Loop detectors are probably the widest-used technology for traffic state estimation. Previous research has shown that loop detector positions within the link significantly affect the estimation of the macroscopic fundamental diagram (MFD) of a given network. This paper examines the biases produced by the positioning of loop detectors on the MFD, using both analytical and simulation methods, as well as empirical data from UTD-19. We confirm earlier results that a uniform distribution of loop detector positions reduces the bias. For non-uniform distribution, we found that: (i) the subsets of the MFD by the loop detector position help estimate whether the loop detector MFD will have a bias; (ii) if the detectors in the network are positioned more downstream with a larger variation, the loop detector MFD is more likely to have a discrepancy in position subsets of the MFD; (iii) a lower ratio of link length to green signal time elevates the MFD bias as well. This research opens the possibility for the bias of MFD induced by the loop detector data to be corrected by only using itself.