Jaguar XJ8 (X308) Misfire
On the X308 a misfire is nearly always one ignition coil, and the quickest proof is to swap the suspect coil to another cylinder and see whether the code follows it. If it doesn't, I look into the plug wells for oil or water, then for an air leak on the intake side and at that cylinder's injector.
How bad is it · Garage, carefully
Drive gently to a garage and keep off full throttle
The engine management cuts power to protect itself, so the car will still move but won't pull properly. On an XJR, stay off boost until it's sorted: a cylinder that's short of fuel under boost runs lean.
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.
A failing ignition coil
Each cylinder has its own coil sitting on the plug. They break down inside on the high-voltage side, or crack in the plastic or the rubber boot, and the spark tracks away to earth instead of jumping the plug gap. A failing coil is often intermittent at first, then constant.
A code for one cylinder, P0301 to P0308, often with a coil circuit code: P0351 to P0358 run coil A to H, so P0356 is coil F, cylinder 6. P1314 points to bank B and P1316 means the misfire is persistent. If several cylinders on one bank all show misfires, suspect the remaining coil that shares their monitoring line: one shorted coil can drag the others down with it.
Move the suspect coil to another cylinder, ideally on the other bank, clear the codes and drive. If the code moves with the coil, the coil is bad; if it stays put, look at that plug and connector. A meter reading proves little here, since the breakdown is in the secondary; an arc tester with the engine running tells you more. If you do measure, zero the meter with the leads touching first. A dead miss can hide above 1,000 rpm, so test at idle: unplug the suspect coil and listen for the idle to change. If a whole bank seems to do nothing, look at the ignition side before the injectors: on the 1998 engine each bank's coils are driven by a module near the brake servo, and its plugs and harness are worth checking.
Replace the coil that carries the fault, not the whole set, and only go further if others start setting codes. From the 2000 model year (AJ27) the coils are four-pin, earlier ones two-pin, so order the right type; cheap pattern coils for the older engine are a false economy.
An air leak on the intake side
Unmetered air leans the mixture and the engine management counts the rough combustion as misfires. On the XJR the rubber joints on the supercharger plenum perish, and a perished joint shows up hardest at speed and as the supercharger winds down. On any X308, a vacuum line refitted badly after throttle body work can stop the fuel pressure regulator doing its job.
Misfire codes on several cylinders at once (P0300 with others), or a miss that only appears once the engine is hot, with plugs, coils, fuel pressure and compression all fine.
Put a vacuum gauge on the inlet: a steady 17–21 inHg at idle is healthy, and noticeably lower points to a leak, compression or valve timing. Go along the hoses with an unlit propane torch or a little starting fluid and listen for the idle changing. If the throttle body has been off, check the small vacuum line to the fuel pressure regulator first.
Renew the perished plenum rubbers on an XJR, or remake the leaking vacuum connection with fresh hose.
A mechanical fault: the head gasket or a timing chain
If a cylinder loses its seal or a camshaft stops turning, no coil or injector will cure the miss. A head gasket can fail on the AJ-V8 without warping the head or the block, and a stopped exhaust cam on one bank, from a failed secondary chain, gives the same weak, rough running.
Rough from the moment it starts, or a whole bank that seems to do nothing, and no change after coils, plugs and injectors.
A compression test before anything is taken apart; a cylinder or a bank well down on the others points inside the engine.
A new head gasket. With the engine apart, renew the chain tensioners and belts too, as the originals are prone to failure.
Oil or water in the plug wells
The plugs sit at the bottom of deep wells in the cam covers, sealed by rubber rings. When those seals harden, oil seeps into the wells and soaks the coil boots; oil itself doesn't conduct, but it ruins the boots and coils over time. Water from an engine wash sits in the same wells and shorts the spark straight away.
Restricted Performance after washing the engine or after a service, or a boot that comes out wet with oil when you pull a coil. Damp alone can do it: after days standing in heavy rain the engine may shake at idle and then clear after a long run.
Pull the coils and look down each well with a torch. Before taking a plug out, vacuum the well clean so nothing drops into the cylinder; a length of heater hose taped to a shop vacuum does the job.
Dry the wells and coils out, and replace the cam cover gaskets and plug-hole seals at least on the leaking bank (the supercharged AJ27 set differs from the AJ26 one). Don't wash a hot engine or use a pressure washer on it.
An injector or the fuel supply
The engine management counts a weak burn from bad fuelling as a misfire, just as it does a missing spark. An injector can still click and deliver the wrong amount of fuel. On the XJR a second fuel pump joins in for heavy throttle at high speed, and a weak one leaves the engine short just when it's needed.
The misfire stays on the same cylinder after a coil swap and new plugs, or it only appears at high speed or high revs under load. An occasional cold miss with no warning lamp points away from the electrics and towards fuel or mechanics, because a real misfire lights the lamp.
Swap that cylinder's injector with a neighbour and see if the misfire follows. A noid light shows whether the ECU is pulsing it (about 3 ms at idle, too short for a voltmeter); a stethoscope tells you it's working. On an XJR, check fuel pressure at high revs, not only at idle.
Worth trying injector cleaner first; replace the injector if the misfire moves with it.
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 pending ones
Note which cylinder (P030x) and which coil letter (P035x) is named, then clear them and see what comes back: there's usually one real fault and the others are knock-on. Codes like P1111 or a bus communication code aren't where to start.
free with a code readerLook in the plug wells
Take the air trunking off to reach the coils (mind the fragile breather pipe), pull the coils and check each well for oil or water. Dry out anything wet before going further.
freeSwap the coil
Move the coil from the misfiring cylinder to one on the other bank and drive. If the fault follows the coil, replace that coil. If several cylinders on one bank misfire together, swap the remaining coil on that bank first.
freeCheck the plugs
Look at the colour of the plug from the bad cylinder and set the gap to 1.0–1.1 mm (0.039–0.043 in). Stick to the heat range in the handbook; a colder plug is no cure for a rough idle.
a set of plugsHunt for an air leak
Gauge on the inlet, then go along the hoses for a change in idle. On an XJR, look closely at the supercharger plenum rubbers; on any car with recent throttle body work, the regulator vacuum line.
a vacuum gaugeSwap the injector and test compression
If the misfire still won't move, swap the injector and check its pulse, then compare compression on that cylinder with the rest. Low vacuum and low compression together point at valve timing; on these engines check the chain tensioners.
a noid light and a compression gaugeWhat it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJ8 (X308) Timing Chain Tensioner ProblemsIf vacuum and compression are low, or the misfire came on suddenly with a rattle, check the timing chain next.
- Jaguar XJ8 (X308) Rough Idle and StallingIf there's no cylinder code and the idle hunts or the engine dies when you lift off.
- Jaguar XJ8 (X308) Won't Start, No Crank or Car DeadIf it won't turn over or start at all rather than running rough, start there.
- XJ8 (X308) fuse and relay boxesIf the code reader goes dead when you plug it in — the diagnostic socket has its own fuse in the rear.
- Jaguar XJ8 (X308) Check Engine Light, Fault CodesIf the code isn't a misfire: O2 sensor, lean, evaporative or catalyst codes.
- 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.