Jaguar XJ40 Misfire
On the XJ40 a misfire is nearly always on the high-tension side: leads arcing in the damp, a rotor or cap breaking down, a coil failing as it heats, or the distributor turned out of position. I'd watch the engine run in the dark and look under the cap before buying anything, and do a compression test if one cylinder stays dead.
How bad is it · Garage, carefully
Drivable gently, but get it sorted soon
Unburnt fuel from a dead cylinder overheats the catalyst, and a failing rotor or coil can leave you stranded. If it cuts out in rain or through deep water, don't keep cranking it.
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.
The distributor sitting out of position
The computer times the spark and advances it as revs and load rise; the distributor only passes it to the right lead. If the body sits a spline or a few degrees out, the rotor tip drifts past the cap post as the advance grows, and the spark misses. Fresh cap and leads can show up an error the old parts put up with.
A miss at a steady cruise above about 2000 rpm or 50 mph, while idle, acceleration and lift-off are fine; worse warm; often after head work or with the distributor recently out.
Bring No. 1 to TDC on its firing stroke and take the cap off: the rotor should point at the No. 1 post, slightly ahead of it because the spark is advanced. Firing order is 1-5-3-6-2-4 and the rotor turns clockwise. A timing light on each lead at the speed where it misses shows the flash going erratic.
Slacken the clamp and turn the body a few degrees, or lift it and move it one spline, to the middle of the range where turning it no longer changes a hot idle; then road-test at the speed that misfired.
Plug leads arcing, worst in the damp
Old leads leak high voltage through their insulation. Under load or in damp air the spark takes the short way to the metal loom holders, a bolt head or the next lead, and that cylinder misses. Standing water in the plug wells does the same.
A shudder or miss under load in a high gear that goes away on kickdown; stumbling or stalling after a wash, in rain or through a puddle; lumpy idle in hot traffic; better when warm and dry.
Run it in the dark with the bonnet up and look for blue streaks and sparks along the leads, then mist them with a spray bottle: a healthy system shrugs the water off. Look inside the plug caps for green corrosion. When it refuses to start in the wet, dry the leads with a hair dryer: if it then fires, you've found it. Pull each lead off its plug and look for green corrosion between the terminal and the plug nipple; a lead that reads open or very high end to end is finished.
A new set of leads, kept apart by their spacers and clear of the metal holders, with plugs, cap and rotor if they are old. Drying the leads and plug wells gets it going in the meantime.
An ignition coil breaking down as it heats
The coil sits in the heat from the exhaust manifold and radiator, and an ageing one breaks down as it warms: fine from cold, then a miss that spreads, popping and banging, and sometimes a cut-out. A coil lead not fully home or a boot not seated on the small terminals gives the same miss.
Runs well for 10–15 minutes, then misses or chugs and gets worse the further it goes; a splutter at a light cruising throttle that a timing light shows on every cylinder at once; a coil too hot to hold.
When it misses hot, pour cold water over the coil: if it smooths out at once, it's the coil. Hold the coil lead about 1/4 in from earth: a fat blue spark is good, thin and yellow is weak. By the book the primary reads about 0.4–0.5 ohm and the secondary 6–6.5 k-ohm at 68 °F, but good coils often read off that, so a meter won't settle it. Push both small boots and the coil lead fully home. On the road, once warm: full throttle in first to 3500 rpm should pull hard; then in second raise the revs slowly to 4000–4500. A stumble only in the slow pull says the coil or amplifier can't build voltage at high revs; a fuel restriction stumbles in both.
A new coil, genuine DBC1140 (the same part as Bosch 0221122450); universal 12 V canister coils have run for years as substitutes, though a cheap one can fail within days. A coil is worth renewing at around 100,000 miles. If a new coil doesn't cure a hot miss, the amplifier under it is next.
The distributor cap and rotor
The rotor carries a potted resistor that can fail with no visible sign, and its plastic can burn through so the spark shorts to the shaft. In the cap the sprung carbon button wears, sticks or falls out, and oily breather fumes coat the contacts.
Hesitation that turns into a shake above 2000 rpm under load; a rough idle at start-up that gets worse until it won't restart; dies at a stop and cranks with the rev counter moving but won't catch; trouble straight after fitting a new cap.
Take the cap off and look for burn marks or tracking on the rotor and cap, a missing or stuck carbon button, oil in the base. A visible spark proves little: a shorting rotor still gives a weak yellow spark that won't fire under compression. A dab of grease in the hole under the rotor is a free test.
A new rotor and cap together, even if they look fine, or cleaned contacts if they are only coated. Oil inside the distributor is worth chasing to the breather restrictor in the hose to the intake elbow. Carry a spare rotor. Cap and rotor are ordinary Bosch parts whatever the engine size, such as cap GB731 and rotor GB905; the Jaguar cap is DDB150.
Damp, dirty contacts at the coil and amplifier
The amplifier sits under the coil. Moisture and grime on their terminals let the spark leak away at the coil tower or across the coil top instead of going to the plugs.
A miss under load or uphill once warm, worse in mist and damp and after puddles; occasionally a stall; a fault code that comes and goes.
Look at the coil tower in the dark for tracking, and check the coil lead is tight with its boot over the tower.
Take off the coil and amplifier, wire-brush and clean every contact and refit. Clean the coil top between the terminals with alcohol, turn the coil lead to face away from the centre and fit its rubber hood. A water-repellent spray over the cap, leads and connectors keeps the damp out.
A burnt exhaust valve
These engines are prone to burning exhaust valves; a piece breaks away and that cylinder loses its compression, so no ignition part will cure it. A valve clearance set too tight is one cause: a valve that can't seat can't shed its heat.
A steady miss at idle that's worse in Drive and smooths out higher up, often from about 1000 to 2500 rpm; it often starts on a long uphill pull; pulling that plug lead changes nothing; a sooty plug.
Compression test warm, all plugs out: at least about 160 psi and within about 10 psi of each other. A cylinder near zero that doesn't rise with a squirt of oil is a valve; a leak-down test from that plug hole can be heard at the exhaust. Repeat a zero reading to be sure the gauge was screwed in tight. Do it with a fully charged battery and the throttle wide open, cranking four or five turns per cylinder. After head work, two dead cylinders side by side suggest valves bent when the head was laid down or one camshaft turned without the other. Swap a suspect injector to another cylinder to see whether the miss follows it.
Head off for a new exhaust valve, valve stem seals and head gasket, with the clearances checked. Until then, unplugging that cylinder's injector protects the catalyst.
Fuel pressure falling away in the heat
A tired pump flutters in hot weather, and on cars with the pump in a plastic canister the seal at its outlet can let fuel run back. The rail pressure sags, the engine leans out under load and stumbles like an ignition miss. The pump's plug on top of the tank can arc and melt too, so the pump cuts in and out.
A miss and loss of power when hot, above about 80 °F; jerking at a steady part throttle as if the key had been switched off and on; pulling plug leads one at a time shows no single bad cylinder.
Put a gauge on the fuel rail and watch it on a hot day under load; look at the pump connector on top of the tank for scorched or melted spade terminals.
Worth trying: a new pump if it has never been done, the outlet seal on canister pumps, a new connector where the old one has cooked.
The wrong spark plugs
The engine is designed for plain copper plugs; double-platinum and multi-electrode types have given an idle misfire that vanished on going back. Anti-seize that melts and runs into the chamber can also short a new plug.
A misfire that started with a new set of plugs, or shows mostly at idle.
See what's fitted and how it's gapped; look at each plug's colour and for oil fouling.
Worth trying: plain copper plugs such as Champion RC9YC gapped at about 0.035 in; Autolite plugs have also given a smooth idle.
An air leak or a poor airflow meter signal
Unmetered air past a split hose, an open port or a shrunken inlet manifold gasket leans the mixture, and the misfire is worst at idle, where the leak counts most. A corroded airflow meter plug or a poor meter earth feeds the computer a false reading and sets FF12.
Misfire with a smell of unburnt fuel at the tail pipe; a rough idle that pulling individual plug leads doesn't change.
Spray starting fluid round the manifold and hoses and listen for the revs to rise; make sure every vacuum hose and port is connected and the manifold bolts are tight. Key off, measure from the airflow meter's earth pins to the bulkhead earth: anything but zero is a fault. Look at the EGR valve: one open at idle is a big leak in itself. An exhaust manifold leak upsets the mixture too, and lets fumes into the cabin. If it runs rich and smoky when warm instead, pull the fuel pressure regulator's vacuum hose: fuel in it means the regulator diaphragm has gone.
Worth trying: seal the leak, tighten or renew the manifold gasket, clean and reseat the airflow meter plug several times; a replacement meter if FF12 keeps coming back.
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.
Watch it run in the dark
Bonnet up, engine idling, lights off: blue streaks or sparks along the leads, at the coil tower or round the cap mean high voltage leaking. Mist them with a spray bottle to bring it out.
freeFind the dead cylinder
Pull one plug lead at a time at idle: each should make it run rougher. The one that changes nothing is the cylinder to look at; if none changes, suspect the coil.
freeLook under the cap
Rotor and cap for burn marks and tracking, the carbon button present and springy, no oil in the base.
freeCheck where the rotor points
No. 1 at TDC on the firing stroke, rotor just ahead of the No. 1 post. If it misses only at a steady cruise, turn the body a few degrees and road-test.
freeTest the coil hot
Cold water over the coil while it's missing, and a fat blue spark from the coil lead at 1/4 in from earth.
freeCompression test
Warm, all plugs out. A cylinder near zero is a valve, and no ignition part will fix it.
a gaugeThen renew the wear parts
Leads, cap, rotor and copper plugs as a set, and the coil if it's old or failed the water test.
leads, cap, rotor, plugsWhat it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJ40 Rough Idle and StallingIf every cylinder fires but the idle hunts or it stalls at junctions.
- Jaguar XJ40 Cranks But Won't StartIf it has died and now cranks without firing.
- Jaguar XJ40 Fuel Pump ProblemsIf the stumble comes at a steady throttle in the heat or with a low tank.
- Jaguar XJ40 Head Gasket Failure, Low CompressionIf one cylinder stays dead and compression is low on it.
- Jaguar XJ40 Check Engine Light, Fault CodesIf the VCM shows a Fuel Fail code with the misfire.
- 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.