When you plug the vacuum line going to the EGR valve on this engine (1992 602 300D) the electronic boost control cuts boost by opening the wastegate on the turbo. I have heard that airflow meter in the top of the air cleaner housing is responsible for the signal that causes the boost to be cut when the EGR vacuum line is plugged.
I heard that one fellow bypassed the system (EGR abnd Air Recirculaing Valve on fresh air side of turbo)) entirely by installing a manual boost controller which ran off manifold pressure. He also had to install a pressure type turbo wastegate in place of the vacuum actuated wastegate.
Another fellow, who is a bit smarter than I, designed a pressure sensing circuit board, installed a pressure sensor rated at 15 PSI in the manufold and somehow electronically defeated the EGR and air recirculating valve without cutting boost.
Both of these methods seem a bit complex and expensive. (as compared to the luxury that most of you guys have of putting a BB in the vacuum line). Has anyone got any more to add to this subject.

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Walter
1992 300D 175K