Jaguar XJ8 (X350) Misfire and Rough Idle
On the X350 a misfire usually comes down to one ignition coil, and a failing coil can even upset the other modules and the gearbox. If the engine only runs rough at idle and is fine in gear, I'd look at the throttle body and for an air leak before buying coils. Rail pressure codes point at the fuel filter.
How bad is it · Garage, carefully
Drive gently to a garage, not on long trips
The engine management cuts power with Restricted Performance, and a car that dies at every stop is a hazard in traffic. A short, gentle drive to get it looked at is fine.
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.
A failing ignition coil, or its wiring
Each cylinder has its own coil on top of the plug, and a failing one stops that cylinder firing. On these cars a breaking-down coil can also radiate interference into the CAN bus, so the gearbox, ABS, parking brake and instrument cluster modules lose touch with each other.
A misfire code for one cylinder, often with Restricted Performance on a long run. A less obvious sign is harsh, jerky shifts that are worst in the first twenty minutes from cold, with lost communication codes between the gearbox and other modules.
Before buying parts, look at the coil connectors and wiring, the earth points, and the battery: at least 12.45 V. If the gearbox is acting up, watch the misfire count per cylinder live before suspecting the transmission.
Replace the coil on the cylinder that's misfiring, with new NGK plugs; for example a Delphi GN10448 coil on cylinder 8 cured both the miss and the gearbox. If the same cylinder keeps missing, its injector can go at the same time.
A dirty throttle body starving the idle
Soot builds up as a ring around the throttle valve and chokes the small amount of air the engine needs at idle. Under load the throttle is open, so the restriction doesn't matter.
Fine in Drive or Reverse, rough and dying in Park or Neutral. Once warm the idle swings, for example between about 1000 and 500 rpm, and the engine stalls at stops. Often no check engine light, yet a scanner shows misfires on several cylinders and a random misfire.
With the engine off, have a helper open the throttle and look at the bore behind the valve for a black ring.
Wipe the bore clean by hand with the throttle held open, then do a proper job with throttle body cleaner. Don't force the valve open by hand, and don't drench the coating with spray.
An air leak on the intake side
Air that gets in past the meter leans the mixture and the engine runs unevenly. A small leak may not be enough to set a code.
The idle is a little uneven and the car flutters at some road speeds, worst at motorway pace, yet there are no codes and the fuel trims read normal.
With the engine idling, go along the intake pipes and hoses with an unlit propane torch or a little starting fluid; where the idle changes, the leak is there. Don't touch the throttle stop screw: idle is controlled by the engine management.
Patch or replace the cracked pipe. An hour's labour put a 3.0 V6 back to running smoothly.
A clogged fuel filter
A blocked filter restricts flow, so the rail pressure swings low and then high and the engine runs lean in bursts.
Misfire, hesitation and a start-then-stall, with P0087 and P0088 (rail pressure too low and too high) on bank 1. Cycling the key a few times before cranking helps it start. Injector cleaner helps for one tankful, then the fault returns; clearing codes buys only 20 to 30 miles.
Read the codes: rail pressure codes rather than dirty injectors point at the supply.
Replace the filter. It sits behind an access panel under the floor on the driver's side of a left-hand-drive car: 10 mm bolts for the panel, a plastic nut for the cover, 16 mm unions cracked slowly to release the pressure. Fit the new one with the outlet towards the engine. Premium fuel and an injector clean are worth trying as well.
Chewed injector wiring on one cylinder
Rodents bite through the wires right at the injector plug, the circuit goes open and that cylinder gets no fuel.
P0208 (cylinder 8 injector circuit open) with P0308 and P0316, a shaking engine and the check engine light on.
Look at the connector on the injector the code names: it can be lying loose with no wires on it.
Fit a new injector connector, solder the joints, cover them with heat shrink and wrap the harness in cloth tape. A basic scanner may not show misfire counters on this engine, so judge by ear and by the light staying out.
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 note which cylinder
A single P030x points at that cylinder's coil, plug or injector. P0087/P0088 send you to the fuel filter, P0208 to the injector wiring. Misfires on many cylinders at idle point at air, not ignition.
free with a code readerCheck the battery, earths and coil connectors
At least 12.45 V at the battery, clean earth points, coil plugs seated and undamaged. Look at the injector plug on the cylinder named while you're there.
freeNote when it runs rough
If it's rough only in Park or Neutral and fine in gear, go to the throttle body. If it flutters at speed with no codes, hunt for an air leak with propane or starting fluid.
freeClean the throttle body
Engine off, throttle held open by a helper, wipe the soot ring out of the bore. Don't force the valve open.
a can of throttle body cleanerReplace the coil on the misfiring cylinder
One coil and fresh NGK plugs. If the code comes back on the same cylinder, look at its injector.
a coil and plugsChange the fuel filter if the rail pressure codes are there
Release the pressure slowly at the unions and fit the new filter with the outlet towards the engine.
a filter, ten minutesWhat it usually isn't
Don't spend money here first:
If none of this finds it:
On jagaux.com
- Jaguar XJ (X350) DSC Not Available, Engine System FaultIf Engine System Fault or DSC Not Available shows with the rough running.
- XJ (X350) fuse boxes and full fuse listEvery box on this car, drawn, with what each position feeds.
- Jaguar XJ (X350) Radio Code After Battery ChangeIf you disconnect the battery for this work, have the radio code ready.
- 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.