why am i having problems with my 1996 mercedes benz e320 passing the emission test

You could need anything from a tune up to a catyltic conveter. Your best bet would be to take it to a good indipendent mechanic and give him the emissions report so he can correct the issue.

E320 with twinning cam engine have a history of e g r pipe clogging up it runs from the intake to the exhaust manifold around the back of the engine

Michael is right on. Probably the EGR pipe or the EGR valve. But you would probably have a Check Engine light on also depending on how bad the restriction is.

if you failed, then you have codes/ reasons, specify what they are and I can direct you from there....

Here's the answer to your problem after I hand to find out the hard way: The 1996 Mercedes e320 (and many other models) CANNOT PASS the current smog check even though THERE IS NOTHING WRONG WITH THE CAR'S ENGINE, NOR THE COMPUTER(S) THAT RUN THE CAR, and that in order for the car to be properly checked for smog emissions, smog technicians must follow the specific sequence of steps announced in the May 1, 2011 Technical Service Bulletin S-B-14.00/17a. This may include a twice repeated Drive Cycle that for each cycle will require 1 to 1 and 1/2 hours each (a two to three hour minimum process for the drive cycles alone). This problem is almost exclusive to the State of California: contact the State Department of Automobile Repair to specifically request this information, as well as a copy of the "former" Appendix J.

Correction: "hand" should read "had" and "Department" should read "Bureau." Good luck.

I think smartwoman answered your question but just in case, have you changed your oxygen sensors lately?