1998 Buick Riviera Owner's Manual

If the overheat warning continues, there’s one morething you can try You can add the proper coolantmixture directly to the radiator but be sure the coolingsystem is cool before you do it.Steam and scalding liquids from a cooling hotsystem can blow out and burn you badly. Theyare under pressure, and if you turn the radiatorpressure cap -- even a little -- they can come outat high speed. Never turn the cap when thecooling system, including the radiator pressurecap, is hot. Wait for the cooling system andradiator pressure cap to cool if you ever have toturn the pressure cap.5-18