Jaguar XJS Misfire, Rough Idle
On the 4.0 six, a misfire or rough idle is usually a failing coil, aftermarket ones above all, or water in the ECU connector, and the two often turn up together. On the V12, look at the plugs, air leaks and the idle and throttle settings before anything costly. I'd start under the bonnet with a torch: plug wells, coil labels, the ECU plug.
How bad is it · Garage, carefully
Drive it gently while you find the cause
Most of these faults are cheap, but a stumble pulling away or a car that won't rev can leave you stuck in traffic. A misfire with smoke or a loud backfire wants looking at before more miles.
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 non-original ignition coil on the 4.0
The AJ16 six has a coil sitting on every plug. Aftermarket coils are the usual weak spot: one failing coil gives a stumble at idle and away from a stop, while the car feels perfectly normal at speed.
Irregular stumbles at idle or a jerk pulling off, smooth on the move, no warning lamp. The ammeter needle can twitch with each stumble, and a stored P1158 may turn up on a scan.
Look at what's fitted. Original coils wear a black 'made in Japan' sticker; swap any odd one for an original and see if the stumble goes.
Fit original coils throughout. A used original Lucas set can cost less than new aftermarket ones, for example six for about $100.
Cracked coils and worn plugs on the 4.0
Even original coils crack with age: the epoxy body splits and the spark tracks to earth before it reaches the plug, and worn plugs make a weak coil give up sooner. The AJ16 is slow to set a code for it.
A miss on one cylinder at idle that smooths out on throttle, no warning lamp, often turning up after a fill-up or a wash. Swapping two coils gives confusing results when both are weak.
Lift the coils and look for hairline cracks in the bodies, then run the engine in the dark and watch the coil tops for sparks. A resistance check proves little; a scope with a current probe is the real coil test. Pull and read the plugs on the misfiring bank.
New original-type coils on the cracked cylinders and a full set of plugs of the specified heat range; the engine runs smoothly on and off the throttle.
Water in the engine ECU connector
On the six-cylinder cars, rainwater follows a vacuum pipe where it passes the bulkhead, right-hand rear corner of the engine bay, and lands on the loom running to the ECU. The pins grow a bluish crust, and the injectors, idle control and ignition amplifier start losing their signal.
Very rough running at a standstill, often after rain or a wash, that may clear itself after a few hours. Hot, a whole group of cylinders can drop out, for example 4, 5 and 6 together.
Hot, pull the injector plugs one at a time: a working cylinder makes the idle dip clearly, a dead one changes nothing. Then unplug the ECU and look for blue deposit on the shell and pins.
Spray the pins with electrical contact cleaner, scrape them bright, then dielectric grease on the pins, the connector and the ECU earth. Look at it again every month or so. If cleaning doesn't stop heavy over-fuelling, the ECU itself may be damaged; a known-good ECU cured one car.
Oil or water standing in the plug wells
On the 4.0, oil spilled while topping up or water from washing can seep through the holes in the cam cover gasket into the spark plug wells. Liquid round the plug tops shorts the spark away, and the burning oil smokes.
Heavy misfire and bucking that starts straight after an oil change or a wash, with smoke from the engine bay.
Lift the coils off and look down each well. Some can be flooded so deep the plug is hidden while the rest are dry.
Push paper towel down each well until it comes up clean, wipe the plug tops, refit the coils. No parts.
Vacuum leaks at idle
Air that bypasses the meter leans the mixture most at idle. On the six, the plastic vacuum tees crack, one of them at the fuel pressure regulator, and even a brand new EGR valve can leak through its stem and seat. On the V12, injector seals leak.
A hiccup or hunting idle that disappears once you're moving.
Put a vacuum gauge on the manifold and smoke-test the hoses. A sealed hot engine pulls about 21 inHg at idle; a leaking EGR valve alone can cost around 4 inHg.
Brass tees in place of plastic, a sealed EGR port, sealed injectors. Then set the hot idle, for example 700 rpm in Park and 580 in Drive.
V12 idle set wrong, so it surges while warming
The V12's idle switch has to close as the pedal comes back. If the idle adjusting screw is wound too far out, the switch only makes at the very end, and the idle runs high and hunts. Low coolant also makes the auxiliary air valve oscillate.
Idle far too high, swinging up and down during warm-up; it isn't an ignition fault.
Top up the coolant first. Then check that the idle switch makes before the pedal is fully released.
Wind the idle screw back about one turn so the switch makes earlier, and seal any air leaks round the injectors. Nothing to buy. On early cars the idle is set at the extra air valve screw, fully warm: half a turn took one car from 950 to 750 rpm.
V12 won't rev once warm: throttle sensor wiring
Warm, the ECU trusts the throttle position sensor; cold, it ignores it. A short in melted or badly joined TPS wiring makes it see a shut throttle, so above about 1500 rpm it applies the overrun fuel cut.
Revs freely from cold, but once warm it won't go past about 1000 rpm and the injectors cut out.
Unplug the sensor or try another: if nothing changes, it's the wiring. Run three temporary wires from a spare sensor outside the loom; if it then revs, the loom is proved guilty.
Renew the three wires, soldered and heat-shrunk, and any melted ones nearby. Then set the sensor with the ignition on, back-probing red and yellow, to about 0.32 to 0.34 V.
Spark plugs cracked or shorting inside
A plug can look fine and still leak its spark to the body internally. A good spark at the distributor cap proves nothing about the plug. On a V12, several going at once leaves it crawling on a handful of cylinders.
After standing, it starts reluctantly and runs as if on four cylinders, with good fuel pressure and spark at the cap. A single cracked plug gives a slight miss and hesitation below about 2000 rpm.
Test every plug in its lead against earth, not just a few. For example, 8 of 12 can be shorting together. Run it in the dark or mist the leads with water and look for sparks tracking down a plug's ceramic.
A fresh set of the specified Champion-type plugs; the engine starts and idles smoothly again.
Mechanical: a head gasket or a stuck valve
If the electrics are clean, a cylinder that can't seal can't fire. A gasket blown between two cylinders or an exhaust valve sticking open gives a miss that no sensor or valve will touch.
On the six, rough idle and a miss in gear that survive a new IACV and EGR valve and a clean smoke test. On the V12, one bank missing on two cylinders, then it dies and backfires like a gunshot on restart.
A compression test across all cylinders before buying any more parts.
A head gasket replacement for the gasket; a stuck valve means taking the head apart, or a replacement engine.
An airflow meter reading wrong on the six
The ECU fuels from the airflow meter. A tired or poorly rebuilt meter under-reads at idle, so the mixture is wrong exactly where the engine is least tolerant, and smooths out at speed.
An irregular idle miss or over-fuelling with no single dead cylinder, often after a run of new sensors that changed nothing.
Hot idle, back-probe the meter output: about 1.6 V is right, a meter reading around 1.2 V idled badly on one car. Unplugging the meter forces a fixed limp-home idle; if that idles better, the meter is suspect.
A good used original meter; aftermarket rebuilt meters are a known source of fresh trouble on these cars.
V12 auxiliary air valve stuck or misset
The auxiliary air valve lets extra air past the throttles while the V12 is cold. Stuck open it makes a high, hunting idle; pressed in too far it starves the cold idle and the engine stalls when put in gear.
Idle hunting between about 1000 and 2000 rpm, or a cold stall when you select Drive or Reverse with the A/C on.
Pinch or plug the valve's hose with the engine warm: if the idle drops, the valve isn't closing. Off the car, it should be part-open cold and fully shut in hot water at about 70 C. Look at the vacuum balance hose too; a collapsed one makes the idle hunt.
Free or replace the valve, set its depth correctly, then reset the cold idle to about 750 rpm.
V12 injectors dirty or their seals leaking
In Park the V12's injector pulses are very short, so one lazy injector or one leaking injector seal shows up as roughness that disappears once the load of Drive lengthens the pulses.
Slightly rough in Park and Neutral, smooth in Drive, with plugs and leads already new.
If it misses in both Park and Drive, look at ignition or a single-cylinder air leak; only in Park points at the injectors. Have them cleaned and flow-tested.
Cleaned, flow-matched injectors split evenly between the banks, new injector seals fitted carefully, and sealant on the manifold gaskets.
V12 throttle plates or linkage sticking
The two banks' throttles are joined by rods to a central turntable. A sticky rod can drag a plate off its stop when the pedal is lifted quickly, and the idle collapses.
It stalls or nearly stalls the moment you lift off the pedal sharply, and idles normally otherwise.
Engine off, work the linkage by hand and feel for binding in the rods and bushes. Unplug the throttle sensor to see whether anything changes, and check the plates are clean and set to their 0.002 inch gap.
Clean and free the linkage, renew worn bushes and reset the throttle plates.
Marelli rotor arcing to earth on later V12s
On the later Marelli-ignition V12, the rotor can break down and arc to earth, dropping spark on a whole bank while that bank's injectors keep fuelling.
A sudden rough idle with a raw-fuel smell from the exhaust.
Take the cap off and look at the rotor for a burn mark or crack.
Fit a good cap and the upgraded rotor, and carry spares. Check the catalysts, which unburned fuel can damage.
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.
Note what happened just before
A wash, heavy rain or an oil change points straight at the plug wells and the ECU connector.
freeLook down the plug wells on the 4.0
Lift the coils off: oil or water round the plug tops is the whole fault.
freeFind which cylinders aren't working
Hot, pull injector plugs on the six, or plug leads on the V12, one at a time. A cylinder that changes nothing is the one to chase.
freeCheck the coils are original
Look for the black 'made in Japan' sticker and swap any aftermarket coil for an original.
free to lookOpen the ECU connector
Blue crust on the pins means clean, scrape and grease them, then find where the water comes in.
a can of cleanerHunt for vacuum leaks
Vacuum gauge and smoke test; plastic tees, the EGR valve and injector seals first.
a gaugeOn a V12, coolant, idle switch and TPS
Top up the coolant, check the idle switch makes before the pedal stops, and read the TPS at about 0.32 to 0.34 V.
freeCompression test before engine work
Only once ignition, wiring and air leaks are clear.
a garage testWhat it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJS V12 Won't Start or Starts and DiesIf the V12 won't fire at all, or dies hot and restarts cold.
- Jaguar XJS Alternator Not Charging, Battery LightIf the ammeter swings for reasons other than a stumble.
- Jaguar XJS Overheating, Cooling Fan ProblemsIf the coolant keeps dropping alongside the misfire.
- Jaguar XJS Transmission Problems, GM400 and ZF GearboxWhite smoke from the right tailpipe and ATF in the air filter box on a V12: the gearbox vacuum modulator.
- 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.