1998 Buick Riviera Owner's Manual - Page 223

If the overheat warningcontinues, there's one more thing you can try You can add the proper coolant mixture directly to the radiator but be sure the cooling system iscool before you do it. Steam and scalding liquids from a cooling hot system canblow out and burn badly. They you are under pressure, andyou turn the radiator if pressure cap even a little they can come out at high speed. Never turn the cap when the cooling system, including theradiator pressure cap, is hot. Wait for thecooling system and radiator pressure cap to if you ever have to cool turn the pressure cap. -- -- 5-18