Whilst trying to diagnose an idle issue my XJS started making a lound ticking sound from cylinder 12. So I checked the compression and got a dry reading of 130psi from the suspect cylinder. I would’ve thought that a dropped valve seat would not hold much if not any compression. The only other cylinder below 170psi was #4 at 130psi. Is it possible to drop a seat and still have compression, or could it just be a sticking valve even something else?
Valve seats can drop different amounts. They can drop just a mm or so and continue to open and close, everything still working and cylinder still firing, only problem is that tappet now has a mm more clearance! It’ll rattle something awful. At the other end of the scale, the valve seat can fall completely out, get hammered to bits by the valve and the piston, and then get chucked out the exhaust tract and end up plugging up a catalytic converter. Then your clearances close to zero and that cylinder quits working.
Different symptoms, same result: Head has to come off.
A lot of overhead cam engines can make a top end ticking sound due to bad lubrication. How is your oil pressure? Lot of my old cars tick a bit at start up. Have you tried changing your oil? And cleaning your oil? What oil weight are you running?
Also, FWIW I noticed my V12 making ticking noises at idle. Got my stethoscope, just loud injectors!