  We report on a comprehensive analysis of the kilohertz (above 300 Hz)
quasi-periodic oscillations (kHz QPOs) detected from the neutron star low-mass
X-ray binary 4U0614+09 with the Rossi X-ray Timing Explorer (RXTE). With a much
larger data set than previously analyzed (all archival data from February 1996
up to October 2007), we first investigate the reality of the 1330 Hz QPO
reported by van Straaten et al. (2000). This QPO would be of particular
interest since it has the highest frequency reported for any source. A thorough
analysis of the same observation fails to confirm the detection. On the other
hand, over our extended data set, the highest QPO frequency we measure for the
upper kHz QPO is at about 1224 Hz; a value which is fully consistent with the
maximum values observed in similar systems. Second, we demonstrate that the
frequency dependence of the quality factor and amplitude of the lower and upper
kHz QPOs follow the systematic trends seen in similar systems (Barret et al.,
2006). In particular, 4U0614+09 shows a drop of the quality factor of the lower
kHz QPO above 700 Hz. If this is due to an approach to the innermost stable
circular orbit, it implies a neutron star mass of about 1.9 solar masses.
Finally, when analyzing the data over fixed durations, we have found a gap in
the frequency distribution of the upper QPO, associated with a local minimum of
its amplitude. A similar gap is not present in the distribution of the lower
QPO frequencies, suggesting some cautions when interpreting frequency ratio
distributions, based on the occurrence of the lower QPO only.
