Xjs 1992 v12, 5.3, marelli
I have been trying to figure out this problem for a while now.
So, car at idle does run rough, it’s not misfiring, but I do feel uneven shaking and it’s getting worse when car is hot(after a drive) it’s more or less the same in all gears (more in Drive ,but I think that’s due to lower rpm on drive)
I don’t feel anything when on the road, but it doesn't mean anything, since v12 can hide small issues pretty well under speed

Ff34 and ff36 randomly pops out sometimes, usually after 5 mins of idling, sometimes gone for days and then back.

What’s been done
Both new 02 sensors
Both new camshaft speed sensors
Injector and ignition loom redone
Injectors cleaned and tested
New spark plugs
Both new coils and ign amps
New dist cap
Tried with another ECU
Vacuum checked , no leaks
New fpr and fuel pump
Every injector is clicking, tried to disconnect one injector at a time - nothing
TPS - correct
Blades adjusted several times

The only thing I know is that when it hot it’s getting worse and it overheats more when it’s already hot(don’t ask me, it’s a 2 years story) forget about overheating, anyway v12 shouldn’t feel like a 2 litre old diesel track, it’s really massaging my a** on idle.

P.s Cats not clogged

Also, I have been dealing with defected new parts a lot, so don’t cross out some new parts that I have put.

Now logically thinking, whats left untouched is
Rotor (it does look good)
marelli ECU (looks like new)
Injectors, yes, shop cleaned them and tested them, but i wouldnt trust, it was not a professional shop

Any ideas?

 

follow up question, could it be because of alternator not keeping up? i have noticed that with everything off, at idle it shows 13.1V, when efan kicks in - 12.5, if i turn on all lights, heater it reads at 11 and im not sure, but feels like bumpy idle gets worse (it could be that i want to think this way)

 

I had a similar issue with my Daimler Double Six and am about to write up on the cause. Rough idle at stop like the engine was sputtering, missing. This was especially apparent after driving for a while. Worse when AC was blowing. Then, the problem got worse and the car felt like it was going to cut out at traffic lights, with a near total loss of power.

It was the fuel injector harness. Exposed wires on three or four, brittle casings on all of them right next to fuel injectors.

 

That would be nice to find, but unfortunately i dont have anything on top or sides of the engine to investigate, everything was replaced and all looms were redone =( And the worst part of it, that the only thing that was bad is a wrap, wires were in perfect shape everywhere, but i rewired it anyway....

Update, its not temperature related, i was wrong, it just feels better with lower rpms, higher RPMs just hiding it. Once it gets up to temp, RPM goes to 700ish, and you can feel it better, drop it to Drive, RPM drops a hair lower and you can feel it even better.
Given that it has nothing to do with CTS, ATS, its the same in all gears(just feels different due to lower/higher RPM), the same in closed or open loops, so o2 sensors are not the case, ECU is not the case.
its even feels like that this shaking rhythm is not aligned with RPM!!?? its not speeding up when i revv it at least up to 1200 (after that im loosing it)