On the return leg of a grocery-getting trip tonight, I noticed the skinny pedal going down much farther than usual for a given thrust level, then noticed that the idle was low and rough.

I pulled over and restarted. Regular operation resumed, but not permanently. Most of the last mile was normal, but it was limping again in the final block. No CEL. Scanned for codes, nothing new. I briefly watched short term fuel trim data while idling and stationary revving; Bank 1 ('A' Bank, we assume) looked normal and was dancing around +/-3%. Bank 2 (B?) was pegged at 19.5% or -19 or something, but not moving.

What should I check next? If there isn't anything that commonly causes half the engine to stop contributing, well, I've been meaning to bring it into The Jag Shop for front-subframe-out service anyway.

Also, the voltage gauge appeared lower than typical too; through OBD it rose kinda convincingly off idle, but only to maybe 13.3 rather than a more healthy 14+. Maybe just a failing alternator?

 

Start with the simple stuff.
Switch coil packs to insure both are good.
Check to see if you have fuel at the schrader valve.
Was it misfiring at all before A bank stop working? Multiple misfires will shut down the ecm to that side... fuel and spark.
Check your relays to insure no water damage on the pins.
I bet we can get you running without a paying a mechanic.​​​​​

 

Looks like this thread documents an identical failure mode, with a resolution. I'll pore over that thread and wiring diagrams tonight.

 

Here are the two usual suspects, copied right from Lady P's post (from 2017) on the current X300 thread "O2 sensor location". These connectors are susceptible to moisture and dirt, as Ahmed found out. Lady P (Parker) always suggested cleaning these in all cases of an ignition problem. Apparently he has abandoned the forums again! Too bad. He is a giver and a never-give-upper. Hope he returns.



I'm betting it is a wiring problem and once resolved you will be better off for it.

 

I undid the black and beige multiplugs under the relays and found no corrosion or apparent damage. The airbox bracket looked like it may have been cutting into the black connector's (PI61?) harness, but pulling back the electrical tape revealed nothing untoward. I also unplugged and replugged all six relays in the area; none looked to be compromised.

What are the brainboxes in front of the relays and behind the RH high beam with the brown multiway plugs? Are they ignition modules or something? Should I unplug and replug them too?

I started the car, idled a few minutes, and revved it up several times. No sign of anything wrong, it ran perfectly. Guess I'll have to road test it. >shrug<

 

Those brainboxes are the front lighting control modules. These cars have a lot of tech, right! And it takes up a lot of space in all these discrete modules tucked away here and there. And requires about 30 miles of wiring. Somewhere along production in 96 the lighting tech was condensed into a single module in the rear fuse box, looking identical to your 95 rear lighting control module in the same location.

Fun fact: the 96+ updated lighting control module does not include the side marker lights or the headlight gallery lights in the bulb fail system.

Keep unplugging and replugging connectors, that simple act could restore connectivity where it may have been intermittent before.

 

May just be worth checking the clip mentioned in this TSB
http://www.jagrepair.com/images/TSB/...%20Harness.pdf

 

Eureka, I have had one of my cars kicking my *** for the last three years on a no start. I'd mess with it, get frustrated and walk away, three times now. Missed this service bulletin and the clip was not only present but had damaged the loom. I have to drain out the gas and replace with new and reinstall a battery but I am confident this is my issue. Thank you Cafcpete you have made this yanks day!

 

I wasn't aware of this rogue clip either, missed another TSB. I don't recall seeing a clip like that but there is so much to see under that hood! I will be looking for it next time I say hello to the V12.

P.S. I've never lifted the hood of any car so much as this one. Not for repairs, but to check in and spend some quality time together. We enjoy each other's company and have a mutual respect for one another!

 

A road test elicited the fault again, after about 10 minutes of normal operation. The transmission light illuminated again too. It went upon restart.
While misbehaving, the B2STFT climbs steadily to 19.53%, stays there, then drops to zero. The exhaust does not stink of raw fuel, so it does seem that the ECU is trying to command the B Bank injectors but unable.
After I parked the car at home, I restarted it, idled and revved. Back to normal, B2STFT swinging around -3 to +7. Still no CEL, but noticed the readiness monitors are now unset (P1000).

My car appears to be rogue-clip-free. Perhaps I need to check the harness plugs at the ECU for notorious water intrusion?

EDIT: I remember now that the previous owner had rodent damage repaired on the injector harness. There could be an intermittent open circuit in the B Bank injectors' common voltage feed. I wonder if I'll be able to track it down before I bring it into the shop on Friday.