I have just had the chains and tensioners done on our XKR (1998, 4.0). I thought I would share a few photos of the parts taken out by the mechanic, as it turns out the noise was due to one of the 2nd gen (I think?) aluminium tensioners failing. How likely this is to happen to other cars is a question for more experienced people than me. It strikes me as an uncommon mode of failure.

The internal parts of the failed tensioner seem to have jammed in some way and it came apart in the box after removal. The plastic slipper is loose on the metal although it did not fall off.

Also, two of the chain guides show significant cracking and look to me as though they were not going to last very much longer. The primary tensioners were in good order to my eye.

The chains showed only light wear so I haven't taken any shots of them.

 

Failed tensioner on the left:

The piece in the end of the upper section is jammed solid in the position shown. Presumably it is supposed to slide inside the tube when pumped up with oil from the engine. The piece in the middle is a tube and was loose in the box. I guess it fits inside the lower part of the tensioner.

 

Chain guides and primary tensioners.

One crack almost the whole way across one guide and a number of smaller cracks radiating from corners. I seem to remember this is the same failure pattern that brought down the comet jets in the 1950's - fatigue cracks starting in the corners of square windows and propagating to the point where the airframes failed.

 

Thanks for the info and pictures. This is about the third aluminum tensioner failure that I remember on the forum so, like most things, it is not fail proof part.

What led you to open up the engine? Rattle, noise of some other kind?

This issue of tensioner is supposed to 'ratchet' up so as to prevent the chain from jumping time. Looks like the ratchet broke.

 

There was a rattle, so I got the mechanic to look at it when it was in for a brake line repair.

It had been quietly making a bit of noise now and then for a while but gradually got worse. It was mostly only apparent at hot idle. When started from cold or when under power it was quiet for the most part. This did not sound the way I had been told to believe a chain would sound, so I was actually glad to find out it was a tensioner and not a big end bearing or a bent valve or something else that would require taking the engine to bits.