They're not junk, the engineers spent a lot of time reducing harshness vibration and noise. The DMF's damping material helps dampen that energy from the engine to the trans and chassis. In a diesel, it does a lot more than a gas since the diesel has a lot more vibration.
They can split, the gears and stuff inside can fail, etc. And when you increase power levels, they can exceed the original DMF's range of normal operation. I'm sure that doesn't help either. A SMF can fail too, but it's very rare. Normally they fail in race applications, the starter ring can chip or break, but these are pretty rare. That said, you only hear when someone's DMF fails, you don't hear all of those thousands more whose DMF are fine.
That said, I would probably replace the DMF when you do a clutch job or replace it with a SMF. I've read a post on tdiclub which an "unnamed guru" said you can't resurface a DMF because it changes the flywheel step. That's among the most scaremongerest statements I've seen from that guru. Any machine shop can resurface the flywheel step, they remove equal amounts of material from the flywheel surface and it's step and it keeps the relationship between the flywheel surface and the step equal. They do it all the time. They can even change the step surface upon request. However, to the best of my knowledge, while the VW TDI DMF can be resurfaced, it cannot be rebuilt. Some DMF can be completely rebuilt, just not the VW TDI one. So if you have worn out the clutch from hard use or have high mileage, the DMF should be replaced.
I need a new clutch soon so there'll be a writeup soon.
Pretty much, the problem is that if you've never driven a similar car, you won't know what's normal vibration and whats a little worse than normal vibration. I think that if they fail, they're bad enough so that you should be able to feel it.
They're not junk but they do fail. And if they fail they can sometimes fail in catastrophic fashion, taking out the tranny too. Pedal pressure has nothing to do with the flywheel, it's a function of the pressure plate.
My wife and I just bought a used 2003 TDi Wagon (130,000KM). Love the car, and have done a lot to it in the past 6 weeks to bring it back to like new condition. Unfortunately, one of the things that had to be done was replacement of the DMF. The Vibration was annoying, and potentially dangerous, so as soon as we could we had a VW specialist here put in a Valeo SMF and clutch. It's sooo much better. I read that sometimes the vibrate when the fail, especially under load (like ours did), and some will chatter and rattle.
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