The purpose of this DIY is to get to the check valves (drain down valves) under the valley pan, under the intake manifold. New replacement parts don't actually exist, but more on that later.
I accept no responsibility if you attempt this and accidentally leave a bolt in your cylinder head, causing irreparable damage.
That said, this is actually quite a simple procedure for those willing to tackle it.
It's a good idea to replace your intake manifold gaskets while everything is apart. I also cleaned the intake runners in the manifold & the head while it was all apart (Seafoam works great for this, soak them & wipe away the grime). Keep a vacuum handy for any loose particles that fall into the runners (unavoidable to an extent).
You will need VAG-COM to adapt the throttle body once you're done.
The first step is to remove the engine covers. If this is beyond you, stop now & back away from the vehicle.
Disconnect your Battery negative terminal (in front of the driver's seat, 13mm)
Covers removed:

Next, you'll need to cut & remove the several black cable ties that hold different parts of the wiring to other assorted parts. There are a few of them on either side of the fuel rail:


Next, unplug all electrical connections & vacuum lines shown, except the injectors as they are easier to reach when the manifold is unbolted and lifted an inch or so.




Once the following is removed, unplug the vacuum lines & remove the solenoid:

These next two (one on each side) will be free from the rail once unplugged:

I don't have a good pic of the throttle body connector, but there is only one:

You'll also need to remove the vacuum line from the back of the manifold, under the throttle body & the one on top of the TB (screw clamp). Both of these are easier to get to with the PCV valve removed. I cut and removed all of the Oetiker clamps on all of the PCV hoses, then removed the PCV hoses.
Loosen the clamp on the TB hose and pull it off of the throttle body.
Pic with all the hoses removed & electrical connections disconnected:

The fuel line can be disconnected at this point (grab a rag or two, also note the PCV valve removed):

At this point, unbolt all of the manifold bolts with a 5mm Allen socket. Some of them are tricky to get to (those near the back). Make sure you have a magnetic pickup tool, you will drop them. You'll note other fasteners on the manifold with a "torx-like" head. Leave those as the hold the two halves of the manifold together.
Once the manifold is loose, lift it an inch or so & unplug the fuel injectors. Finally, lift the manifold out, while pulling it towards the front of the vehicle. Make sure it's not caught on any vacuum lines or electrical connectors.
Also find the one Allen bolt in the back of the manifold holding the only metal PCV line to the manifold itself (driver's side, shown below):

You can also see the vacuum line under the TB hose (passenger side) that needs removing (mentioned earlier).
Vacuum all of the loose stuff out of everything. Clean the runners if you like.
Installation is the reverse of removal.
I went on to remove the valley pan cover to "replace" the check valves underneath (the two Allen head items here):

I'll explain what happened with these in a reply post.
I accept no responsibility if you attempt this and accidentally leave a bolt in your cylinder head, causing irreparable damage.
That said, this is actually quite a simple procedure for those willing to tackle it.
It's a good idea to replace your intake manifold gaskets while everything is apart. I also cleaned the intake runners in the manifold & the head while it was all apart (Seafoam works great for this, soak them & wipe away the grime). Keep a vacuum handy for any loose particles that fall into the runners (unavoidable to an extent).
You will need VAG-COM to adapt the throttle body once you're done.
The first step is to remove the engine covers. If this is beyond you, stop now & back away from the vehicle.
Disconnect your Battery negative terminal (in front of the driver's seat, 13mm)
Covers removed:

Next, you'll need to cut & remove the several black cable ties that hold different parts of the wiring to other assorted parts. There are a few of them on either side of the fuel rail:


Next, unplug all electrical connections & vacuum lines shown, except the injectors as they are easier to reach when the manifold is unbolted and lifted an inch or so.




Once the following is removed, unplug the vacuum lines & remove the solenoid:

These next two (one on each side) will be free from the rail once unplugged:

I don't have a good pic of the throttle body connector, but there is only one:

You'll also need to remove the vacuum line from the back of the manifold, under the throttle body & the one on top of the TB (screw clamp). Both of these are easier to get to with the PCV valve removed. I cut and removed all of the Oetiker clamps on all of the PCV hoses, then removed the PCV hoses.
Loosen the clamp on the TB hose and pull it off of the throttle body.
Pic with all the hoses removed & electrical connections disconnected:

The fuel line can be disconnected at this point (grab a rag or two, also note the PCV valve removed):

At this point, unbolt all of the manifold bolts with a 5mm Allen socket. Some of them are tricky to get to (those near the back). Make sure you have a magnetic pickup tool, you will drop them. You'll note other fasteners on the manifold with a "torx-like" head. Leave those as the hold the two halves of the manifold together.
Once the manifold is loose, lift it an inch or so & unplug the fuel injectors. Finally, lift the manifold out, while pulling it towards the front of the vehicle. Make sure it's not caught on any vacuum lines or electrical connectors.
Also find the one Allen bolt in the back of the manifold holding the only metal PCV line to the manifold itself (driver's side, shown below):

You can also see the vacuum line under the TB hose (passenger side) that needs removing (mentioned earlier).
Vacuum all of the loose stuff out of everything. Clean the runners if you like.
Installation is the reverse of removal.
I went on to remove the valley pan cover to "replace" the check valves underneath (the two Allen head items here):

I'll explain what happened with these in a reply post.