Jaguar XJS V12 Won't Start or Starts and Dies
A V12 that cranks but won't fire usually has one break in a short chain: pump prime, main relay, spark, then the firing signal to the injectors. Listen for the pump, look for spark at the coil lead, then listen for injector clicks — each answer halves the search. After long standing, fuel and gummed injectors come first; after recent work under the bonnet, look first at what was disturbed.
How bad is it · Safe to drive
Nothing gets worse while you work through it in order
These faults stay as they are until found, but one that cuts the engine once hot can stall it in traffic — keep trips short until it's found. The real risk is disturbing healthy connectors while hunting: it's easy to end the day with a broken wire or a jumper left in a sensor plug — two faults of your own.
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 fuel pump stops or never runs: inertia switch, relays, ECU plug, the in-tank pump
Key on, the main relay in the boot powers the ECU and injectors and feeds the pump relay. If the main relay doesn't pull in, nothing in the injection system wakes up. The inertia switch in the passenger footwell cuts the pump after a knock. An old in-tank pump can also start to stick once warm, and a burnt plastic connector on top of it inside the tank adds resistance, so the pump gets less voltage than it needs.
No hum from the boot for the first seconds after key on; a car that ran fine the day before; recent work in the footwell or near the battery. Or it ran all day, stalled after a short stop and now cranks with spark but no fuel — and a knock on the tank brings it back.
Press the inertia switch button down. Then feed 12 V to the white trigger wire at the main relay: if the pump and ECU come alive, the fault is upstream of the relay. If it cuts out on the move, run a voltmeter on long leads to the pump terminals and watch it while driving.
Reset the inertia switch; unplug the injection ECU and clean its terminals; replace a main relay with blackened contacts. The main relay carries a diagonal red stripe and an extra spade — a lookalike that fits the socket won't power everything. An original in-tank pump that clicks and sticks gets replaced — on these cars it can come out without dropping the tank; clean the burnt connector and renew the filters so the new pump isn't strained. A pump relay overloaded by a dirty filter can cut out after a drive and recover once cool. A pump that won't run on 12 V fed straight to it is dead — check its earth first, then drain the tanks and clear the strainer before the new one goes in.
The ignition amplifier and its connectors
The amplifier fires the coil by switching its negative side to earth. Corroded terminals, a frayed crimp on its earth tab or a dead module kill the spark — and with no spark the V12 ECU fires no injectors either.
No spark and no injector pulse; often only on a cold, damp morning, after which it runs all day. 12 V on both coil terminals with the key on is normal. On a Lucas amplifier, heat soak on a hot day can bring the same no-start for a while. Mounted on the manifold, an amplifier with tired heat-sink paste can drop the spark for a moment as the revs fall once warm — the rev counter dips towards zero.
Unplug, clean and reseat the amplifier connectors, then look at every crimp on the earth tab. On Lucas boxes, disconnect the internal condenser as a quick test — a shorted one kills the spark.
Clean the corroded connectors; if a wire inside the box is frayed from repeated module swaps, replace the whole amplifier with its harness. A dab of heat-sink paste under a new module helps it last. If it cuts out when warm, a module moved off the hot manifold to the cool space between grille and radiator can cure it; fit a genuine AC Delco module rather than a cheap copy.
No 12 V at the coil: the feed wire and the ignition switch
One wire from the ignition switch feeds both coils and the amplifier. A break, a corroded switch contact or a melted wire there leaves the coil starved — dead, or weak only while cranking.
Zero volts at the coil with the key on; or the coil drops several volts more than the battery while cranking; or everything dies as the key comes back from START to II. A feed wire that has slipped off and only rests on the coil terminal gives a spark too weak to start a stone-cold engine after a few days' standing, and stalls it once it moves further.
Measure the coil positive while cranking: it shouldn't fall much more than about 1 V below the battery. If it does, work back to the bulkhead plugs and the switch.
Repair the broken or melted feed, clean the corroded switch contacts or replace the switch, and reconnect any bulkhead junction plug that's come apart. Check what's fitted in the bulb-check socket — a plain relay there can short this feed. Fit eyelet terminals screwed to the coil rather than push-on spades — wires in the vee get knocked whenever you work there.
The white coax signal wire to the ECU
A shielded white wire carries the firing signal from the ignition side to the injection ECU in the boot. Engine heat breaks its core or braid, usually between the amplifier and the bulkhead, and the injectors stop while the spark carries on. On Marelli cars from 1989 it runs from the Marelli ignition module in the passenger footwell to the injection ECU in the boot — cut it there and the car won't start at all.
Spark at the plugs but no injector clicks; it fires on starter spray and dies; sometimes it cures itself until the next time. On a Marelli car the injectors click when you work the throttle with the key at II, but not while cranking.
Test continuity from the amplifier end to the ECU plug — pin 18 on Lucas cars — and wiggle the harness while you watch the meter. Test the injector click while cranking, not by working the throttle — a long screwdriver to your ear on an injector will do.
Cut back to sound braid and splice the core properly, with the shield earthed at the engine end only — or renew the under-bonnet section. Where heat has cooked the insulation at a splice on the bulkhead, cut out about 6 inches either side and solder in new wire under heat-shrink.
Injectors gummed up by standing
A car that sits leaves varnish and sediment in the tank, lines and injectors. One injector starves and its cylinder drops out — easy to miss on a V12, which still pulls on eleven.
The car has stood for years, the fuel has been changed, and it still misses at idle — or the injectors buzz on a test lamp yet the engine barely catches.
Pull one plug lead at a time: if the idle barely changes, that cylinder wasn't working.
A heavy dose of injector cleaner in about a quarter tank of fresh fuel, then a long idle, revving occasionally once warm — the miss can clear in about half an hour, rail untouched. If nothing clicks or flows after a decade or more of standing, the injectors have to come out for professional cleaning and new seals.
The crank sensors — Marelli cars from 1989
From 1989 the Marelli ignition reads engine position from a TDC sensor at the front pulley and a speed sensor at the flywheel. They and their plugs fail intermittently when hot, then completely — no spark, no injection. Earlier Lucas cars have no crank sensor at all.
Dies after twenty minutes, restarts once cool, then one day won't start; no injectors audible while cranking. A sensor with too wide an air gap can also refuse to fire whenever the battery is slightly below full charge.
Watch the rev counter while someone cranks: a flicker means the signal arrives; a still needle means it doesn't. No injector clicks while cranking with spark present points at the same pair — both must report to the Marelli module; check the rear sensor plug is actually connected. A meter can't pass a sensor — one reading about 700 ohms was still dead; only a scope proves the signal.
Clean both sensor plugs first — one by the B-bank thermostat housing, one behind the B-bank cam cover; corrosion alone can kill the spark. The joint at the front of the block, where the sensor's short lead meets the main harness, sits low in the valley where dirt and water collect — reseat it, and renew both halves of the connector if it keeps coming back. Then replace the front sensor, and the flywheel one if it still won't fire — the two take the same part, DBC12507. Put back whatever you disturb while hunting: a broken air temperature wire or a jumper left in the coolant sensor plug adds faults of its own.
Cuts out once hot, restarts when cool — coil wire, distributor pickup, trigger board
Heat opens a marginal connection or a tired sensor in the ignition chain. A cracked boot on the coil-to-distributor wire, a pickup coil whose output fades when hot, or the reed-switch trigger board on early D-Jetronic cars — each works cold and quits warm.
Runs normally for 10–20 minutes, then stumbles, surges and dies, with the rev counter bouncing towards zero; won't restart until it has cooled for anything from minutes to an hour; often only in summer. A new coil and amplifier module change nothing.
Test straight after it dies, before it cools. With an analogue AC voltmeter on the pickup leads, crank cold and note the reading — for example about 1–1.5 V — then crank again right after a hot cut-out: nothing, or far less, condemns the pickup. Flex the coil-to-distributor wire and look hard at the boot on the coil positive. A trigger board that passes a cold bench test can still fail hot.
Repair the coil-to-distributor wire and its connector; while you're there, reconnect a broken vacuum-advance pipe and free a gummed centrifugal advance. Replace a pickup that loses its output hot. On a pre-HE D-Jetronic car, fit a sound used three-wire trigger board — a hot-failing one feels exactly like an ignition fault.
Cap, rotor, HT leads — and a new coil that's weak
A corroded or cracked cap, a missing carbon button or leads swapped between banks stop the spark reaching the right cylinders. A brand-new coil can also be too weak to start the engine, though it sparks on the bench.
A weak orange spark instead of a fat blue one; trouble that began after the cap or coil was renewed; backfiring once it catches. Or it idles cold but stalls the moment the throttle opens, until fully warm.
Pull the cap: look for pitted contacts and the sprung carbon button in the centre. On Marelli cars, match each bank's coil plug and lead to its bank. Crank for a full 20 seconds while watching the spark: a two-second test can pass while the spark comes and goes.
Fit a quality cap and rotor and set the timing; if the spark stays weak, refit the old coil to compare. On 1990–91 Marelli cars the lower coil is the A bank, marked red, and the upper the B bank, marked yellow. Look at the advance while the distributor is open — a dead vacuum unit and a sticking centrifugal advance often come with a burnt cap — and at the coil: one weeping oil is failing.
Runs only with the pump relay bridged — the ECU's pump circuit
The ECU switches the pump relay by pulling its trigger wire to earth. On the early 6CU unit, and later ones too, that circuit fails inside the ECU — dry solder joints hidden under the coating.
The engine starts only with a loop in the pump relay socket or the orange trigger wire earthed; new relays change nothing.
Ease the pump relay off its base and earth the orange wire on pin 85. If the pump runs and the car starts, relays and pump are fine — the ECU's output isn't.
Fit a used or rebuilt ECU. A wire from an ignition feed to the relay keeps the car moving meanwhile, but it masks the fault rather than fixing it.
Plugs fouled by short runs
Starting the engine for a minute and switching off before it's warm soots the plugs. A fouled plug can carry the spark down its insulator instead of across the gap, so every check passes and the engine still won't cough.
Fuel at the rail, injectors pulsing, spark on the leads — and no response even to starter spray.
Pull a plug from each bank after cranking and look at the nose for soot and wet fuel.
Fit new plugs — access on the V12 is fiddly, so start each plug by hand. Then run the engine fully warm before switching off.
Hard to start hot after a half-hour stop — fuel boiling in the rail
After a hard run the fuel rails soak up heat while the car stands. If rail pressure leaks away — through a pump check valve that's missing or leaking, or a regulator — the fuel boils, and the injectors spray vapour until cool fuel flushes through. Today's ethanol fuel boils sooner. The thermal vacuum valve on the rail is there to cut vacuum to the second regulator once hot, which raises rail pressure and with it the boiling point.
Starts cold every time and after a short stop; after 20–40 minutes standing hot it fires, stumbles and backfires on throttle, then idles rough for a minute or two before it settles. Blocking the vacuum pipe to the regulator lets it start inside the bad window.
Fit a gauge to the rail and watch it for half an hour after a hot shutdown: pressure falling away points at the check valve or a regulator — one that leaks fuel into its vacuum side does it too. Pressure that holds proves less than it seems — a rail can sit at about 38 psi and still hold vapour. Then test the thermal vacuum valve on the rail (EAC 5086, superseded by EBC 4600) in a pan of water: it should pass vacuum below about 155°F and shut above it. About half of used ones are dead.
Replace a dead thermal vacuum valve — test any used one before fitting; boiling a stuck one for a few minutes has revived it for a while. Fit an in-line check valve after the pump if yours has none — many aftermarket pumps don't, and adding one made hot starts much better; replace a regulator that leaks into its vacuum hose. Turn the key two or three times before cranking to flush hot fuel back; ethanol-free fuel in summer, a full tank and an open bonnet after a hard run all help a little.
A clogged pickup screen in the sump tank
Fuel reaches the pump through a screen in the small sump tank under the battery. Sediment clogs it until the engine starves after a short run; dirt falls off when it stops, so it restarts and dies again. Pulling through a blocked screen also wears the pump.
Starts and idles well for 15–30 seconds, then runs rougher and stalls — repeatably; revving holds it for a while. Gets worse over days. Bridging the pump relay changes nothing.
Empty the fuel filter into a clean jar and look for rust scale, then drain the sump tank and pull the screen. A gauge on the rail shows pressure sagging as it dies.
Clean or renew the screen — on later cars the upgraded one is CBC5649 — and add an in-line filter ahead of the pump so it's easy to clear next time. If pressure is still short, the pump has worn. A Jaguar in-line pump's terminals are wired opposite to most, so check polarity before blaming the new pump.
The ECU's injector drive — early 6CU units
Inside the injection ECU the injector drivers and their protective zener diodes age. A failing hold circuit opens the injectors too briefly; a shorted zener holds one bank's injectors open and floods it.
Starts only after many quick pushes of the pedal, idles at about 450 rpm and dies when the throttle opens — though every injector clicks. Or fuel runs from one bank's exhaust and thins the oil.
Clicking injectors don't prove the pulse is long enough. Put noid lights on each injector group. On the pump relay control wire a meter should read nothing for a second or two at key on, then 12 V. With the ECU open, compare the two zener diodes on its heat sink against each other.
Fit a known-good ECU — a later 16CU in place of a failing 6CU fired the car at once. A shorted zener can be replaced with a suitable transient-suppression diode, for example a 5KP43; don't just leave it out, it protects the output transistors. Change the oil if fuel has thinned it.
The ECU vacuum pipe knocked off
The injection ECU in the boot reads engine load through a long vacuum pipe from the inlet manifold. With the pipe off or split it has no load signal and gets the mixture badly wrong.
Dies after a couple of minutes at idle, restarts after a wait and dies again; idle sits at 1200–1300 rpm instead of 900–1000. Usually soon after work at the front of the engine, such as a radiator flush.
Follow the pipe from its push-on fitting at the front of the A bank all the way back to the ECU.
Refit the pipe securely and renew it if it's split.
The injector harness — cooked by heat, or rebuilt wrongly
The ECU fires the injectors in four groups of three — 1A 3A 5A, 2A 4A 6A and the same on B. Heat cracks the original harness, and after a rebuild a swapped pin or a bridged joint takes out a whole group.
Starts and runs smoothly for a few seconds, then drops onto fewer cylinders and won't pick them up with throttle. The dead injectors fall into groups of three.
Click-test each injector while cranking — a noid light is better. A whole dead group points at its common wire and the plug pinning, not at the injectors.
Repair or replace the damaged sections and correct the plug pinning. A new harness runs to about $500; route it high, tie it to the rail and insulate it from the heat.
The coolant temperature sensor — dies after a minute, restarts cold
The ECU sets fuelling from the coolant sensor. An open-circuit sensor or a loose plug makes it flood the engine until it dies and won't restart hot.
Starts, runs badly for a minute or two, dies; restarts only after hours of cooling; a fuel smell in the oil.
Measure the sensor across its pins, then at ECU pins 5 and 19 with it plugged in — the readings should match. Open circuit when cold means a dead sensor. Follow the wiring right into the plug — frayed wires inside the connector do the same. Look closely about three inches from the sensor, under the factory sheathing — the wires can be broken there.
Worth trying: replace the sensor and fit a good push-on connector; change the oil if fuel has thinned 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.
Ask how long the car has stood
After months or years of standing, fuel before ignition: stale fuel, a dirty sump tank and gummed injectors are likelier than an electrical failure.
freeListen for the pump at key on
A short hum from the boot means the main relay and pump circuit woke up. Silence: press the inertia switch down, then look at the main relay and the ECU plug.
freeWatch the rev counter while it cranks
On Marelli cars from 1989, a flickering needle means the crank signal is arriving; a still one points at the crank sensors.
freeTest for spark at the coil lead, then at the plugs
Hold the coil's HT lead about a quarter inch from metal and crank for a good 20 seconds: a bright blue spark all the way through clears the coil and amplifier — one that comes and goes doesn't. One dead plug proves little on a V12 — it can run on the other eleven.
a few poundsMeasure the coil positive while cranking
More than about 1 V below the battery: the feed wire, bulkhead plugs or ignition switch. Zero: a broken feed.
freeSpark but no start: listen for the injectors
A stethoscope or a test lamp on an injector plug while someone cranks. No pulse with good spark: the white coax wire.
a few poundsTry starter spray in the intake
It catches and dies: fuel side. Not even a cough: ignition — cap, plugs, timing. Catching on spray doesn't prove the spark is strong enough for petrol.
a canTry cleaner and a long idle before opening the fuel rail
A bottle and an afternoon — it either cures it or proves the injectors must come out.
a bottleDies once hot: test the moment it stops
Spark, injector clicks and pump hum right after a hot cut-out — before it cools, the fault is still there to find. No spark: coil wire, pickup, amplifier. Spark but silent injectors: the coax wire or crank sensors. No pump: the pump and its relay.
freeHard only after a hot stop: put a gauge on the rail
Read the rail pressure at shutdown and again after half an hour. A falling reading points at the pump's check valve or a regulator; a steady one sends you to the thermal vacuum valve — or block its regulator vacuum pipe as a quick test.
a gaugeStarts, then dies within a minute or two: vacuum pipe and sump screen
Check the ECU vacuum pipe is on at the front of the A bank, then empty the fuel filter into a jar — rust scale sends you to the sump tank screen. If it drops onto fewer cylinders instead, click-test the injectors by groups of three.
freeOnly now change parts — one at a time
Put back whatever you disturb before touching the next thing, or you'll create new faults. An old car often hides two faults at once.
freeWhat it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
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- Jaguar XJS Battery DrainIf a tired battery is part of the story: a weak one can crank and still starve the ignition.
- XJS fuse boxes and relaysWhere the relays and fuses sit.
- Jaguar XJS 3.6 and 4.0 Won't StartIf yours is the 3.6 or 4.0 six-cylinder: crank sensor, EMS relay and pump relay come first.
- 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.