P0400 keeps returning

[Robert Squires]

'96 E320
* First time - replaced the popular broken vacuum feed line to switchover valve, reset P0400 code (no other codes), obvious problem - thought I was done,
* Second time - car returned, replaced EGR valve and checked EGR pipe for unobstructed flow (good flow, can hear compressed air, in intake manifold, blown into pipe w/ EGR valve removed), reset P0400 code (again no other codes).
* Before I turn it loose this time maybe I should try to do a "system" check. When energized, the switchover valve "switches over" the vacuum. But how do I test that the SOV is ever energized? What are the conditions, for the HFM M104 engine, that call for exhaust gas recirculation?
* Would self-adaptation values or a sluggish oxygen sensor ever trigger a P0400 code?
* Is there a diagnostic "flowchart" in WIS (or any other useful information for that matter) for this code in this model car? Document number?

[myarmar]

Robert, P0400 is not for the SOV, it is triggered if manifold vacuum doesn't drop enough when EGR valve is opened by the ECM. All electrical components are fine, but maybe EGR vacuum hose is bad, or the valve diaphragm leaking? Did you try to activate EGR with the hand vacuum pump? I had the EGR code in my 600SEL and cleaned everything many times. It kept coming back. Then I figured that the hole in the intake manifold was all plugged with carbon. I used a paper clip sized wire to clean it and it was tight in there. Then I found the hole is much bigger - probably close to 1/4 inch, so I did not clean all of the carbon. After I cleaned it completely - no more codes for EGR.
Mike

[Arthur Dalton]

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It is the SOV that allows the vac to get to the EGR in order for it to open.. The ECM sends an electrical sig to the SOV, not the EGR. So, a bad SOV can cause the code b/c the vac sensor never sees a change if the SOV does not get an ECM signal or does not open when it does gets a signal.
ECM sig mapping for SOV activation is only above coolant temp of 48C, with engine rpms above idle and below WOT w/load mapping from ECM.
Absolute manfold pressure is monitored during those conditions by Diagnostc module for emmissions specs.

[myarmar]

SOV has it's own fault for electrical circuit - P1400. But yes, it could be SOV mechanical failure.
Mike