Jaguar XJ40 Rough Idle and Stalling
On the XJ40 a rough, hunting or stalling idle is usually dirt and air rather than electronics: a carboned or sticking idle speed control valve, a gummed throttle body, an air leak or a poor airflow meter plug. I'd clean the idle valve and throttle body, look for leaks, then run the idle reset routine before buying any part.
How bad is it · Garage, carefully
Drivable, but expect it to stall when you stop
Most of these faults only upset the idle, but a car that dies at junctions or on turns is a hazard in traffic. If it cuts out dead at speed, get it to a garage rather than carry on.
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.
Carbon and sludge in the throttle body, idle valve and breather
The breather feeds oily vapour into the intake, and over the years it gums the throttle plate, the idle valve bore and its plunger. The valve can no longer move freely or open far enough, so the idle hunts, sags under load and dies.
Stalls a few seconds after selecting a gear, on full lock or coming to a stop; idle hunts or sits high; often worst cold. Code 17 with a wandering idle points here too.
Take the idle valve off the throttle housing and look at the plunger and its bore; clean the throttle plate, the breather hoses and the air temperature sensor in the elbow while you're there. On the 4.0 the idle control valve sits on the manifold between cylinders 2 and 3: unplug it at idle, and if the loping stops, take it off and clean the baked carbon out of its passages.
Clean the bore and plunger and put a smear of lithium grease on the grooved shaft; clear the breather hoses; then run the idle reset.
Idle set up wrong: throttle gap and the reset routine
The computer trims idle with the stepper, but only around a base setting made by hand. If the throttle plate gap or the bypass screw is off, or the reset is done cold or with the wrong pauses, it surges or sits high. After any cleaning or valve swap the routine has to be redone.
High idle, or a rhythmic surge, that started right after cleaning the throttle body or changing the idle valve. An idle bouncing between about 1100 and 1500 rpm straight after a throttle body clean usually just needs the bypass screw turned in. Around 500 rpm in Drive is too low and stalls at junctions.
Warm engine, in Park, A/C off. Throttle plate gap about 0.002 in, even all round. Key on about 5 s, unplug the idle valve, key off about 15 s, plug in, key on, unplug again, start, then set the hex bypass screw to roughly 550–600 rpm and reconnect. Check too that the throttle cable lets the plate open fully. A throttle plate loose on its spindle, or a spindle slipping in its bell-crank, gives wildly swinging revs. If the idle can only be corrected with the bypass screw, the valve itself is not doing its job.
Redo the reset with a stopwatch on a hot engine. Hot idle should then be roughly 700 rpm in Park or neutral, about 600 in Drive, around 800 cold. On 1993–94 cars the old routine may not take; a set-up on dealer diagnostic equipment is worth trying if it still surges. A car that has stood for months may simply need a long run before the idle settles.
An air leak: intake gasket, a split hose or the idle air adapter
Unmetered air lifts the revs; the computer sees high rpm with a closed throttle, cuts the injectors, the revs fall and it starts again — the classic surge. A big leak gives a rough idle below about 1500 rpm and popping. The intake gaskets on these cars give up with age.
Hunting or high idle, a squeal or whistle from behind the throttle body, popping on restart, a rough hot idle with code 26; on the 3.6, hunting can also come from a chafed wire opening the supplementary air valve at random. FF6 with an idle fault often means the intake bellows isn't seated at the bottom of the throttle body. A loping idle that starts right after cam cover work points at the breather tube or a vacuum line knocked off.
With the engine idling, close the throttle plate by hand: if it keeps running, air is getting in elsewhere. Spray carb cleaner round the elbow, the gaskets, the hoses and the base of each injector and listen for the revs to change. Look under the manifold behind the throttle body for the brass nipple on the idle air adapter.
New intake gaskets, new lower injector O-rings greased with silicone, new vacuum hoses (bend them to find hidden cracks), the adapter nipple pushed back and wired in place, a crushed idle hose stub sleeved open. Seat the bellows fully, and put a screw-sealing compound on the idle air screw, since air can leak past it too.
The airflow meter, or just its plug
The airflow meter's signal voltages are tiny, so a slightly corroded plug is enough to make the idle surge and drop. A failing meter, or one going bad when hot, does the same.
Revs surge up and fall almost to stalling at lights; code 6 or FF12 on the dash. A rough warm idle with hesitation and code 4 can come from nothing more than an air filter box packed with debris — nesting rodents are a classic — fouling the meter.
Unplug and replug the meter several times, then clean both halves with contact cleaner. If it runs better with the meter unplugged, suspect the meter. Check the wires at the computer end as well as at the plug.
A cleaned plug, or a swapped meter — a known-good spare is a better test than bench readings, which can look fine on a faulty one. Open the airbox first: empty it, then clean the meter with airflow-sensor cleaner and treat its plug.
A worn or badly sealed idle stepper motor
At high mileage the stepper can bind internally, so cleaning the plunger doesn't help. A valve mounted on a thin o-ring leaks air past its seat and sets FF47.
Idle still poor after a thorough clean; a stepper coil code or FF47; high idle that never settles.
Watch the plunger move as the key is switched on and off. Check the valve is bolted down hard on a proper thick o-ring. Part numbers: JLM1741 on the early 3.6, JLM12185 on the 3.2 and 4.0; the plug sits differently on 1991 and 1994 cars.
A new stepper, then the reset routine. A Renault Megane 1.6 stepper is a known cheap substitute, but not every one behaves; the small screws holding the old one snap easily.
The throttle position sensor
Oil from the breather can fill the sensor and its track wears, so the computer loses the closed-throttle signal and the idle drops out. On the 2.9 the idle contact in the throttle switch can lose contact.
Dies at idle or at stop signs with no warning and no code; idle creeps up and won't settle.
Measure the sensor output: it should sit in the idle range closed and sweep smoothly. Look inside for oil. Test the wires at the computer as well as at the plug.
Drain and clean an oil-filled sensor, or replace it (DBC2626 or DBC2889).
Fuel pressure: the regulator, a tired pump or a blocked tank vent
A split diaphragm in the pressure regulator feeds fuel down its vacuum hose into the manifold, or passes far too much pressure to the injectors: it runs rich, lopes and hunts at idle and can stall. A pump past its best gives low pressure and a rough idle that dies when you open the throttle; a worn one can quit once hot and only restart after it has cooled. A blocked tank vent starves it on long hot runs, with surging before it quits.
A loping, hunting idle with a rich smell or black smoke; rough idle with hesitation, cut-outs on a trailing throttle; or surging then stalling on a hot run that restarts after it has cooled. Dying as soon as the throttle is pressed with black, sooty plugs and Fuel Fail 26 is classic low pressure; white plugs and stalling with FF44 can be a blocked filter rather than the oxygen sensor.
Pull the vacuum hose off the regulator and switch the ignition on a few times: fuel in the hose means the diaphragm has gone. Rail pressure should be about 30–40 psi; 10 psi means a failing pump, and far above that points at the regulator. Loosen the filler cap and drive: if the trouble stops, clear the two vent hoses under the car between the differential and the boot. At the rail inlet expect about 38–44 psi with the key on, 40–46 psi at idle with the regulator hose off and 30–38 psi with it on, and pressure that holds after switch-off. On in-tank pump cars, about 10 psi with a new pump points at the blue seal between pump and canister having pushed out of its seat. Pinch the return line when the idle plays up: if it smooths out, the regulator is weak. On 1990 and earlier cars with the external pump, fuel can boil at the pump on hot days.
A new regulator, or a new vacuum hose on it if that has split (on the 3.6 the regulator has to be lifted off the rail to fit it); a new pump; or cleared vent hoses. Pump screens in the tank are worth cleaning while it is out. Reseat or bypass the canister seal; change a filter whose insides rattle, and clear rust from the pump screen and the tank.
Ignition: coil, leads, cap and rotor
A coil can work cold and break down as the engine bay heats up, giving a stumble at idle and then a cut-out. Leads leak current through their insulation, especially when damp, and misfire the idle.
Sputter and pop at idle that gets worse as it warms; sudden rough idle with puffing at the tailpipes; trouble after a wash.
Strong blue spark at the coil lead and at each plug; look for tracking on leads and inside the cap. On early cars check the distributor body hasn't been set so late that the advance runs out — that gives a rough idle and a hard stop near 2000 rpm. If it has died on the road with no spark at all and the crank sensor is good, the ignition amplifier is next.
A new coil, a full set of leads, or cap and rotor (a new rotor arm can be faulty out of the box); a new ignition amplifier where the spark had gone completely; on the early car, the distributor turned back to its correct position.
The oxygen sensor or its relay
Once warm, the computer trims the mixture from the oxygen sensor; a lazy or dead one sends it the wrong way, so the idle stumbles and FUEL FAIL 44 comes up. On later cars the yellow-based oxygen sensor relay also feeds the fuel pump relay, so a tired relay gives starts that fire and die.
A stumble and a puff of black smoke at idle once warm, with FF44 or FF26; or a hard cold start that fires and dies at about 500 rpm, then runs perfectly all day.
Read the codes, then the sensor on a warm engine: its voltage should swing between about 0.1 and 0.9 V, and one that never climbs past about 0.6 V is lazy. Swap the yellow-based relay on the firewall for a matching one from the same row and see if the fault moves.
A new oxygen sensor, or a new relay (on a 1994 car, the third from the passenger side on the firewall). A seized sensor may need a new threaded boss welded into the pipe.
Cuts out dead when hot: the crank sensor
When the crank sensor's signal drops out, ignition and injection stop together, as if the key had been turned off. It often fails with heat and comes back once it cools.
Runs well, then cuts out at running temperature and needs a wait before it restarts; or won't run below about 1000–1200 rpm. The V12 behaves the same way.
Watch the rev counter while cranking: about 200–300 rpm means the signal gets through; nothing means the sensor or its plug. The sensor sets no fault code. Look for oil from a leak coating it, and for hardened, cracked insulation on its lead that breaks the signal as the engine rocks on its mounts.
A new crank sensor, or its wiring repaired; it is cheap enough to keep the old one in the boot as a spare.
Shaking at idle in gear: hardened front engine mounts
The engine runs evenly, but worn front mounts pass every pulse into the body once the gearbox loads the engine in Drive.
Heavy vibration at idle in gear that eases in Park or Neutral; the engine itself sounds smooth.
Watch the engine as a helper shifts between Park and Drive with the brakes held; look at the rubber of both front mounts for cracks and sag.
Fit new front mounts with the engine supported on a jack and wooden boards under the sump; the left side is easier with the wheel and oil filter off. Mounts changed part-way through production, so order by VIN.
Lumpy idle that needs the throttle held: slipped cam timing
If a camshaft sprocket loses its locating circlips, its splines can strip and the cam slips far out of time. The engine still runs, badly and over-rich.
Very lumpy idle and a stall unless the throttle is held open, no fault codes, starts only with flooding tricks; perhaps a single tink on cranking.
Before buying more sensors, take the cam cover off and check the cam timing marks and the sprocket fixings.
New sprocket and retaining plate; if the valves have hit the pistons, head work follows.
Low voltage at idle from a failing alternator
The management needs a steady supply; an alternator that stops charging at low revs lets the voltage sag, and the idle sags with it until it stalls.
The charge needle drops below 12 V at idle and the idle sinks to around 450 rpm; fine above about 950 rpm.
Watch the battery gauge at idle and in Drive, or measure at the battery: about 13.5 V or more is healthy.
Worth trying: repair or replace the alternator, and load-test the battery.
The coolant temperature sensor or its wiring
The computer fuels by this sensor; a flaky one or cooked wiring makes it run rich, and the trouble grows as the engine warms.
Starts and idles fine, then goes rough and stalls warm and is hard to restart. A hunting idle until the engine is warm, or stalling just after a cold start, points the same way.
Check the sensor's plug and wires on top of the water pump for hardened, cracked insulation, and measure the sensor against temperature. As a guide, about 2500 ohm at 25 C falling to about 250 ohm at 90 C.
Worth trying: a new sensor or repaired wiring.
Stumbles after standing: fuel draining back
If the pump's check valve or the pressure regulator doesn't hold, the rail empties while the car stands, and it stumbles or nearly stalls until pressure builds.
Fires and nearly dies after a few hours parked, or struggles after a hot soak; an immediate restart is fine.
Switch the key on for 2–3 seconds two or three times before cranking: if it then starts cleanly, pressure is draining away.
Worth trying: priming with the key as a stopgap; an in-line check valve or a new regulator to cure 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.
Read any codes first
On these cars a FUEL FAIL number points at a sensor signal, not the pump: code 6 or FF12 means the airflow meter, FF47 the idle valve.
freeReseat and clean the airflow meter plug
Several times, then contact cleaner. Try a drive with the meter unplugged to compare.
freeHunt for air leaks
Close the throttle plate by hand at idle, then spray cleaner round the elbow, gaskets and hoses and listen.
freeClean the throttle body, idle valve and breather hoses
Keep fluid out of the throttle sensor. Grease the idle valve shaft lightly.
a can of cleanerSet the throttle gap and run the reset
About 0.002 in, then the key-and-plug routine on a hot engine with the 15-second waits.
freeWatch the charge gauge and check fuel pressure
Under 12 V at idle points at the alternator; fuel in the regulator's vacuum hose, at the regulator; under 30 psi at the rail, at the pump or the vent.
a gaugeOnly then buy parts
Stepper if cleaning didn't help, throttle sensor if it dies with no code, oxygen sensor or its relay with FF44, coil if it worsens as it warms, crank sensor if it cuts out dead.
a stepper, sensor or coilWhat it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJ40 Fuel Pump ProblemsIf the pump drones, surges or the engine starves in hot traffic.
- Jaguar XJ40 Cranks But Won't StartIf it stalls and then won't fire again.
- Jaguar XJ40 MisfireIf one cylinder seems dead, or it misses under load or in the damp.
- Jaguar XJ40 Alternator Not Charging, Battery LightIf the charge gauge drops below 12 V as the idle sags.
- XJ6 (XJ40) fuse boxes and relaysWhere the relays and fuse boxes sit — other years from the same page.
- Jaguar XJ40 Head Gasket Failure, Low CompressionIf the idle shakes and the coolant keeps going down.
- Jaguar XJ40 Low Oil PressureIf the oil light flickers at idle as well.
- Jaguar XJ40 Transmission Problems, Limp ModeIf the transmission light comes on with the stalling.
- 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.