Engine coolant flush-filtering using external gas pressure and blocked radiator fill port

A method for rapid and efficient cleaning of an internal combustion engine cooling system includes: (a) forcing the coolant liquid from the cooling system to the exterior of that system, (b) treating the coolant liquid from the cooling system to the exterior of that system, (b) treating the coolant liquid in a zone or zones outside the cooling system, such treating including removing contaminant from the coolant liquid, (c) returning the treated coolant liquid to the cooling system, (d) the forcing step including supplying pressurized gas to the cooling system to drive coolant liquid therefrom, (e) the cooling system including a heat radiator including a container having a coolant liquid fill opening, and the forcing step including employing the gas to drive coolant liquid from the radiator via the container fill opening, (f) and including the step of controllably venting fluid including gas from the container via the fill opening, during the step of returning the treated coolant liquid to the cooling system.