We report the magnetization, ac susceptibility, and specific heat of optically float-zoned single crystals of Fe1-xCoxSi, 0.20≤x≤0.50. We determine the magnetic phase diagrams for all major crystallographic directions and cooling histories. After zero-field cooling, the phase diagrams resemble that of the archetypal stoichiometric cubic chiral magnet MnSi. Besides the helical and conical state, we observe a pocket of skyrmion lattice phase just below the helimagnetic ordering temperature. At the phase boundaries between these states evidence for slow dynamics is observed. When the sample is cooled in small magnetic fields, the phase pocket of skyrmion lattice may persist metastably down to the lowest temperatures. Taken together with the large variation in the transition temperatures, transition fields, and helix wavelength as a function of the composition, this hysteresis identifies Fe1-xCoxSi as an ideal material for future experiments exploring, for instance, the topological unwinding of the skyrmion lattice.