Wiki article summarizing VW TDI emissions cheating so far, click here. Anyone logged in can edit it by clicking the "edit" button on the upper right of the page. It's a reference so please post any comments or questions in the forum. It's a work in progress so feel free to update it!
Air intake or interior cabin air filter DIY replacement mk4 TDI engine
How to change the engine air intake or interior cabin air filterDifficulty: 1/5
Introduction: This article is divided into three sections: replacing the intake air filter, cleaning the snowscreen, and replacing the cabin air filter on a VW Jetta TDI
The snowscreen is an air intake snorkel with a screen that keeps out large debris and snow out of the air intake box. If it gets clogged with mouse nests or leaves, a spring loaded auxiliary intake flap opens, preventing the engine from air starvation. If the aux flap spring is rusted or the door is stuck, the engine will be air restricted. This aux air is somewhat warmed by the engine and is not as good as cold intake air but if it's working it prevents the engine from stalling.
Mann makes the OEM VW air filter so it's a good filter. The cold climate air filter has an extra foam layer on the bottom.
The replacement interval is 40,000 miles. Change the cabin air filter as needed, once a year is suggested.
Parts (click links to compare current prices)
1 new engine air filter - either:
Cold climate engine air filter (with extra foam layer) VW# 1J0 129 620 or Mann filter from kermatdi
Non cold climate engine air filter VW# 1J0 129 620
1 new activated carbon interior air cabin filter
a brush or compressed air to blow out screen
you can also get a filter pack from kermatdi
Intake air filter replacemen
All you have to do is unscrew the top cover to the air box. (2 phillips screws). Mine were rusty so I used a dremel to cut a groove in them and then used a standard screwdriver to remove the screws.
If you need more clearance, remove the MAF electrical plug and unclamp the accordion like hose if you want. I also suggest vacuuming out the bottom of the air filter box and cleaning the snowscreen while you are in there. Make sure the tabs all fit into their slots and that the air filter edge gasket is seated properly.
These cars are known for slow MAF failure. If your car has been losing power, a faulty MAF may be responsible, see 1000q: limp mode, 1000q: constant low power or can't rev, and 1000q: a4 MAF FAQ for more details. If you determine that the MAF is bad and need to remove the element, a special VW tool is required to remove the MAF screws. You could also use the dremel to cut a groove into it and just replace them with normal screws. Most MAF come as a whole unit with the plastic and require no special tools to remove.
Very easy but here's a video anyways. There's unnecessary music in the video so don't bother turning up the volume.
Cleaning the snowscreen
Note that in the below pictures, the air box and battery were removed for clarity.
Remove (1x 10mm) nut circled in yellow and pull out the intake snorkel. The snowscreen is at the end of the snorkel. Use water or compressed air to clean it out and then replace it. I do not recommend cutting the snowscreen out since it prevents larger debris from reaching the air filter.
Here is the spring loaded dooropened partially.
Cabin air filter replacement
Remove the 4 phillips screws (one is circled in red) and pull up and away the rubber weather stripping on top of the firewall.
Pull out the plastic trim and the cabin air filter. During reassembly, the black plastic frame has 2 tabs on the top front edge and 3 tabs on the lower rear edge. 1 of the tabs is marked in red.
Vacuum out any dust/dirt and replace the filter and trim. The ribbed cylindrical object below the air filter is the blower fan. If a leaf gets caught in there it'll rattle.
I prefer activated carbon filters since the activated carbon absorbs odors.