Jaguar XJ8 (X308) Restricted Performance
On the X308 the throttle body is behind most Restricted Performance warnings — worn position sensors inside it, or dirty plugs on it. I'd read the code, charge the battery and clean the throttle body plugs before buying anything, then look for cracked breather pipes.
How bad is it · Garage, carefully
Drive gently to a garage — but stop at once if you hear a clunk
The car limits power on purpose and usually gets you home. A clunk or a Low Oil Pressure warning with it can mean the timing chain has jumped — then switch off and have it recovered.
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.
Worn position sensors inside the electronic throttle body
The throttle body carries the pedal and butterfly position sensors, each read by two tracks, one rising and one falling, that must agree. The tracks wear or crack. When the two signals disagree, the engine computer cuts power to protect itself. The sensors aren't sold on their own.
Codes such as P1226, P1121 or P0121; a dead pedal or fail-safe that comes on when the engine is warm, often clearing after a restart. A worn spot can bite at one exact throttle opening, so the warning comes at the same speed every time.
Read the code first. On a 1999 car, look at the date code on the sensor on the passenger side of the body — anything from 10T5 to 11T25 means the body has to be replaced.
Fit a complete throttle body — new is safest, because many faults in a used one set no code. A reconditioned unit or re-greasing dried bearings has also brought a sticking body back.
Dirty or loose connectors at the throttle body and engine computers
The throttle body plugs on these engines weren't gold-plated at first, and Jaguar later changed them. A film of oxide is enough for the computer to lose a signal and limit power. The large plugs on the engine and gearbox computers and the ground studs suffer the same way.
Restricted Performance with no code, or straight after work near the air box or a long layup.
Unplug, clean and refit the throttle body plugs several times, then the big computer plugs. Wire-brush the ground studs behind the headlamps and on the bulkhead. Don't forget the ground studs on the left front inner wing near the ABS unit, and the plug of the second intake air temperature sensor, which can work loose.
Clean both halves with electronic contact cleaner or propanol, a little dielectric grease in the female half, new plug clips. On 1999–2001 AJ27 engines bulletin 303-58 covers P0121 from this connector.
Unmetered air — cracked breather pipes, injector seals or intake trunking
Air that slips in after the airflow meter makes the engine run lean. The computer reads it as a fault and limits power. The concertina breather pipe cracks where it meets the cam cover, and the injector O-rings harden.
Lumpy idle, often a check engine light alongside, and the warning coming and going at random. Revs held to about 3000 rpm, with a judder above that or above 50 mph.
Look at the full-load and part-load breathers and the anti-resonance pipe on the intake. Spraying cleaner around the manifold can miss a leak — a smoke test finds it. Follow the thin vacuum and breather pipes round the four-way connector on the right of the throttle body.
Replace the cracked breather (tape holds it for a short drive), clear the small orifice in the part-load breather, renew the injector O-rings and seat the clips properly. On an XJR a cracked intake trunk does the same.
A dead throttle body relay
The throttle motor has its own relay. When it stops switching, the code reads like a failed throttle unit or computer.
P1229 or P1251; cut-outs a couple of seconds after starting, or fail-safe with traction and stability control failed on the first start of the day while later starts are fine; speed or revs held low, for example 30 mph or 1700 rpm.
The throttle motor relay is the same part as the fog lamp relay: swap the two and see whether the fault moves. Do it before pricing a throttle body or a computer.
Replace the relay; a dealer may add a software update to the engine computer at the same visit.
A gummed-up throttle bore
Deposits around the butterfly upset the idle air the computer expects, so idle wanders and the warning can flash with it.
Idle stuck above 1000 rpm and rough, sometimes right after a half-done clean.
Disconnect the battery before moving the butterfly by hand, then look at the edge of the plate and the bore.
Clean it until it's spotless, not just better. An independent will clean one; a dealer will usually only replace it.
A failing coil, or the coil relay and ignition modules
A weak coil misfires under load and the computer limits power to protect the catalysts.
Spitting and coughing on acceleration, or judder when the engine is warm; a code naming one coil such as P0355, or oil in the plug well. Coil codes on both banks with P1367 point higher up.
Swap the suspect coil to another cylinder. For codes on both banks, swap the coil relay and the ignition modules and see whether the codes move. Look at the rubber boots: perished or cracked ones leak spark.
Replace the coil the codes follow and stop any oil leak into the plug well. With perished boots on most coils, a full set with new plugs cures it.
A timing chain that has jumped
Early plastic tensioners crack and let a chain skip teeth. The cam timing goes wrong and the computer sees misfires or crank and cam signals out of step.
A solid clunk, misfire codes across one bank such as P0300–P0304 with P1316, or P0336 and P0340 together; sometimes a Low Oil Pressure warning follows.
Stop cranking it and look at the chain and tensioners before anything else.
Replace the chains and tensioners. Shut down quickly and the engine can survive with nothing else damaged.
A body control module fault holding the brake lamps on
The engine computer watches the brake signal. If the module that drives the brake lamps holds them on, the engine sees the brake applied while you accelerate and drops into fail-safe, and cruise control drops out.
Fail-safe a few kilometres into a drive, cruise control cutting out, and the main brake lamps lit with your foot off the pedal while the high-level lamp behaves.
Have someone watch the lamps. If the main ones stay on with the pedal switch unplugged, the switch isn't to blame. Look for damage near the module, especially after a front-end accident.
An auto electrician repairing or replacing the body control module cured it.
Water in the plugs after an engine wash
The knock sensor plugs at the front of the engine and the crank sensor low on the bellhousing take on water. The computer sees a bad signal and limits power until it dries.
The warning appears right after rinsing the engine bay; the car often still drives normally.
Think back — was the engine washed, or driven through heavy rain? If a long run doesn't clear it, unplug, clean and reseat the connectors on top of the engine.
Worth trying first: a long warm drive, sometimes two or three hours, and the warning clears on its own. Don't hose this engine down.
An early throttle body covered by service action S514
The first throttle bodies on 1998 cars had a design fault. The engine keeps idling but stops answering the pedal, or stalls when you lift off.
A 1998 car that dies at motorway speed and restarts normally, or stalls off the throttle or under braking.
Look for the sticker that marks a replaced body and check the service history against S514 by VIN.
Worth pushing the dealer: depending on VIN they reprogram the computer or fit the revised body, free even on old cars. It applies to the early AJ26 management, not to later AJ27 cars.
A sticking brake pedal switch
The brake switch also feeds the cruise control. If it sends an implausible signal, the engine drops into Restricted Performance.
Occasional warnings with little real loss of power, often alongside stability control messages; a restart resets it.
A Jaguar-level scanner names the switch; a generic reader may not. With the seat fully back the switch comes out with two small spanners.
Worth trying: clean and lubricate the switch and its rods, or replace it.
A dirty airflow meter or air filter
A contaminated airflow element under-reads, the mixture goes wrong and the computer protects the engine.
A protective-mode warning after flooring the throttle and lifting off, or after the air box has been opened.
Look at the filter, then the meter element just behind the air box.
Worth trying before a new meter: spray the element and wires with throttle cleaner, and fit a new filter.
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 code, and leave the reader plugged in
Restricted Performance is always triggered by a fault code, but it can be brief or Jaguar-specific. Restricted Performance and Engine Fail Safe are different messages — note which one you get.
freeCharge and load-test the battery, then hard-reset
A weak battery can crank fine and still upset the computers. Both leads off, touch them together briefly, reconnect; UK cars may then ask for the radio code.
freeIf the engine was washed, drive it dry
A long warm run often clears it. If it doesn't, carry on below.
freeClean the throttle body and computer plugs and the ground studs
Electronic cleaner or propanol, not WD-40 — it leaves a film and attacks insulation.
freeCheck the breathers and intake for air leaks
Full-load and part-load breathers, the intake trunking and the injector seals. A smoke test finds what spraying misses.
a few poundsOn a 1998 car, check whether S514 was done
Look for the sticker on the throttle body and ask a dealer to check the VIN.
freeOnly now replace the throttle body
If the code is P1229 or P1251, swap the throttle motor relay with the fog lamp relay first. A new body is the safer buy; a used one can hide faults that set no code.
£120–280What it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJ8 (X308) Engine Failsafe ModeIf the dash says Engine Failsafe Mode, especially with the cruise dropping out.
- Jaguar XJ8 (X308) Rough Idle and StallingIf there's no warning on the dash but the idle hunts or the engine dies when you lift off.
- Jaguar XJ8 (X308) Timing Chain Tensioner ProblemsIf there's a clunk or misfire codes across one bank, check the chains there before driving further.
- Jaguar XJ8 (X308) MisfireIf the code names a misfiring cylinder, follow the coil and plug checks there.
- Jaguar XJ8 (X308) Won't Start, No Crank or Car DeadIf it stalls and then won't restart, or a weak battery is throwing up warnings.
- Jaguar XJ8 (X308) Transmission ProblemsIf the dash says Gearbox Fault or TRANSMISSION FAULT, or the car is stuck in one gear.
- XJ8 (X308) fuse and relay boxesTo find the engine management relays before you swap them.
- Jaguar radio code generatorFree. For when the radio asks for its code after the hard reset.
- 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.