[W126 Coupe] Camshaft Snapped! Intermittent Rough Idle -Me too-HELP!

[A figment of the Imagination]

Does anyone know if the pistons on *these* engines
simply generate more than an unusual amount of thrust,
that translates into stress that ultimately wears out
lifters, rocker arms, cams, camshafts, journal
bearings, bearing bolts, to the point that a weak spot
in the metal could develop that ultimately severs the
camshaft or that loosens a piece of metal to jam the
camshaft and promote a fracture?  



For instance, if you had been driving around for a
while with a high idle speed problem and a cylinder
that was not functioning correctly causing rough
idle/acceleration, this could have translated into
uneven stress along the camshaft.  This differential
in force could have contributed to metal weakening,
ultimately leading to fracture at the shearing point
from two opposite forces.  So, it's important to
carefully identify where the break occurred.  Once
that is done, you will need to identify what created
the differential in force along the camshaft.  You
should check for worn out rocker arms that could have
been so worn out that they became loose and then
jammed your camshaft while the engine was rotating. 
Other causes could be broken/loosened springs,
lifters, journals, journal bolts....
  
If the problem actually occurred during idle instead
of while driving (after the loud noise), a very high
idle speed could have provided all the force that was
necessary to snap your camshaft.  At the notorious
15,000 rpm high idle speed, there would be plenty of
torque for shearing and there would be plenty of
vibration for movement of a flattened rocker arm (or
other metal element) when it was coupled with a poorly
functioning cylinder (or cams) to begin with.  As
mentioned, amplification of some resonant frequency
could have contributed to the damage.


It would helpful if you could describe any unusual
metal sounds, snapping, clanging, loud high frequency
tapping, thunks,  overwhelmingly rough
idle/acceleration during idle and/or the drive (after
the initial noise)?


Regarding the extent of the damage:

Based on what you have written, my guess would be that
you may have lucked out on what could have been much
worse.  While I have no real idea, MBZ engineers could
have additionally designed the camshafts to be made of
a softer metal to deliberately shear/fracture to
minimize compressive forces against valves and lifters
 per cylinder, specifically more so, for the cylinder
where the fracture occured.  

The answer to extent of the damage really depends on
how intact the rest of the camshaft was and how
tightly the journals were secure to the cylinder head.
 While you are examining the system, if you find that
*all* your journals came undone, you may be looking at
stripped out journal bolt sockets and extensive damage
throughout the system (valves, rods, rocker arms,
lifters, cams, cylinder, heads, etc....) If the
journals were intact, you may want to look for broken
or worn out elements from rocker arms, lifters,
springs, other metals.  Any loose metal, as stated
above, could have jammed the camshaft prohibiting it
from normal rotation.

So the next thing to look for would be the fracture? 
Is it a tranverse fx or a spiral one?  Transverse fx
are straight across.  Spirals have a slight angle to
them.  Look at the cross-section of the shaft for
signs of shearing.  A transverse fx could point to a
hammer like compressive force such a throwing of the
timing chain.  A spiral fx could point to something
jamming your camshaft, like a loosened, misplaced
rocker arm (from a poorly firing cylinder, loosened
journal bolts, stripped journal bolt sockets,
mistiming from the distributor cap, loose plugs,
wires, etc...everything on forward creating uneven or
overly amplified destructive resonant
frequencies/harmonics).

You will definitely have to identify the type of fx
because it may help us all learn about ways of
preventing such a catastrophe.  It will also allow you
a way to go in and properly correct the root of the
problem.

On the flip side, the MBZ camshafts may have a serious
material or craftsman ship flaw.  If this can be
proven, you may be eligible, if some warranty still
existed, for compensation.  You may want to inspect
the camshaft and try to isolate if any seams came
loose.



Here's some food for thought:


What seems to cause most timing chains to break?  Does
this problem relate to inferior
camshafts/housings/springs/rocker
arms/lifters/assembly/timing chains, guides?

Could camshafts simply get jammed while the engine is
turning and thus induce timing chain breakage?  If so,
what are the possibilities to cause this? I've already
discussed some theoretical causes.



Finally:

IMO, I still consider your 560 SEC as a masterpiece
worthy of keeping. In general, I think it is a
trade-off...the more time and energy you work to put
into the car, the greater and sweeter is the ride,
when it is all done.  Your car deserves it.  So, I
recommend fixing it.

Without any question, these SECs still have the
finest/smoothest ride of any car I've ever driven. 
Just an opinion, of course.

Good luck.

-Figstir


	
		
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