Air intake grille modification - B5 Passat
VW Passat air intake - front upper grille modification
Some Passat had blocked off air intake grilles, some had open grilles. There doesn't appear to be any logic to which part got onto which car - it could have been as simple as the guy on the assembly line grabbing something from the box on the left vs. box on the right.
(Update via VW parts research and Modern TPS Manufacturing Engineering Knowledge)
There were two grille assemblies used on the 2001-2005 Sedan and Wagon versions of the Passat (including 4 motion).
1) Grille Part Numbers: 3B0853651K 3FZ ; 3B0853653J 3FZ - which HAVE the HOLES in the Grille. These Grilles fit Vehicles with 5 and 6 Cylinder Engines.
2) Grille Part Numbers: 3B0853651L 3FZ ; 3B0-853-651-H-3FZ; 3B0853651H 3FZ; 3B0853651J; 3B0853653J 3FZ - Which DO NOT have the holes in the Grille (as shown below), These grilles fit Vehicles with 4 Cylinder Engines both Gasoline (Petrol) and TDi (Diesel).
FYI about modern manufacturing techniques:
You may come across vehicles which have been repaired using the incorrect grille, which could give the impression that grille use from the factory was random, however being very familiar with manufacturing processes (including VW), variation of this type would not be possible for manufacturing assembly to accomplish without a specific authorization from Engineering.
Vehicles are assembled according to a computer generated, bill of materials (BOM), constructed by Vehicle Design Engineering. The BOM defines each specific component to be used to construct the vehicle and the quantity of each component used on the vehicle. The BOM (computer data) is integrated with automated (computer generated - CG) component parts ordering from suppliers (MRP) as well as automated (CG) manufacturing orders (MO's) for assembly. Parts acquisition and assembly planning are not functions of the Assembly department(s), they are specifically Purchasing/Planning functions (PIC). Manufacturing assembly works from the automated (CG) MO initiated by the planning department, which is based upon the automated (CG) BOM created by Design Engineering. The MO specifies which specific component parts are provided to which specific manufacturing assembly work cells.
With the advent of lean manufacturing (TPS 1990s), component parts delivery from suppliers, runs on a system called Just in Time (JIT), meaning there is only enough inventory from automated (CG) MRP orders to build each complete vehicle. MRP acts upon a 1:1 ration of customer orders based upon the vehicles specific BOM, and no excess inventory is maintained. If a component part failure occurs during manufacturing assembly, orders must be made for replacement components which are charged as an additional expense to either manufacturing or the supplier (depending on the failure cause). Since manufacturing and their suppliers live on very thin profit margins, failures are a very rare occurrence, usually measured (for automotive) at or under 25 parts per million (0.0025%).
A blocked off grille will help give more water splash protection but an open grille will give less resistance to the engine air intake. Enough water getting into the intake air could damage the turbo or hydrolock the engine, causing severe damage. Since the air intake is at a high spot and the intake air box has a water trap, I feel that opening the grille won't cause any problems, especially because many Passat have open grilles stock from the factory.
On the other hand, plenty of air still gets to the cold air snorkel through the lower grille. If it didn't then your engine would have problems running normally. Opening the grille should reduce air resistance but there might be added turbulence from being broken up and passing past the grille slots (instead of staying in 1 mass and moving through the top large grille). In any case, this lets me see if there are leaves or other junk blocking the intake snorkel.
For a very good air intake you would have to make a custom cold air snorkel with a velocity stack underneath the air filter like the one pictured below. Although you'd only get a few horsepower at this level of modification, every little bit adds up. I do not suggest putting on an aftermarket oiled air filter on your TDI because it runs by throttling fuel not air and VW MAFs tend to be very sensitive to contamination. See 1000q: MAF FAQ for more details on the hot film sensors and why they are different than old style sensors.
Lastly, the same exact air filter used on your 138 hp VW Passat TDI is also used on the 1.8T Passat and Audi A4, various V6, and even the high performance Audi S4 and V8 engines. If it flows enough for an engine with twice the horsepower, it flows enough for your TDI engine.
Air grille modification procedure
Dremel out some of the holes.
After - I didn't remove all the slots to prevent weakening the cheap replacement replica grille's plastic. The replica grille looks flawless from a foot away and was half the price of a new OEM, but copycat parts tend to have more brittle plastic.