I am new here but have owned four Jags in the past including a XJS coupe. I just picked up a red convertible 91 with the V12. It sat for 7 years in a wet enviroment UGH.... It ran very rough and was making a rattling sound that sound internal. I first replaced wires, cap, rotor and plugs. It runs more smoothly but is very sluggish. I am thinking it is a vacuum leak or maybe the injectors need a good cleaningwith the poor running. Here is my problem I could use help with. The rattling sound is something that has me puzzled. It sounds like it is coming from the crankshaft area. I do not think it is the famed valve seats issue. Not a valve noise clicking. I disconnected all the belts to see if maybe it was a water pump or alternator or steering pump noise. Noise still there. There is a brand new starter on the engine. Is it possible the starter bendix is hanging up? Or would a transmission make that noise. I hear the noise above the car and underneath the car on a hoist. It is not rod knock. I have good oil pressure and the compression was 210 pound on each cylinder. Any ideas before I pull the engine for rebuild.

 

Have you used the screwdriver stethoscope to narrow down where the noise is coming from? Touch the screwdriver to the block and the sound should get louder the closer you get to the source.

The V12 does have noisy tappets if they have not been adjusted, or it could be timing chain. The bottom ends on the V12 are usually pretty bullet proof have you changed the oil and if so what was its condition?

 

I have used a stethoscope and seems like the noise is internal. I was also thinking it be a timing chain. Pasenger side of the block seems a little louder. Not tappets or valves. What does a loose timing chain sound like? I am also thinking would a timing chain cause the engine to not want to rev smoothly or quickly. The sound is like something like a loose part is flying around hitting something. Not crazy loud but very noticable.

 

If the oil has not been changed, drain it and get some of the bottom sediment to test for particle magnetism or not. Isolate the area of the noise as best you can so you can determine what is in that area. With a stethoscope, you should be able to tell if it is the engine or the transmission for sure. Listen around the flex plate and torque converter. Does the noise change any increasing rpms..if so what is the change? Does it change when put in gear, etc.
Are there any other indications of a problem like excessive pressure in the crankcase, emulsified oil, oil in the coolant, anything out the tailpipe etc.
Once you have a list of symptoms you may be able to find a direction to determine what the problem is before removing the engine and then finding out that it wasn't necessary.