Jaguar XJ6 Series 3 Misfire, Rough Running
When a Series 3 runs on half its cylinders, it is usually one cylinder or one group of injectors, not the ignition. The six injectors work in two groups of three through a resistor pack, so three dead at once points at its plug or the earths; a single dead cylinder is usually its injector — even a new one that clicks — or its valve. I'd find which cylinders are dead before buying any ignition parts.
How bad is it · Garage, carefully
Drive gently, and not far
A cylinder that gets fuel but doesn't fire wastes it and does further damage — unplug its injector if you must drive. A heavy thump when cranking, with a fuel smell, means stop: fuel may be sitting in a cylinder.
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.
Half the engine dead: the injector resistor pack or its connector
The six injectors are not fed one by one: they work in two groups of three, 1-5-6 and 2-3-4, through a power resistor pack (DAC1211) and its multi-plug. Lose the feed to one group and three cylinders stop at once, with spark still on all six. A dirty or half-seated plug at the pack, or a poor earth at the rear bolt of the fuel rail, is enough.
Suddenly running on three cylinders and unable to idle, an hour after it ran perfectly; spark on every plug, but three injectors silent.
Engine idling, listen to each injector with a stethoscope or a long screwdriver — hot as well as cold. Unplug an injector and put a test lamp across its plug: it should flicker while cranking. Light on the good group and nothing on the other points at the pack, its plug or the earths; no flicker anywhere points at the ECU or its wiring. An injector reads about 2.4 ohms; never hold 12 V across one for long.
Unplug the resistor pack connector, clean the terminals and push it fully home; clean and tighten the earth wires at the rear of the fuel rail. A new pack only if the feed is still missing after that.
One injector stuck or leaking, even though it clicks
A clicking injector proves only that the solenoid moves. It can still dump fuel or barely open — a brand-new one included, and old ones stick after the car has stood for months. The cylinder drowns or starves while ignition and compression are perfect.
A miss that appears almost overnight and feels like five cylinders; one plug darker or sooty compared with the rest; unplugging that one injector at idle makes no difference.
With spark and compression proven, swap the suspect injector with one from a good cylinder. If the miss or the black plug follows it, you've found it; if it stays, the fault is in the feed to that cylinder.
A new injector for the faulty one — and test a new one before blaming everything else. Injectors stuck after a long lay-up can sometimes be freed with injector cleaner; a professional clean and flow test for the whole set often smooths the idle as well.
Rough only until warm: the airflow meter arcing, or the coolant sensor
On a cold engine the airflow meter's track can arc and send the ECU a false reading, so it over-fuels: misfiring, black smoke and no power above about 3000 rpm under acceleration, all clearing at once when warm. The coolant sensor that feeds the ECU does the same job from the other side — read wrong and the mixture is wrong until the engine reaches about 70–75 °C.
Cold start and idle normal, but stumbling and smoking when you pull away cold; uphill is hopeless until it warms up, then it suddenly runs clean.
Look under the rubber boot of the airflow meter plug for a capacitor fitted earlier — it can fail too. Measure the ECU's coolant sensor on the coolant rail (not the gauge sender): several kilohms cold, about 300 ohms hot. If it misfires with 300 ohms hot, the sensor is innocent — check fuel pressure and compression next.
The capacitor modification: a non-polarised capacitor across airflow meter terminals 6 (negative) and 8 (positive), soldered in — Jaguar's own bulletin fix. A new coolant sensor if its reading is wrong.
The fuel pressure regulator leaking fuel into the manifold
The regulator holds the rail at about 36 psi with a spring and a diaphragm; its vacuum pipe goes to the inlet manifold. A torn diaphragm sends raw fuel down that pipe into the engine. It runs rough and smells of fuel, and parked nose-down the fuel pools in cylinder 6 — enough liquid to lock the engine and blow the head gasket on the next crank.
Rough running soon after a new regulator went in; a strong fuel smell; a heavy mechanical thump when cranking.
Pull the vacuum hose off the regulator and look inside: any fuel in it, or fuel spurting from the port while cranking, condemns the regulator. A new one deserves the same check before it goes in.
A sound regulator. If it has thumped on cranking, stop: plugs out, crank to clear the fuel, and do a compression test before running it — a blown gasket next to cylinder 6 means the head comes off.
Unmetered air: a split duct or a vacuum hose off
The ECU fuels for the air the airflow meter measures. Air that gets in after the meter leans the mixture and gives a lumpy, hunting idle — metered air only raises the revs. The flexible duct between the airflow meter and the throttle body splits, and a pipe from the air distribution block can simply fall off.
A shaky, hunting idle that smooths out with a little throttle; rough for a few seconds after a hot restart; sometimes a pop from the intake.
Squeeze and flex the duct from the airflow meter to the throttle body and look for splits. Trace every vacuum pipe against the diagram. Nudging the airflow meter flap a little at idle smooths an engine that has a leak; a vacuum gauge on the regulator pipe should sit steady at about 18 inHg. Smoke blown into the intake shows the leak.
Reconnect or renew the hose, replace a split duct, then set the hot idle again — about 750–800 rpm in Park with the throttle gap correct.
One cylinder low on compression: a burnt valve or a tight clearance
A valve that no longer closes leaves one cylinder doing nothing while ignition and fuel are fine. On a high-mileage 4.2 a burnt exhaust valve is typical; on later cams a clearance closed up to 0.008 in or less is enough to kill a cylinder at idle.
Pulsing and hesitation under load, smooth when revved in Park; one cylinder far below the others on a gauge; pulling its lead changes nothing.
Compression test with the throttle held wide open: all six within about 15 percent. Repeat wet on the low one — no rise points to the valve, not the rings. A leak-down test that blows air out of the exhaust confirms a burnt valve. Check that cylinder's valve clearances: 0.012–0.014 in.
Worth doing: reset the clearances first. A burnt valve means the head off and a machine-shop valve job; don't keep driving on it — a broken valve can wreck the head and piston. Until then, unplug that cylinder's injector.
The whole engine stumbling hot: the ignition amplifier
This is not a one-cylinder miss: the spark weakens on all six. Inside the Lucas amplifier box sits a GM-type HEI module, and it fails with heat, so the car runs fine cold and stumbles or dies pulling away once warm. Coils that keep failing point the same way — or at an overcharging alternator.
Fine at speed, but dies pulling away when hot unless you feed the throttle in very gently; a new coil cures it for a while, then it comes back.
If the fault only shows hot, suspect the amplifier before another coil. Measure the coil's windings, look for oil weeping from it, and check the charging voltage at speed.
Worth trying: a genuine GM module in the amplifier box — AC Delco D1906, GM 10482820 (equivalents NAPA ECHTP45, Wells DR100, Niehoff DR400); cheap aftermarket modules tend to fail quickly.
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.
Find the dead cylinders
At idle, unplug each injector in turn: the revs should drop the same for every one. A cylinder that changes nothing is the dead one — three at once means a group of injectors, not the ignition.
freeRead the plugs
One black or wet plug points at that cylinder's injector. All six sooty points at the mixture: airflow meter, coolant sensor, fuel pressure.
freeListen to the injectors and test their feed
A stethoscope on each injector, then a test lamp across the plug of a silent one while cranking. No flicker on three: the resistor pack, its plug, the earths at the rear of the fuel rail.
freeSwap the suspect injector
Move it to a good cylinder. If the miss follows, replace it — new or not.
freeCheck spark on the suspect cylinder
A known-good plug with a wide gap on that lead should give a fat blue spark. If not, look at the lead, its terminal and the cap tower.
freeLook for air leaks and fuel in the regulator pipe
Flex the duct from the airflow meter to the throttle body; trace the vacuum pipes. Pull the regulator's vacuum hose: fuel inside means the regulator.
freeCompression test, dry and wet
Throttle wide open, all six within about 15 percent. A low one with no rise wet: valve clearance, then a leak-down test.
a gaugeOnly cold: the capacitor and the coolant sensor
Misfiring and smoking only until warm: check for the capacitor at the airflow meter plug and measure the ECU coolant sensor.
a capacitor or a sensorOnly hot, and the whole engine: the amplifier
If all six fade together once warm, fit a good GM module before another coil.
a moduleWhat it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJ6 Series 3 Won't Start, Cranks But Won't FireIf it won't fire at all, or dies and won't restart hot, start there.
- Jaguar XJ6 Series 3 Fuel Tank Switch, Changeover ValveIf it runs well on one tank and starves on the other.
- Jaguar XJ6 Series 3 Overheating, Losing CoolantIf it is losing coolant as well as missing, look at the head gasket there.
- XJ6 Series 3 fuse boxesThe fuse panels of the Series 3.
- 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.