If I understand you correctly, you no longer have the coolant overflow tank in your car, which was probably removed to make way for your FMIC.
Without it, when the coolant expands due to heat, which obviously will happen when the engine warms up, any "excess" coolant in the cooling system will flow out to the ground instead of into the overflow tank.
This results in an overall loss of coolant, which means besides polluting the environment, you will be spending more money on additional coolant to replace that coolant which flowed out onto the ground.
And, if you lose enough coolant and don't realize it, your engine will overheat. How bad it overheats, and for how long, will determine the amount of possible engine damage you will ultimately have.
If the coolant went to the overflow tank, it would return to the engine when the engine and coolant cooled.