It (clear line) should not have anything in it, should be clear and open! This tubing passes a boost signal thru the valve on the firewall to the ALDA and allows it to increase fuel flow of the IP in proportion to boost level. The intake is also likely to be covered with this crud. Comes from the EGR valve which should be blanked off or at least have the vac tubing plugged or removed. Clean all of the crud from 'banjo' bolt at intake all the way over to the ALDA, if it goes that far.

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1961 190Db retired
1968 220D/8 325,000
1983 300D 164,150