Thanks for that info myarmar. Intake manifold pressure at idle is a condition for indicating an EGR malfunction. But intake manifold pressure is evaluated before and after the EGR valve is open at 1900-2600RPM. When i perform the test at idle I am looking for a change in intake manifold pressure to verify that in fact the intake manifold is increasing in pressure (less vacuum) because the EGR is opening the intake to exhaust. My test was to determine that indeed the EGR is allowing exhaust into the intake. This is observed by:
1. Engine stumbles (as it should when the EGR valve opens)
2. By reduced engine vacuum when the EGR opens.

Myarmar, your conclusion that cleaning out the EGR pipe did not fix the P0400 check engine code is why i don't want to jump to any conclusion either. regarding your case: did you test the other parts of the EGR system, e.g. the switchover valves and vacuum lines going to the EGR control port? Although your valve and pipe are OK you still need the EGR vacuum controlled system to work.

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1972 450SL
1982 300D Turbo