1993 XJS 4.0L with 109,000 miles on it…..
Here is the sequence of events…..
Blown head gasket between 5 and 6 - 6 plug sooty, always had a small amt of smoke at startup, always thought to be valve stem seals but who knows….best we could determine was a faulty injector sticking open causing too much raw fuel…. Hydraulically locked cylinder blowing the weak part, that is the head gasket…...
Rebuilt head, relapped vales, milled very little to get corrosion down
Replaced head gasket, all injectors, fuel P regulator, plugs
Replaced wires, cap and rotor.
Car ran fine
Month later, got the bogging coming and going - hard to go up hills, had to limp home at 35MPH but made it, felt like backfiring at times. This happened TWICE as sometimes it ran fine, sometime not!
No codes yet.
Had a spare crank position sensor so replaced that (new one) as preventive measure
Talked to a few reputable mechanics and we agreed it was fuel pump or cats. Since it ran well sometimes and bogged other times and had trouble up hills the fuel pump was replaced along with the fuel filter, got a FF23, reset computer/disconnected battery….ran fine for a bit then almost stalled while driving and popped a code 44 or 45 (o2 sensor) and didn’t stall, picked up speed like back to normal once the check engine light came on. Disconnected battery, code went away but came back. Replaced o2 sensor, no more o2 sensor code. Drove car 1200 miles, FF23 again came back. It almost seems like it misses when it wants to, no frequency like every 10 secs…maybe every 2-10 minutes…kinda feels like the tranny is slipping when it feels like it skips a beat but has no regularity.....at times (definately startup cold) if you go put yur hand by the exhaust or listen there you can hear the miss....catch is its not always there only when it wants to be.
I read this, worth a shot?? How do you check a relay or do you just buy a new one or can you swap one?:
the first thing to check is the Oxy sensor relay, one of six on the firewall in the engine bay, the one with the YELLOW base.
THOUGHTS ???????????
Think it’s the CATs????????? OR?????

 

Have you downloaded the S91 Publication for AJ6 4.0 Engine management????

I uploaded it a while ago as a 13MB .pdf. It is in the 'sticky' section somewhere.

It has a trouble shooting 'tree' to diagnose faults. The use of the 'datalogger' function of PDU would help but there are workarounds.

bob gauff