I'm more than a little disappointed reading this thread.
How could VW get something so rudimentary, so wrong?
Nearly every I4 engine I've seen or owned, going back to the late 80s, was chain drive. It's simply...superior in every way. Timing belts are awful, which is why the industry has abandoned them.
Then...somehow, there's VW who insists on a belt for the diesel. Didn't make much sense to me. The primary reason to choose a belt is for noise consideration. But...it's a diesel...? It's already loud. Mercedes and BMW have used chains on their USDM light diesels for decades.
This isn't to say there haven't been minor issues along the way. My 1989 Nissan 240sx (single cam in those days) had a habit of the plastic chain guides getting brittle and breaking off. This is a somewhat common issue of chain-drive motors from the 80s and 90s(cough cough BMW). But it really just causes some chain slap on start up, and sometimes a bit of noise at idle.
Sometimes chain tensioners, which are/were typically controlled by oil pressure, would "fail" or become sticky over time. I replaced the chain tensioner on a friend's 1998 Corolla (1.8 I4 dual cam). I think it took like 45 mins? 10mm wrench and a youtube video. Likely booked at ".3 hrs"
Larger, more complex engines stuck with timing belts for a bit longer. My 1994 Nissan pickup truck (VG30E 3.0L V6 motor) had a belt. The early, "high maintenance", ones were rated for 60k, and they increased it to either 90k or 120k later on.
Then I got my 2000 Toyota Tacoma. 3.4L V6. The 4-bangers (in those trucks) had been using chains for 10-15 years at that point, same with Nissan. Timing belt interval was 100k, I believe.
Then I moved onto 2005 Tacoma 2.7L 4-cyl. Chain. And later upgraded to a 4.0L V6 (chain) truck of the same year.
2005, the first year of that model iteration, is when Toyota did away with the belts entirely. As with Nissan, and most of the industry, by that time, I assume. Again, I'm assuming materials availability finally caught up with the engineering, and allowed them to finally make the jump to chains entirely.
The reason they didn't use chains on the V6 engines prior was due to the added complexity and noise. A 4cyl has one chain that's pretty direct. Crank sprocket, cam sprocket, tensioner.
Here's the timing setup on a Nissan KA24E motor. An engine from the late 80s.
On V motors, you either have to have multiple chains or...some really complex routing.
This is a Toyota 4.0L v6 variable valve timing motor, by the way. Introduced in ~2003.
And then you've got fancy-pants Cummins with their timing GEARS
Now, I wasn't in the design room when Cummins made the 5.9 in the 80s, but I assume they had reliability in mind, as that's somewhat important to their customers - truckers. I'm sure you're familiar with the high regard for reliability that the Cummins motor receives. This is one of the many reasons why. Also, if you'll notice, that smaller, recessed gear to the right of the 2 large gears -
that's the power steering. Yes, the power steering is gear driven off the timing gears. Also, one of the gears at the bottom is for the fuel pump, and the other is water pump. Cummins didn't play games with silly timing belts -
for a reason. You can literally drive this truck home with no battery,
and no alternator. Sold from 1989-1997 until they added computerization from 1998-2002, but kept the same block until 2009 (?) and continue to use gears presently.
The only benefits of a belt are that it's much cheaper, and a much easier to produce system. In the past 5-10 years, it's very occasionally seen, pretty much only on very cheap cars/motors. Economy cars primarily - think: Ford Fiesta. Literally everything else has a chain.
But why would you burden the customer with a belt if you didn't have to? Talking to someone about timing belt intervals is like when you grandmother says to you "be careful driving in the rain so you don't get your
points wet." Huh? What are you talking about? "Points"?? Burdening your customer with a cumbersome $1500 maintenance job, that they're going to ignore anyway, that you could prevent? It seems like a win-win. Customer doesn't have to pay unnecessary main't fees, AND car manufacture doesn't get bad rap of "unreliable cars smashing valves after 5 years".