2006 XJ8, felt a single bump last Wed while sitting at a stoplight, thought it might be the air suspension but nothing further happened.
Thurs, 3 bumps intermittently while driving through the city.
Fri, clear rough running at idle, rev countering bouncing around at startup. Bumps are obviously misfires/detonations.

1st scan gave me about 40 ECU errors, usually pairs of 'high-low' errors. ghostbin.com/paste/H6zBN
I cleared the codes and tried again, this time got 362 errors, including Cylinder 3 misfire. ghostbin.com/paste/tLI1f
3rd and final scan, all cylinders reported misfire and I had to stop midway because of how rough it sounded. ghostbin.com/paste/FN5fh

Obviously something important has failed and continues to deteriorate. Does anyone have any idea how I could begin to diagnose this? Should I leave it to a mechanic instead? Usually I'm ok at working on the car, but all the errors have me completely bewildered at where to look.

Am I at least right that the problem is more likely mechanical rather than electronic, given the worsening with time? I've not tried to start the car since.



 

Use a smart phone with the Torque app to connect to the vehicle PCM via the OBD port with a Bluetooth interface to read the fuel pressure.

You can also use the app to read STFTs and LTFTs to see how much the PCM is attempting to compensate for either lack of fuel or a massive vacuum leak.

Under no circumstances should the vehicle be operated under conditions that cause the MIL to flash as catalyst damage is occurring.

 

So many errors at once typically mean a serious electrical power issue e.g. bad battery, bad battery connection.

 

Good point on battery. Must be sure that the battery is fully charged and then clear and retest. Maybe even disconnect the battery, charge it and start all over. Good luck
RyeJag

 

As they said, start with the battery. Even if the vehicle is able to start without hesitation doesn't mean that the battery have enough voltage. That happened to me and I was looking at everything and everything changed when I finally replaced the battery. That amount of fault codes are not a common thing just like that unless that has something to do with your battery.

 

After checking the battery - charge if needed and run codes again. A full charge may narrow some codes. I would then check the easy stuff first; like check the air intake tube from engine to airbox (they sometime develop holes or cracks) check the air filter, check and clean the MAF, Look at the VVT seals for oil leaks, check for any vacuum leaks - use carb cleaner or water in a spray bottle and spray around all plastic pipes and listen for change in RPM's that would identify a vacuum leak and might as well check the oil dip stick o-ring since you're under the hood.

Next I would check injector wires and connectors, spark plugs, spark plug seals and coils, fuel pressure etc.

 

Agree with the others about the issue, most likely being electrical.
This many codes, at the same time, first indicates electrical, then possibly a fuel issue to me.
But this is just a guess at best,