Jaguar XJ6 (X300) Rough Idle or High Idle
On the X300 a rough, high or stalling idle is nearly always the throttle body and what bolts to it: gum from the breather, a tired return spring, the idle valve or the position sensor. I'd press the throttle shut by hand, check the battery, clean the throttle body and let the ECU relearn before buying any part.
How bad is it · Garage, carefully
Drivable, but expect it to stall or creep when you stop
Most of these faults only upset the idle. A car that dies at junctions, or pulls hard against the brakes on a high idle, is a hazard in traffic — fix it before long trips.
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.
Gum in the throttle body
Oily fumes from the cam cover breather coat the bore and butterfly. The plate stops closing fully, or sticks shut, so the idle runs high, hunts or dies.
Idle hangs at 1200–2000 rpm, a stiff pedal after the car has stood, a lazy rise when you press it, or a crust line behind the butterfly.
Take it off: four 10 mm bolts, plug the coolant hose, no gasket. Clean bore, blade and the small bypass hole; leave the grey coating in the bore and keep cleaner out of the position sensor.
A proper clean, or a cleaned used throttle body. Expect the idle to settle only after a drive. To stop it coming back, deal with the source: the wire-mesh oil separator inside the cam cover loads up and lets oil through, so wash it out, and clean the small part-load breather orifice at every service. A catch can in the hose to the throttle body, mounted below it with a drain, helps. Never block that hose — the crankcase has to breathe or the oil seals suffer. If the plate has shifted off its home stop, reposition it and lock its screws with threadlocker.
A weak or unwound throttle return spring
The ECU only drops to idle when the throttle sensor reports fully shut. A tired spring leaves the plate a hair open, and the idle latches high.
Idle sticks at 1400–1600 rpm once warm and drops to 800 the moment you blip the pedal; slow fall to idle when you lift off. A throttle cable that drags where it leaves its outer sleeve does the same, holding the plate just off its stop.
With the idle high, press the throttle shut by hand. If the revs fall, it's the spring. Don't touch the hex stop screw on the throttle body, and don't set a butterfly gap with feeler gauges — on the X300 neither is adjustable in service; that gap procedure belongs to the older XJ40.
Wind the middle spring on the throttle body one turn tighter, fit the uprated spring NBB 3131AB from recall R493, or add a light supplementary spring that can't jam at full throttle.
The idle speed control valve
A stepper valve on the throttle body meters the idle air. It gums up, wears its tip or loses a winding, and the ECU gets no feedback that it has.
Stalls cold as the gauge starts to rise, sags to 500 rpm at stops, or creeps up towards 3000; comes and goes over months.
Unplug it and test both coils for continuity; clean the valve and its seat. The screws are threadlocked and snap easily. Never put battery volts straight across it.
Clean it; if that doesn't hold, a new or good used valve (JLM12185, gasket JLM11490). Reset the plunger by cycling the ignition, not by screwing it in.
The throttle position sensor
If the sensor never reads its idle value, the ECU won't take control of the idle at all. Wear, heat and dirt shift its reading.
A steady fast idle of 1400–1900 rpm hot or cold, no engine braking downhill, or high idle only once the engine is hot.
At idle it should sit near 0.5 V; for example 526 ohms closed and about 4.4 kohms wide open across the signal pins, sweeping smoothly. Clean it carefully — cleaner run down the shaft can ruin it.
A new or good used sensor; one that only misreads hot can be stood off on a paper gasket of about 0.015 inch. A new one may need setting with Jaguar equipment.
An EGR valve stuck open
Carbon holds the EGR valve open, and exhaust gas floods the intake just when the engine needs clean air — at idle and on lift-off.
Stalls as you lift off and come to a stop, or a pulsing idle at every temperature; sometimes P1400.
Ignition on, engine off: the shaft should cycle and settle smoothly, and close again some seconds after switch-off.
Clean the shaft until it moves freely, or fit a new valve. Recall R493 also covered the EGR valve and an ECU reflash.
Idle learning lost after a clean or battery change
The ECU learns its idle position and only relearns slowly. After a clean or a battery change the old value no longer fits.
High idle straight after a throttle body clean, or a week later, with nothing mechanically wrong.
Rule out the spring and gum first, then let it relearn before buying parts.
Battery off for about 15 minutes, then a 30-minute drive; it can take a few drive cycles. A high idle just after a cold start is normal and should drop by itself.
The coolant temperature sensor, or the airflow meter
A sensor that reads warm too early makes the ECU cut the fuel and the cold idle support while the engine is still cold.
Starts well, then within minutes the idle sags and it stalls at stops until fully warm. Unplugging the sensor lifts the idle. Here the coolant sensor is the classic cheap culprit after a new throttle position sensor has changed nothing.
Measure it cold and after a short drive; for example about 2.8 kohms at 70 °F but under 0.9 kohms at 99 °F is out of range. Make sure it is the right sensor. Frozen lambda readings on a scan point to the airflow meter.
The correct coolant sensor; on a stubborn car, a second-hand airflow meter as well.
An air leak — breather hose or intake
Unmetered air leans the mixture out. A cracked breather hose under the bonnet is enough to stall it when hot.
Stalls or sputters hot and won't restart for a few minutes. Gearbox codes such as P0727 and P1775 can appear only because the engine died on the move. A leak can also make the idle hunt: revs rise until the ECU cuts fuel on overrun, fall, and rise again. The concertina hose from the air cleaner can split on its underside, where you neither see nor feel it.
Look over the breather hoses, intake bellows and the joints between airflow meter and throttle body; check the throttle body bolts are tight. With the engine idling, spray starting fluid or lighter fuel along hoses and joints and listen for the idle to change. The throttle body sits on the manifold with no gasket — clean both faces and seal with a non-setting jointing compound on refitting.
Replace the cracked hose or seal the leak.
Cutting out on the move: crank sensor, its wiring or the ECU plug
This is a different fault from a poor idle: the engine loses its speed signal and dies outright. Heat or a chafed wire does it.
Dies after 20 minutes of fast driving and restarts only after it cools; or stumbles badly around 3000 rpm and the rev counter drops to zero while cranking.
Read the codes (P0335, P0340). Wiggle the crank sensor harness with the engine running. Clean the white ECU connector behind the right headlamp and look for road-salt corrosion.
Another crank sensor — a new one can fail too — a repaired harness wire, or a cleaned ECU plug and fresh relays.
Low battery voltage and poor earths
The idle electronics need a healthy supply. Below about 12.5 V before starting, the idle can stall after a cold restart.
Stalls after the car has stood, an old battery, or warning lamps flickering at the same time.
Measure the battery before starting; charge it overnight and see if the stalling stops. Clean and grease the battery, bulkhead and engine earths.
Worth trying before any part: a charged or new battery and clean earths.
A failing ignition coil or oily plug wells
Each cylinder has its own coil on the plug. One coil missing every few dozen revolutions gives a lumpy idle that may never set a code, and oil seeping into the plug wells lets the spark track to earth.
A lump in the idle in Park and Neutral that smooths out with revs, worse when warm; sometimes a slight flatness on the move. New coils are not above suspicion.
Lift the coils and look for oil in the wells and tracking on the boots. A miss without a code can need a current probe and scope to catch.
Worth trying: clean and dry the plug wells, fit new plugs, and replace the coil that shows the miss; cure the cam cover leak that put oil there.
Cracked exhaust manifolds upsetting the oxygen sensors
The six-cylinder manifolds can crack. Air drawn in through the crack reaches the oxygen sensors, which then report a lean mixture that isn't there, and the ECU corrects the idle the wrong way.
An erratic or lumpy idle that has survived the throttle body clean and the usual sensors, often with a ticking from the manifold when cold.
Look and listen along the manifolds from cold for a tick or soot trail; read the fuel trims on a scanner — one bank pulling far richer than the other points at a leak before its sensor.
Worth trying once the cheaper causes are ruled out: weld or replace the cracked manifold.
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 the codes and the idle
Warm idle should be about 700–800 rpm. A generic reader shows the codes but can't reset the idle.
a code readerClose the throttle by hand
If a high idle drops, it's the return spring or gum at the butterfly. Try this before anything else.
freeCheck the battery and earths
At least 12.5 V before starting. Clean the earths and the white ECU plug behind the right headlamp.
a multimeterClean the throttle body off the car
Bore, blade, bypass hole and idle valve. Keep cleaner off the position sensor.
carb cleanerLet it relearn
Battery off for 15 minutes, then a proper drive. Judge the idle after that, not straight away.
freeTest the EGR valve and look for air leaks
Ignition on: the EGR shaft should cycle and settle. Breather hoses, bellows, throttle body bolts.
freeMeasure the sensors
Throttle position sensor near 0.5 V at idle; coolant sensor cold and warm.
a multimeterOnly then buy parts
Idle valve or throttle position sensor, matched to what the checks showed.
a valve or sensorWhat it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJ6 (X300) Battery Drain, Flat BatteryIf it stalls after standing and the battery is low before you start.
- Jaguar XJ6 (X300) Alternator Not ChargingIf the voltmeter drops at idle as the revs sag.
- Jaguar XJ6 (X300) Immobiliser Problems, Won't StartIf it cuts out and then won't fire again at all.
- XJ6 (X300) fuse boxes and relaysWhere the relays sit — 1996 and 1997 from the same page.
- Jaguar XJ6 (X300) Cranks But Won't StartIf it stalled and now spins over but won't fire again.
- Jaguar XJ6 (X300) Fuel Pump ProblemsIf it loses power or cuts out and the fuel pump is the suspect.
- Jaguar XJ6 (X300) Heater Not WorkingIf the gauge reads low and the heater blows cool as well.
- Jaguar XJ6 (X300) ABS Light On, ABS PumpIf the throttle hangs open or the engine drops to idle on a car with traction control, the actuator cables are covered on the ABS light page.
- 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.