I was invited to watch the technicians at my VW dealership perform the 32,000 km (20K mile) service inspection on my car last week. One of the most interesting things that I learned is that Phaetons, along with all other VW products sold in ‘cold’ climate regions, come equipped with snow screens on the engine air intakes.
The purpose of the snow screen is to block frozen snow and ice particles from entering the air filter plenum. If snowflakes or ice pellets entered the plenum and then settled on the filter, they would melt, thus making the filter wet, and this would compromise both the strength and effectiveness of the filter. The snow screen traps particles at the entry to the plenum. They will eventually melt and the water will disappear through a drain hole at the bottom of the plenum.
If the snow accumulation is heavy enough to block the snow screen, the increased vacuum in the plenum will cause a spring-loaded relief valve to open up and admit air into the plenum from the area under the hood, where no snow could be. This is generally harmless, but will cause a slight drop in peak engine performance because the engine will now be using quite warm air from under the hood, rather than cold (ambient temperature) air that is collected from just behind the grille.
The snow screens work very well. In fact, one common problem is that they work TOO well – they tend to plug up with insects, small seeds, bits of grass, and other stuff that works its way along the air intake system to the air filter plenum.
I was a bit surprised to see that the saturation indicator (production code 1L2) on each of my air filters was fully red, implying that the air filters were 100% dirty. The saturation indicators are little vacuum gauges located downstream of the air filter media that measure delta P between the throttle body intake and ambient air pressure.
Air Filter Saturation Indicators
These are only visible when the covers have been removed from the side of the engine.
The techs removed the two air filters, and to tell you the truth, the filters themselves didn't look all that bad. Once we had a look at the snow screens, though, it was obvious to all of us that the snow screens needed to be cleaned.
To get access to the snow screen, the top cover of the air filter plenum is removed (this is the part that has the saturation indicator built into it), and the air filter is removed from its holder. The snow screen is then found at the lower forward area of the air intake plenum. It is held in place with one Philips screw. When removing the snow screen, take note of how it slides into the little rails molded into the lower plenum assembly
Finding the Snow Screen
Left side of engine, snow screen removed
Right side of engine, snow screen removed
The snow screen itself is easy enough to clean – just rinse it backwards with running water, then gently clean both sides of the very fine screen with hot soapy water and a soft brush. If there is any accumulation of sand and debris in the lower part of the air filter plenum, that will need to be removed with a shop vacuum before washing the lower plenum with hot soapy water, otherwise, the debris might clog the water drain when you wash out the lower plenum assembly. There is a water drain in the lowest point of the plenum, and the water should freely and rapidly run out from this drain when you are washing the plenum interior.
Cleaning the Lower Plenum and Snow Screen
Don't forget to reinstall both snow screens, and to reset the two saturation indicators back to zero by gently twisting the dial until the red indication disappears.
I do not know if snow screens are provided on cars that are built for warm weather climates. Every VW I have ever purchased in Canada or Switzerland has been equipped with a snow screen.