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I seem to remember that my VW with the more sensitive VE pump had a 10 micron filter spec'd by VW engineers, and they are some of the most anal bastages to ever build a car.
Even a 5 micron I could understand... But 2? Why?
There are some articles (google the subject) explaining why high pressure common rail systems will wear out injectors and pump internals much faster than normal if insufficient filtering is employed.
The VE ran what max pressure? 16K psi iirc. Compared to 23,200 on the CP3, I'd say the CP3 is far more sensitive to small particles. In a Cummins study, they found that particles in the 5 to 7 micron range that passed harmlessly through lower pressure systems, caused accelerated wear in the much tighter tolerances in the 23,200 psi systems.
While the FSM lists a 3 micron filter for the CRD, I think that was the original Racor that was installed in the earlier versions sold overseas. The paper filter in the factory system sold on US models is probably closer to 10 micron nominal, rather than absolute. In plain English, it's a bean counter special, and will likely work for a few years, but not really good enough for a modern common rail system. Iirc, Bosch specifies 5 micron filtering for the CP3 pump and associated injectors. If you look at some of the Bosch Yellow series technical manuals, you'll find a wealth of information about EDC diesel engine management systems, emissions controls and some information posted here recently on NOx formation appears to have been copied from one of those Yellow series manuals.
If you google the ISO fuel tests, you may run across George Morrison's name, who posted frequently on TDI club.com. George identified the problem with dirty fuel in his customers storage tanks causing early injector and pump wear (fleet systems where these costs are enormous) and the need for micron filtration levels and water reducing practices. I read one article that stated that George was instrumental in getting the excellent Cat 2 micron filter built. There were some early efforts by some on tdiclub to get one built, but Cat ended up building it and some early posts on that site, circa 2000 had george, dbw, laundyt(who occasionally posts here) running tests on the CAT, Stanadyne and Racor filters. In general, they found that those 3 brands lived up to their advertised claims, while others did not. It was interesting reading.
Also, Old Navy has several posts on this subject as he was also involved in some of the early TDI club testing. It's fairly informative reading.
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