on compression and air in through the plug hole. Air in the exhaust means a valve; bubbles in the radiator mean the gasket.
a leak-down tester or a garageHead off and to a machine shop
Check flatness, inspect the water passages for erosion, and have valves, seats and stem seals done while it is off. New head bolts, valve clearances reset, every disturbed hose checked.
gasket set, bolts and machiningWhat it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJ40 MisfireIf compression is even on every cylinder and it still misses.
- Jaguar XJ40 Rough Idle and StallingIf the idle hunts but the compression figures are healthy.
- Jaguar XJ40 Cranks But Won't StartIf it cranks without firing and the gauge shows normal compression.
- Jaguar XJ40 Oil LeakIf the oil weep by the distributor comes without coolant loss.
- Jaguar XJ40 Overheating, Radiator and Cooling FansIf it runs hot or loses coolant without gas in the coolant: fans, hoses and the radiator.
- 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.
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