I am not entirely sure what I am doing in this game. I have 14k points and could carry on indefinitely, but I cannot increase my coolant level. I think that when I amassed a load of cash and quickly upgraded through a few grids without upgrading my coolant level, it made it impossible to do so now. But I am not entirely sure as the game just seems to be about holding down the overdrive button to get 700 points, waiting a few seconds and then doing the same thing again., over and over.
Edit: It seems that my heat/s needs to be between 0.5 and -0.5 to fill the coolant level bar.