  The influence of the isospin-independent, isospin- and momentum-dependent
equation of state (EoS), as well as the Coulomb interaction on the pion
production in intermediate energy heavy ion collisions (HICs) is studied for
both isospin-symmetric and neutron-rich systems. The Coulomb interaction plays
an important role in the reaction dynamics, and strongly influences the
rapidity and transverse momentum distributions of charged pions. It even leads
to the $\pi^-/\pi^+$ ratio deviating slightly from unity for isospin-{\it
symmetric} systems. The Coulomb interaction between mesons and baryons is also
crucial for reproducing the proper pion flow since it changes the behavior of
the directed and the elliptic flow components of pions visibly.
  The EoS can be better investigated in neutron-rich system if {\it multiple}
probes are measured simultaneously. For example, the rapidity and the
transverse momentum distributions of the charged pions, the $\pi^-/\pi^+$
ratio, the various pion flow components, as well as the {\it difference} of
$\pi^+$-$\pi^-$ flows. A new sensitive observable is proposed to probe the
symmetry potential energy at high densities, namely the transverse momentum
distribution of the elliptic flow difference [$\Delta v_2^{\pi^+ -
\pi^-}(p_t^{\rm c.m.})$].
