Daddns wrote on Feb 2^nd, 2013 at 5:54am:

The VNAV is mostly working for me.  In about half a dozen fully-automated flights, I have not experienced the "immediate descent" that others are reporting upon VNAV engage. I am still yet to fully explore the VNAV, but so far, the only thing I have found thus far that doesn't work is the crossing altitudes for each waypoint in the flight plan. As an experiment, I set different flight levels for several consecutive waypoints to see if the VNAV would climb/descend the aircraft accordingly.  It didn't, but that could just be my misunderstanding of how it is all supposed to work. What it definitely does do, is climb and descend the aircraft to the level set on the VNAV page 2 screen - and maintains the speeds set on those pages. More experimentation to come. One thing I have found essential to getting VNAV to engage is to have a "cost index" value set on the performance page.  I don't know what the value should be, but I just enter 1 (one).  I believe the entered value is not used in any way at this stage (apart from preventing VNAV engagement). For those having problems with VNAV engagement, can you please confirm that: 1. You have a route loaded and it is active; 2. You have some value entered for the "cost index"; 3. You have an altitude (or flight level) entered into the VNAV pages;

Testing again, this time paying even more attention to the CI. Yes, I had put it in there (values between 25 and 90 for me), route had been loaded and activated, preflight seemed pretty much complete.

I had this "VNAV DES" thing happening again. I re-entered and EXECuted my cruise level, so VNAV reverted to CLB.

New test: After VNAV engaging in CLB, I re-entered the CLB speed restriction (250/10000 by default) and EXECuted that, hoping for the best. No, the FMC still ignored the restriction; putting me at ECON CRZ speed even in the climb, when still under 10.000 ft ...