Magnitude of [Delta rho (T, H)/rho (0)] and insulator to metal transition temperature (Tat) in the colossal magnetoresistance (CMR) bulk ceramics response of La0.67Ca0.33MnO3 (LCMO) manganites are observed to be changed significantly as a function of higher sintering temperature (close to melting) and long hours sintering schedule. Magnetoresistance and magnetization (up to 5 T) of LCMO are found to be decreased considerably as the sintering temperature is increased from 1400 to 1600 degreesC. Further reduction of CMR response is also observed for long time (a few tens of hours in O-2) at higher temperature. However, on the other hand the value of T-IM is significantly enhanced and reaching close to room temperature under the same sintering schedule to achieve drastic reduction of CMR response. Microstructural analysis revealed the presence of clean and clear intergrain strong connectivity at 1600 degreesC sintering temperature for period of 17 h. This process enhances the physical densification of the bulk and promotes the creation of new conduction channels and reduction of intergrain effective contact area for transport current under magnetic field. Hence, drastic reduction of resistance associated with the CMR response is due to this reason. The disruption in the crystalline order observed by TEM at the grain (formulation of anti-phase) boundaries promotes local spin disorder which may alter the conductivity of polarized electrons. It is also observed that short-range local magnetic order is disrupted by the presence of grain boundaries which may be responsible for the reduction of magnetoresistance and increase of T-IM point towards room temperature.