Jaguar XJ6 (X300) Misfire, Ignition Coil Failure
On the X300 a misfire is usually one of the six coils on the plugs, and after that the plugs themselves or oil in the plug wells. I'd find the dead cylinder by unplugging the coils at idle, swap the suspect coil to see if the fault moves, and only then look at injectors, sensors and compression.
How bad is it · Garage, carefully
Drive gently to a garage, not on a long trip
A steady miss pumps unburnt fuel into the exhaust. If the engine lamp flashes, stop: the catalysts can overheat, and a misfire can also push the gearbox into limp mode.
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. The epoxy body cracks with age and heat, and the spark tracks to earth instead of the plug. It often goes suddenly, and a weak coil may only miss under load or when hot.
Shaking idle as if on five cylinders, a stumble pulling away, a P0301–P0306 code, or a flashing engine lamp. A coil can fail without setting any code.
At idle with the coil cover off, unplug the coil connectors one by one: the one that makes no difference is the dead cylinder. Swap that coil with a neighbour, clear the codes, and see if the fault moves.
Replace the coil that carries the fault. Genuine or good used original coils last; cheap aftermarket ones can fail within months. Beware new coils that copy the original look: unmarked units in generic boxes have misfired from new, and good used originals cured it.
Wrong, worn or badly gapped spark plugs
The AJ16 is fussy about plugs. A plug one heat range too cold, or a gap that has opened up, gives a lumpy idle and loads the coils.
The misfire started right after new plugs, or the old plugs are brown, sooty and well past their change interval.
Pull all six and compare colour cylinder by cylinder. Set the gap to the manual figure of 0.9 mm (0.035 in) and fit the same grade in all six. On the supercharged XJR, for example, NGK BKR7E proved too cold and Denso K20PR-U11 cured it. Early 4.0 cars were listed with a plug that proved too cold, and some parts books still list it, so check what was fitted against the later recommendation. As a quick test, close the gap a little: if the miss eases, a coil or its boot is breaking down at the normal gap.
A fresh set of the right heat range, correctly gapped, with anti-seize kept thin and off the insulator.
Oil or water in the plug wells
The plug wells sit inside the cam cover. When the well seals or cam cover gasket leak, oil pools around the plug and the coil boot, and the spark arcs through it. Water after washing or standing does the same.
A coil comes out wet with oil, burn marks on the boots, or the misfire started after the engine was washed or stood for weeks.
Lift each coil and look down the well. Oil means the seals; water means dry everything and look for where it gets in. The plug-well seals are separate rings and are not always in the cam cover gasket kit, so a new gasket alone can leave the wells filling with oil.
New cam cover gasket and well seals, cleaned coils and boots, and a new coil only for any that are burnt.
An injector plug not seated
The injector plugs sit in a tight loom under the inlet manifold. A plug whose clip has locked short barely touches the pins and drops the injector at random.
One cylinder skips at a warm idle, the lamp flashes, then it runs fine after a restart.
Wiggle the injector harness with the engine idling and listen for the miss. Unplug, clean and push each plug fully home until the clip locks.
Reseat and lock every injector plug. It costs nothing, so do it before buying a coil.
A clogged or sticking injector
Dirt blocks the small filter basket inside the injector, or the injector stops opening on short pulses. The cylinder runs lean at idle while the coil and plug are fine.
Misfire only at idle in neutral, smooth at any throttle; or a steady miss with no lamp after the car stood with old fuel. A misfire with smoke on a cold start, after the car has stood, points to an injector leaking fuel into the cylinder after shutdown.
Listen to each injector with a screwdriver for a steady click. Pull the plugs after a run: a dry plug points to no fuel, a wet one to no spark.
Have the set cleaned and flow-tested by a specialist and refit a matched set; replace any that won't respond.
A failing crank position sensor
The ECU times spark and fuel from this sensor at the front of the engine. A sensor that drops pulses, or wiring chafed by the belt, makes the engine miss and cut out without pointing at one cylinder.
Random misfire across cylinders, worse when hot or in the wet, sometimes cutting out over a few seconds.
Look at the sensor lead near the crank pulley for bare or chafed wire. A meter only catches a dead sensor; fitting a known-good one is the real test.
A new sensor on its bracket, with the lead routed clear of the belt.
Rear O2 sensors on the wrong banks
After exhaust work the sensor plugs can be crossed. The ECU then trims fuel on the wrong bank, and it feels like a cylinder dropping out.
Miss at idle and light load, surging at 25–40 mph, no lamp and no codes, and it all began after exhaust work.
Trace each sensor lead to its plug and confirm it goes to its own bank.
Swap the plugs back. No reset is needed.
A worn coil connector or coil wiring
Each coil has a two-pin plug: one pin is a feed common to all six coils, the other goes to the switching stage in the engine computer. Heat on top of the engine hardens the plug and the wires behind it, and a plug that is not fully home after a plug change makes contact only some of the time.
The miss stays on the same cylinder even after the coil has been moved, or comes and goes with a bump or with heat; the plug on the rearmost cylinder is the hardest to push home.
Unplug, inspect and reseat every coil plug until it clicks. If a known-good coil still misses on one cylinder, check the two wires from that plug back for breaks and a good contact at each pin.
Worth trying before buying more coils: reseat the plugs, and replace a hardened plug with a new pigtail if it will not hold.
An air leak after the air meter
Air that gets in past the air meter isn't measured. The mixture goes lean and one cylinder stumbles at idle, often only once warm.
A lean code such as P1176, a 'bump-bump' at warm idle, or fuel trims far from zero.
Tighten every duct clamp and screw between the air meter and the throttle body, and the inlet manifold bolts. A smoke test finds what you can't see.
Worth trying first: snug the ducting and manifold bolts and give the trims a drive to settle.
A sticking EGR valve
An EGR valve that sticks part-open lets exhaust into the inlet at idle. That dilutes the charge and gives a misfire with no single cylinder to blame. These engines had an EGR recall.
Rough idle, hard starting and low-speed misfire that comes and goes, with several misfire codes or none.
Ask whether the EGR recall was done. Take the valve off and look for carbon holding it open.
Worth trying: clean the valve, or replace it, once coils and plugs are cleared.
A burnt exhaust valve
After long fast running an exhaust valve can stop seating and burn. Compression on that cylinder drops and no coil will fix it.
A bad miss that started after a long run at high speed, a new coil changes nothing, and the miss feels smaller at high revs.
Compression test, then a leak-down test on the low cylinder before any big spend.
Worth knowing: the cure is a valve job with the head off; have the injectors tested at the same time.
An exhaust leak ahead of the O2 sensors
A small crack in an exhaust manifold or a blown gasket draws in air ahead of the O2 sensor. The sensor then reads the mixture wrong and the computer trims the fuel to match, which feels like an occasional miss at idle; a crack can close up once the engine is hot.
An idle miss with no lamp that remains after plugs, coils and cleaning, and a ticking from the manifold when cold.
Listen and feel around the manifold joints on a cold start; look for soot marks at the gaskets and cracks across the manifold.
Worth knowing once the usual suspects are cleared: repair the leak before replacing O2 sensors.
A faulty coolant temperature sensor
There is no separate cold-start injector: the computer lengthens the fuel pulse from the coolant sensor reading. A sensor that reads warm when the engine is cold leaves it short of fuel, which feels like a misfire.
Roughness and hesitation only when cold, fading as the engine warms, often with no code.
With a scanner, compare the coolant temperature the computer sees with the real temperature of a cold engine. Use the two-wire sensor that feeds the computer, not the single-wire one for the gauge.
Worth trying for a cold-only miss: the sensor is cheap and quick to change.
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
P0301–P0306 name the cylinder; P0300 is random. On cars outside North America the lamp may not light for a misfire, so read the codes anyway. A flashing lamp means stop: the catalysts can overheat.
a code readerFind the dead cylinder
Coil cover off, engine idling. Unplug the coils one at a time; the one that changes nothing is the dead cylinder.
freeSwap the coil
Move the suspect coil to another cylinder and clear the codes. If the code follows the coil, buy a coil. Look for cracks or arcing in the dark.
freePull the plugs and look down the wells
Same grade in all six, gap 0.9 mm. Oil or water in a well needs fixing before new coils go in.
a set of plugsReseat the injector plugs
Unplug, clean and push each one home until the clip locks. Wiggle the loom at idle to catch a loose one. Do the same for the six coil plugs: a hardened or half-seated one gives the same intermittent miss as a weak coil.
freeCheck for air leaks
Ducting after the air meter, the throttle body joint and the inlet manifold bolts.
freeCheck the crank sensor lead
Look for chafing at the front of the engine, especially if the miss is worse in the wet.
free, or a sensorCompression test
Do it before buying injectors or pulling the head.
a gauge or a garageInjectors tested
If it misses at idle only and everything above is clean, have the injectors cleaned and flow-tested.
specialist cleaningWhat it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJ6 (X300) Rough Idle or High IdleIf the idle is high or hunting rather than missing on one cylinder.
- Jaguar XJ6 (X300) Fuel Pump ProblemsIf it loses power on every cylinder under load rather than missing.
- Jaguar XJ6 (X300) Cranks But Won't StartIf the miss turned into a car that spins over but won't fire.
- Jaguar XJ6 (X300) Check Engine Light, Fault CodesIf the lamp is on with O2, EGR, air pump or fuel cap codes rather than a misfire.
- Jaguar XJ6 (X300) Instrument Cluster, Dash LightsIf the dash lamps flash and the gauges jump while the engine misses.
- Jaguar XJ6 (X300) Oil LeakOil in the plug wells comes from the cam cover seals, and how to stop it.
- 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.