Jaguar XK8 Misfire, Coil Packs and Spark Plugs
On the XK8 a misfire is most often a weak coil-on-plug unit or water or oil sitting in the plug wells. I'd read the codes, look down the wells and swap the suspect coil with its neighbour before buying anything; plugs, charging voltage and the engine computer come after. An uneven idle with lean codes is more often an air leak at a split breather or intake tube.
How bad is it · Garage, carefully
Drive gently and get it sorted soon
Unburnt fuel from a dead cylinder overheats the catalytic converters and can break them up inside. A short, gentle drive is fine; hard driving with a known misfire is not.
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.
Weak or failed coil-on-plug units
Each cylinder has its own coil sitting on the plug, and they weaken with heat and age. A tired coil often misfires only under load or once the engine is hot, so the car can feel fine at idle and on a gentle drive.
A miss under hard acceleration around 3500–4000 rpm, an uneven idle when hot, or rough gear changes. Codes P0301–P0308 name the cylinder; P0351–P0358 point at a coil circuit. Often there's no lamp at all. The code doesn't always name the right cylinder, so look at its neighbours too. Misfires logged on a whole bank can come from the coil group: the four coils of a bank share a supply relay, an earth and a feedback wire, and one shorted coil can pull that feedback wire down from 5 V to about 1.3 V. Misfires on both banks at once point at one of the two ignition modules.
Swap the suspect coil with a neighbour, clear the codes and drive. If the misfire moves with the coil, you've found it. With no code, renew coils one at a time and note which change smooths it. Read the codes with the battery fully charged. Without a scanner, with the engine running, unplug each coil's feed in turn: the one that makes no difference is the dead one.
Replace the failed coils with a genuine part — for example AJ810455, metal-bodied, four pins — at roughly £29 from a parts supplier against about £56 at a dealer. At high mileage, with two or three gone already, I'd do the whole set at once. If a misfire keeps coming back after renewing coils one at a time, do all eight together with the plugs.
Water or oil standing in the plug wells
The coils sit in deep wells under covers. With a cover missing, half its screws gone or a cam cover gasket weeping, water or oil collects round the coil and shorts it.
The misfire started after the engine bay was washed, after heavy rain, or after cam cover work. A cylinder code such as P0308 on a car that has been standing. Also after the engine has been steam-cleaned, or a reduced-power message on a cold start that clears as the engine warms.
Lift the coil covers and the coils and look down each well. Soak up any water or oil with a paper towel on a screwdriver before anything else.
Dry the wells, renew coils and plugs that have sat in water, then refit the covers with every screw. If it's oil, the cam cover gasket and plug tube seals need doing too. A battery reset clears the stored messages afterwards.
Worn, wrong or badly fitted spark plugs
These engines are fussy about plugs. Old plugs break down, a brand that doesn't suit can stumble even when new, and too much grease in the coil boot can stop the spark until it warms up.
A stumble at low revs or a soft miss at cruise with no codes, or misfire codes on several cylinders at once. Trouble that starts straight after a plug change.
Check the type, gap and torque: NGK PFR5G-11E gapped 0.039–0.043 in on the 4.0, PFR6G-13E at 0.047–0.051 in on the supercharged engine; 25–29 Nm for the plug, 4–6 Nm for the coil bolts. Beware counterfeit NGK plugs with a thicker centre electrode. If the idle stays poor on the 4.0, Jaguar later widened the gap to about 0.052 in.
Fit a fresh set of the right plugs; a change of brand can make a stubborn stumble vanish completely. A fuel trim reset with the dealer tester may be needed afterwards. A car that only does short runs benefits from a long drive at decent revs.
An air leak after the airflow sensor, or a blocked breather
Air that gets in after the airflow sensor isn't measured, so the engine runs lean, worst at idle where a leak is a big share of the air. On this engine the brittle plastic breather tubes, the concertina intake tube and the hard vacuum lines are the usual leaks. A plugged part-load breather in the left cam cover also builds crankcase pressure, enough to push out the rear main seal.
An uneven or hunting idle, a stumble or stall when hot, and lean codes P0171 and P0174, often with Restricted Performance for a few seconds that clears as you pull away. A hiss that changes when you press on a hose.
On a scanner, compare fuel trims bank to bank: high at idle and near zero at higher revs points to a leak, and combined trims beyond 7% either way count as a fault. Feel and listen round the breather tubes from both cam covers, the underside of the concertina intake tube and the vacuum lines below the throttle body, or have it smoke-tested. Push a straightened paperclip through the part-load breather at the front of the left cam cover. On the supercharged 4.0 check the bolts of the elbow into the supercharger, which work loose: a lumpy cold idle first, later barely driveable off boost. Take the flexible intake section near the supercharger off to look — it can crack with pieces missing and still look whole in place.
Renew the split part, for example the left-hand breather vent tube AJ84880 or the intake tube, and refit a loose regulator vacuum hose. Clear the part-load breather as in TSB 600-03. If no leak turns up, clean the airflow sensor with dedicated sensor cleaner, spray only, and wipe the throttle body. On the XKR, check the duct seals behind the supercharger are seated. Refit the elbow bolts with locking washers and mild thread-locker; a cracked corrugated intake pipe is about $70 from a dealer.
Cam timing slipped a tooth
Worn timing chain tensioners can let a cam sprocket jump. The whole bank then runs late, which shows up as a misfire that new ignition parts don't touch.
A misfire on one bank, low compression on that bank only, or a miss that appeared after timing work. All four cylinders of one bank logging misfires together, often with P1316 and Restricted Performance, coming back a few minutes after the codes are cleared, while the coils test fine.
Compression-test both banks and compare; if one is down, check the cam timing before spending more on coils.
Renew tensioners, guides and chains and reset the timing; caught at one tooth, the engine survives. Swap coils between banks first to rule them out, but don't keep driving once the timing is suspect — another tooth can bend valves. Fit the later metal-bodied tensioners if the originals are still in.
Low voltage from a failing alternator
The engine computer and the coils need steady voltage. A dying alternator can make the engine feel as if it's missing on several cylinders and bring up a reduced-power message.
Rough running that comes and goes, sometimes with an odd noise from the front of the engine, and no single cylinder to blame.
Put a meter across the battery with the engine running before you buy any ignition parts. A low or wandering reading points here.
Replace the alternator; the misfire goes with it.
A broken wire at the airflow sensor
One of the five wires near the airflow sensor plug can break inside its insulation, especially where an aftermarket cone filter pinches the loom against the cam cover.
A sudden stumble and cutting out on acceleration, barely running at idle, with P0103.
Pull gently on each wire behind the sensor plug and test it end to end; the insulation can look perfect over a broken core.
Solder and heat-shrink the wire, and put the factory air box back so it can't pinch again.
A failed knock sensor
Each bank has a knock sensor in the V under the intake manifold. A fault in the sensor or its wire makes the engine computer fall back to a safe map, so the engine stutters under load and the dash shows Restricted Performance.
A sputter for a second or two pulling away hard, Restricted Performance, codes P0327 or P0328; traction control may drop out at the same time.
Read which bank the code names and check that sensor's plug and wire before buying the part.
Replace the sensor on that bank. The part is cheap, but the upper intake has to come off; on the XKR the job runs well past the book time. Tie a string to the plug before pulling the old lead out so the new one can be drawn back through.
The engine ECU, when the codes never move
On early cars the capacitors on the engine computer board can leak and eat the tracks, so it stops driving some coils properly.
Coil codes such as P0352, P0353, P0355, P0358 that stay on the same cylinders after coils, plugs, ignition modules and even the throttle body have been swapped.
Only when every swap has failed to move the fault. The damage is often visible only under a magnifier.
Send the ECU away for repair and refit it; it can run perfectly afterwards.
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.
Read the codes, including the stored ones
P0301–P0308 name the misfiring cylinder; P0351–P0354 are the bank A coils and P0355–P0358 bank B. No code doesn't clear the coils — the lamp only lights when the misfire could damage the catalysts.
freeLook down the plug wells
Lift the covers and the coils. Water or oil in a well is the answer before any part is bought; check the covers have all their screws.
freeCheck the charging voltage
A meter on the battery with the engine running. Low voltage can imitate a misfire on several cylinders.
freeListen for air leaks and clear the breather
If it idles unevenly with lean codes, check the breather tubes, the intake tube concertina and the vacuum lines for splits, and clear the part-load breather on the left cam cover. A split tube costs far less than an airflow sensor.
free to a hoseSwap the suspect coil with a neighbour
Clear the codes and drive until it comes back. If the code moves with the coil, buy that coil; if it stays, look at the plug and the connector on that cylinder.
freeFit the right plugs, correctly
Twist each boot to free it from the plug, use a fingertip of grease and no more, and torque to 25–29 Nm. If a coil bolt spins, the brass insert in the cam cover has come loose — refit it with epoxy rather than forcing it.
a set of eightRenew the weak coils, or the set
Genuine coils, not unbranded copies. At high mileage I'd change all eight rather than chase them one by one.
about £29 eachOnly then compression, timing and the ECU
If the fault stays on the same cylinders whatever you swap, compression-test both banks and then have the engine computer looked at.
workshop timeThe fuses named above — where they sit
The highlighted position is the one the procedure talks about. Pulling a fuse is a stop-gap, never a repair: check what the circuit feeds before you pull it.
Circuit descriptions from the Jaguar fuse tables, 2000 edition. Drawing by jagaux.com from the factory electrical guide. Fuse layouts change between model years — read the legend on the lid of your own box before pulling anything.
What it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XK8 Restricted Performance, Limp ModeIf the misfire brings up Restricted Performance and a power limit.
- Jaguar XK8 Engine Failsafe ModeIf the message says FAILSAFE ENGINE MODE and cruise control drops out.
- Jaguar XK8 Timing Chain Tensioner ProblemsIf one bank misfires and compression is down on that side.
- Jaguar XK8 Oil LeakIf there's oil in the plug wells — the cam cover and plug tube seals.
- XK8 (X100) fuse box diagramsThe ignition coils supply fuse, drawn.
- 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.