MP4.2+6.0

For my fellow W12 owners, check this out. Maybe background 4.2 relevance as well.

I have been going through the car looking for anything out of order before the CPO warranty runs out. I looked at everything underneath very carefully, with both the forward (motor) belly pan and mid (tranny) belly pans removed. Similarly from above in the motor and plenum areas, and underneath further back like the tranny tail seal and the rear diff and exhaust. All looked in very good shape--parts, bolt ups, mounts, no leaks, etc.--other than:

1) The u connecting links to the sway bar are starting to show just a bit of deterioration of the embedded rubber bushings on both sides, at the lower bolt up point where the bar attaches. About a $75/side list part for dealer OE and easy to change, so not a huge deal. CPO covers these (unlike many other suspension and CV wear parts apparently), if they conclude they are bad (enough).

2) Minor oil staining (no dripping) limited to passenger side valve cover toward the rear; looks like spark plug valve cover seals in one or two cases. Getting taken care of as part of the 75K spark plug change applicable to W12's. Car has used precisely zero (!) oil since its last service. Still shows identical full on dipstick cold, with none ever added between changes over the whole life of the car. I like that!

3) Right (U.S. passenger side) torque support (ties in with motor mount set up). Whoa! First, I found a lot of brown staining on the outside of the alternator case from underneath. That was odd and surprising. So, I looked from above at the area and likewise, big obvious crack almost all the way around. Oil leakage actually seems to come from elsewhere (lower) than the visible crack. It is fluid (oil) filled to act as a damper, hence the staining. In the pictures below, the part is just to the right of center in the left hand pic, just below a shiny looking hose clamp in the pic--has some blue Sharpie marks on it that are obvious in the close up pic. In the close up pic (now rotated 90 degrees clockwise), the crack is obvious. Above it in the picture you see the alternator case with some dry dark brown staining. Service advisor quickly agreed it appears to be a blown mount element.

A multi hundred dollar part at dealer OE list, and no doubt a pretty big hassle to change.

For you W12 owners, you might want to look at yours too. Even more so if still under warranty. Surrounding symptoms were maybe a tiny bit of shake beyond what you expect for super smooth W12, and maybe a bit of suspension like noise shifting from reverse to drive; hard to know if that was the support part or some other common D3 noise. You can just barely see it if you look down between the right side intake manifold and the air filter box. If you see staining like on mine, or maybe obvious big cracking, not good. I searched the board, but found no history on the W12. I did find a couple of posts about D3 4.2 engine mount replacements (seemingly always on the same right side), and then many a post about engine mounts for many models of older Audi's.

Currently in at dealer for the fix.

Also note in the past (sill under the 4/50 warranty) I did find a bad radiator core doing the same close look underneath with the covers off. Showed up as a bit of fluid on the front edge of the belly pan, and a very slightly damp edge of the lower radiator core. Very small/pinhole type leak when I caught it. That would have been another meaningful bill. Upper front control arm bushings were also previously replaced under the relevant TSB and 4/50 warranty.

To confirm, post only deals w/ physical inspection items. Obviously I separately looked at full scan results.

s4master1

tell the to replace both main mounts together or it will break again soon.

MP4.2+6.0

Do you suggest that from D3/4.2/S8/W12 experience, or more generally?

I looked at the drivers side as best I could--it's even busier w/ stuff like P/S and A/C on that side--and saw none of the same warning signs. Since under warranty, I can only get them to do so much most likely.

Technically, I'm thinking one side will be compressed under high load, while the other will be under expansion pressure (unless I'm not seeing that one is loaded from above and one below). So it seems like the forces acting on the two sides will vary. A lot of force, especially w/ the highest torque W12, but different given the basic rotational nature of a motor acting against its mounting points if all uniformly on the "lower" side.

mishar

Right engine mount load under strong acceleration: to the weight of engine/transaxle (part of it because it is shared) you should add maximal engine torque multiplied by the first gear ratio divided by distance between mounts (left to right), than subtract maximal engine torque multiplied by first gear ratio and final transmission ratio divided by 4 and by distance between mounts (front to rear) and engine/transaxle mass multiplied by acceleration and divided by 2 and by vertical distance between engine/transmission center of mass to the mount. Than you add horizontal forces ...

Left engine mount has same weight and opposite force from drive shaft torque (pulling up) and same force from front wheel drive torque puling up.

On top of all that come inertial forces when jumping bumps and lateral forces in curves.

s4master1

yes, from experience.
very basic explanation, the engine is sitting about 1 inch lower than when new due to fatigue in the mounts, this causes the engine to "hang" from the front mount causing it to break.
no technical equation needed as mounts are not designed based on the torque/weight/lateral/horizontal/vertical/inertia characteristics of the powertrain they carry, they merely isolators.
the only way to get affected by torque is if you step on the brakes while accelerating the engine, even at that point if one mount is completely broken you will hardly see the engine trying to jump like the old cars used to do.
if you can ask them to save the old mounts and compare the height to the new units and you will see how much have they collapsed.
on the 12 cylinder engine you can drive with broken mounts and hardly feel any vibration, also they have an electrical actuator that "softens" the mounts while at idle and "stiffens" when under acceleration.

MP4.2+6.0

At rest the motor is resting on top of the mounts, which then transfer the basic weight down to the front subframe, correct ? (ignoring the front motor mount and the tranny related mounts). Neither side is a true hanging mount system is it, where the motor at rest is bolted into an element of the body via a mount from above? That's the way my very first 100HP C1 was--it didn't have any subframe at all--but I always have thought with the subframes the motor was fundamentally resting on top of the (side) mounts. Not debating here, just trying to connect the dots conceptually.

mishar

I've checked your pictures and my ETKA and I think you are looking at third engine support, one that doesn't carry engine weight and has not oil in it. Main engine mounts are further back. This one is there to counter torques from front wheel drive and drive shaft. It is not that expensive.

mishar

You are not right. Simple low of physics, action and reaction, tells you that torque on drive shaft to the rear differential and front drive shafts has their reactions and they create forces on engine mounts. No braking required. Actually it wont make any difference.

Jumping wise, mounts are constructed so that even without any rubber they still limit engine movement in all directions (though with lots of noise). If you want to test this try to unbolt left main mount and start.

s4master1

you choose the worst example of physics.
the engine is so big and powerful compared to the weight of the car that it barely moves before the car starts moving.
many times I/we had replaced completely broken mounts and the only complaint is vibration at idle, you can unbolt both engine mounts and the engine won't be jumping around in this car.
let me put it this way since you want to get too technical on a simple isolator.
if you install a 500-1000cc engine like a motorcycle or small car engine, it will need to be "tie down" all the way around not just at the bottom,
in the case of a small engine on a heavy car like this, the engine is trying to spin around before the car starts moving, simple power to weight ratio difference.
designing a mount based on all the factors you stated is important but not critical for the simple fact that the car can be sold in a very hot climate or a very cold one, you have to consider both scenarios and go somewhere in the middle.
but on a 6 liter engine you can just wedge the soles of your old shoes between the engine and subframe and be fine driving it.

s4master1

the engine rest on the main mounts but it hangs from the front one when the main ones collapsed, that is the reason it will break soon if the main mounts are not replaced together, your dealer knows this as they always get replaced together.