uh you can if you want..but unless it's really really starting to build up and all black then you should have it flush b4 putting new one in..but if it's low then just poor new one in..make sure they are what toyota recommend or atleast have the same model number on it..engine coolant i would buy the pre-mix kind..it's red also, so i just pour in and mix with the old one..no problem at all..