So.........................
The saga getting my '88 5spd V12, 150k km (bought Oct, 2022) back onto the road after some needed maintenance over the past several months continues!
To recap:
It's been at my mechanics since last Nov while he slowly addressed some pre-purchase agreement items (leaking cam covers, new heater core) and sorted a clutch issue (ended up going with a replacement Getrag 265 transmission kit from Simply Performance).
A week ago they fired it up after its been sitting inside for 6 months. The engine was 'hunting' at idle and a faulty injector was diagnosed. It did this briefly last fall but then self-cured. Injector replaced and hunting issue resolved.
It idles fine with healthy oil pressure and good compression. However, above around 2,000rpm there is a definite 'diesel clatter' coming from the top end. They went over it from head to toe with a stethoscope and the noise is coming from the right bank and can be heard in the exhaust downpipe.
The noise is variable. Sometimes quieter, sometimes louder. The working hypothesis is that coincidently an exhaust valve spring (#4 cyl?) has broken. The variable loudness could be accounted for by the spring oscillating and occasionally working better or less well, depending on the relative orientation of the two spring halves.
I have attached a recording with the car stationary, hood up,

It has been a bit of a head scratcher, so wondering if any of this strikes a chord with you V12 owners out there? Next step is they are referring the matter to a colleague who specialises in engine rebuilds etc. to get his thoughts.
If the diagnosis is correct, another question is whether replacing just the one is okay, or do them all as PM. Doing just the one seems to be the current thinking.

Thanks in advance for any and all thoughts.

Quentin

 

Hi, a blown exhaust manifold gasket can give quite loud , almost mechanical noise if gone, but may get quieter as the engine heats up.These are individual gaskets and not a nice job to change with the engine in situ.
These engines are prone to dropping valve seats if severely overheated but that noise is , from memory, a lot harsher. Has the car had a recent plug change? Its not uncommon for debris / foreign objects to fall down the plug holes during a change. Never heard of broken springs on a v12 but its not difficult, albeit a bit time consuming, to remove the inlet manifold, cam cover , camshaft and tappet block etc and that will reveal any issues. Good luck.

 

Thanks for your thoughts.
Yes, plugs changed soon after purchase, but has been driven since then with no noise. My mechanic has gone back in and looked around using a borescope to double check nothing has ended up where it shouldn't from work they have done on it over thge past few months. Nothing showed.
The noise is definitely akin to an old disel clatter. Will check with garage if they have ruled out blown exhaust manifold gasket.

Quentin

 

Never had a valve spring fracture on any of my many Jags, and they have been "driven as designed" mostly.

A loose spark plug will give that sound, fact. Too many people simply dont take that extra few minutes and really clean that area before removing the plugs, and dirt and other rubbish will prevent those taper seat plugs sealing. They may well "feel" tight, but are they????

Exhaust manifold gaskets, as mentioned, maybe.

Something in the cylinder, more of a Knock than a rattle. Had that with one where the owner would not listen, and dropped a washer in there, oops.

 

Grant,
Thanks for your thoughts.
interesting that you have never had a valve spring fail. My mechanic has never seen a failure on a V12 either. He is really thorough and diligent, so incorrect spark plug seating seems unlikely, but will raise the question.

He has ruled out leaking exhaust manifold and down pipe gaskets. Hasn't ruled out valve seat entirely but mostly.

Second opinion due on Monday.

Quentin

 

OK, got that.

Another way left thought, and in 55 years I have had 2 only.
The porcelain crimp of the spark plug was ill formed at manufacture, and compression pressure was escaping through the gap, BIT, more of an odd :whistle: than metallic noise.

Ill seated spark plugs, yep, I am guilty on a few, its so easy to get it "not quite right" at times.

Dropped valve seats on V12 are mainly on the B Bank, #5 or 6, AND, they CLATTER, often heard 2 houses away, and the engine runs like crap at all times.

Cracked Tappet Bucket, YES, had a good few of them, mainly on the XK 6cyl, BUT the V12 runs the same bucket, so anything is possible I suppose.

BUT again.

A Sticking bucket in the Tappet Block is a real thing. I use a "brew" of 10% ATF (cheapest out there) in the engine oil. Meaning, you have 10ltr sump, so 1ltr ATF, and let if IDLE ONLY for an hour or so, let it cool down, and repeat, then drain that oil as Hot as you can without burning yourself, new filter, fresh oil. Those I had are now quiet, and they got quieter with driving, the more of the latter the better.

 

Thanks Grant.
Car runs fine, no misfire, power good. Apparently each valve spring is an outer and an inner? I understand the smaller inner one has been known to break on occasion, but right now remains a theory. Noise is definitely diesel-esque as opposed to bag o spanners, so hoping not valve seat related.