So the car has been fine, noticed a very slighted rough idle at start over the last month so but within seconds it smooths out. Figured maybe she is just getting tired but only has 79K on her, FYI, 2006 XJR.

Anyway, get in the car yesterday morning, start her up and way rough idle, engine light comes on and I immediately plug in my scanner. Pops up 3 fault codes, the pesky p0401 (pops up every 100 miles or so) but two I have never seen, the 302 and 307, cylinder misfire. By the time I transfer the numbers to my phone so I can check them on the computer it smooths out. The rest of the day, probably 6 more starts, she is fine. Its 7am here, probably wont leave today until 2ish so I will see again but I needed to ask my Jaguar brethren, first thoughts ?

I have passed on some of the normal maintenance (for the mileage) over the last 5000 miles as I was on the fence about keeping or trading the car but after new air struts, tires, wheels, waterpump, thermostat Ive opted to keep her for the long haul.

Just need to figure out this issue and get it corrected.

Sorry for the long post but its been awhile since Ive had any issue.

TIA !

 

Stick to the full code names P0302 and P0307 then you can see what others did to fix them (Search). Usually bad coils.

I'd want to fix the P0401 as well...

 

The first thing I would do is move the plugs 2/7 to another cylinder then see if the misfire moves to the new cyl then I would move the coil packs see if the misfire moves then this way you can find out if it's the plugs or coil packs but I would change the plugs for new ones anyway at that mileage what's p0401 as I cannot find that fault code

 

Thanks guys...... it hasn't happen again but that doesn't mean its ok. I think it is time for plugs for sure. The p0401 is an EGR related code which I will check the wires then clean and see if that clears that up permanently.

 

Plugs or coils paxks on plugs. Assuming file trims are ok.

I've just had an occasional roughness at idle, but no error codes or pending errors, but was having really bad gear changes. Turned out new coils fixed the gear changes, and the cold roughness. Now the coils themselves are 4 wire and are meant to detect misfires, but i understand that the software is lenient on reporting misfires. I was most amazed at the effect on gear shift, i was having flare on up *****, sometimes false neutrals. So glad it was not a transmission issue.
i replaced the whole set, do not know the quality, as a set of 8 were $120 on amazon, and one OEM Denso is $100.

I'm trying to make sense of the mode $6 OBD codes that might show realtime miss counts.

 

My car once began doing that. Rough upon starting then smoothing out. Then it began running rough longer and longer, then constantly. Turned out to by the cylinder coil.
Tom

 

Ill probably just replace the coils, hasn't done it again since I posted 6 days ago but with just turned 80K I guess its part of general maintenance. While I'm at it is probably spark plug time also.