Citroen XM Forum

Not quite.

I do not think any of the valves touched the pistons as there is/was not one mark on the crowns, no "indents" in the soot on top of them. We both think that the sudden forced slowing/stopping of the engine (it was at idle reportedly) broke the rockers with the shock and the valves instantly retracted on their springs avoiding contact with the pistons. I think this is backed up by the valve stems being bent at the top, marking on the cam lobes and the shattered drive on the Vac pump (that still works)...but who knows exactly.

On these when pistons hit valves they don't always mark the pistons.. You need a good thud to mark them. They don't tend to mark because the rock finger tips snap.. The pistons gave to hit valves for any upward break to happen..

Why no marking in the carbon/soot deposits on top of the pistons then ?......for me this is the conundrum.

Because you have run the engine before you even took the head off so it's covered it up again

Is the vacuum pump seized? I know of cases on HDI engines where a seized vacuum pump has broken the camshaft or stripped the teeth on the timing belt.

citroenxm wrote: Fri May 28, 2021 8:44 am Because you have run the engine before you even took the head off so it's covered it up again

I would have expected to see a few compression markings in it ?

xantia_v6 wrote: Fri May 28, 2021 10:22 am Is the vacuum pump seized? I know of cases on HDI engines where a seized vacuum pump has broken the camshaft or stripped the teeth on the timing belt.

No, it still works and does engage on the small tab that is left on one of the teeth.

Stickfinger wrote: Fri May 28, 2021 9:30 pm

citroenxm wrote: Fri May 28, 2021 8:44 am Because you have run the engine before you even took the head off so it's covered it up again

I would have expected to see a few compression markings in it ?

You wont see any witness mark due to both the pistons and head being flat, so the valve contact area is large and even.

There has been metal to metal contact, hence all the cast components (cam followers, cam pulley, vac pump drive dog) have shattered.

Belt debris trapped between the timing belt and cam pulley caused the cam timing to retard until the pistons and valves met. The engine was still rotating, thus multiple cylinders were affected.

If sudden stoppage caused this type of failure it would happen everytime one stalled the engine.

what a ball ache....the space at the side of the springs greatly restricted my available tools !.....I had to modify my spring compressor and use a large nut to be able to manipulate the valve stem collets into place.
What pillock thought 12valves engines were a good idea

Never had a problem with my spring compressor.

Amuters with lesser quality tools I reckon..

Yep....totally.