My XJ-S is 1990 5.3 V12 pre facelift.
My V12 make sharp knock noise like detonation when I down the throttle pedal, and the noise start at 1,500 rpm and at over 2,000rpm the noise stop.
I prospected possibility fuel delivery system trouble or electric distribution system trouble, so I've changed following parts but my V12 has made the noise even now.

>Fuel Filter / Fuel Pump /Fuel Regulator
>Distribution cap / rotor.
>Ignition coils / Spark plug / Plug code

Now I'm continuing troubleshoot, and I funded the noise stop when I disconnect following the circuit with the black socket in the LH Engine room. Dose anyone know that the circuit functionality? and why stop the noise when I disconnect the circuit?

 

If I had to guess (and I am guessing)...maybe oxygen sensor?

Can you trace the front (left) side of the wire and see where it goes?

Thanks,

John
1987 XJ-S V12

 

Hi J_C_R

Thank you for your advice. I try to trace the wire and see where it goes, but I couldn't find out where it goes because it is wrapped with other cables....
Anyway your advice is helpful. It may be oxygen sensor cable. Because I've hard Marelli ignition system has functionality to adjust ignition timing by information from oxygen sensor, and if there is nothing information from oxygen sensor Marelli ECU change ignition timing to max value of delay.
So when I disconnect this cable, from oxygen sensor information to the ECU cut same time. and then the ECU decide to shift ignition timing to delay, as the result stop knocking.
It is just my assume, but I make sense why the noise stopped.

I suspect that the problem is leaded by power register of injector(Shortage of fuel injection). I'm continuing troubleshoot.

 

I foud following article!
Jaguar V12 Marelli Jumper

The line I asked is Marelli Jumper.
The Marelli uses an advanced timing map requiring premium octane fuel. With the jumper removed, the Marelli system switches to a more retarded map that allows the engine to run safely on lower octane fuel.

 

Good find. That knock will eventually make a hole in a piston, so leave it disconnected!