  We study the physical properties and molecular excitation of the different
warm gas components found in the protoplanetary nebula CRL 618. We revise our
previous Herschel/HIFI observations, which consist of several 12CO and 13CO
lines in the far-infrared/sub-mm band. These data have been re-analyzed in
detail by improving calibration, the signal-to-noise-ratio, and baseline
substraction. We identify the contributions of the different nebular components
to the line profiles. We have used a spatio-kinematical model to better
constrain the temperature, density, and kinematics of the molecular components
probed by the improved CO observations. The 12CO and 13CO J=16-15, J=10-9, and
J=6-5 transitions are detected in this source. The line profiles present a
composite structure showing spectacular wings in some cases, which become
dominant as the energy level increases. Our analysis of the high-energy CO
emission with the already known low-energy J=2-1 and J=1-0 lines confirms that
the high-velocity component, or fast bipolar outflow, is hotter than previously
estimated with a typical temperature of ~300 K. This component may then be an
example of a very recent acceleration of the gas by shocks that has not yet
cooled down. We also find that the dense central core is characterized by a
very low expansion velocity, ~5 km/s, and a strong velocity gradient. We
conclude that this component is very likely to be the unaltered circumstellar
layers that are lost in the last AGB phase, where the ejection velocity is
particularly low. The physical properties of the other two nebular components,
the diffuse halo and the double empty shell, more or less agrees with the
estimations derived in previous models.
