I'm getting OBDII check engine code P0400, EGR flow malfunction. Here is what i've done/checked:
1. Reset the check engine, but it comes back
2. Applied vacuum to the EGR valve and it opens and closes properly. Engine stumbles a bit when it is open.
3. Measured intake manifold vacuum at idle, 20"Hg. With EGR opened (with my vacuum pump) the manifold vacuum drops to 17" Hg.
4. Tested the EGR switchover valve with 12V and it switches and ports vacuum correctly. Same with the changeover valve that is used for the EGR line to the intake maniflod pressure sender unit.

Note: In order for the EGR flow malfunction to be triggered no failure in the electrical activation of the EGR switchover valve can detected so i can assume that electrically everything is OK. I also noticed with my OBDII scanner that the MAP fluctuates with different throttle positions.

Is there something else to test? Also is it a sign that the EGR pipe is partly clogged if the intake vacuum only rises from 20" to 17"Hg with the EGR open? The engine stumbles noticeably as expected with the EGR valve open but no where near stalling out as may? be expected. I don't look forward to taking the EGR pipe apart for nothing especially if the info above points to that not being a problem.

Any thoughts?

__________________
1972 450SL
1982 300D Turbo