If the radiator cap’s vent valve is stuck closed from corrosion contamination, the cooling system cannot pull coolant back from the reservoir.
1. Verify that the upper and/or lower radiator hose is collapsed.
2. Use a trim stick to gently attempt to lift the vent valve.
3. If the vent valve is stuck, change the coolant as described in Group 14 of the appropriate service manual. Then replace the radiator cap with part number 1350A199 or equivalent.
NOTE: Do not replace the thermostat for a collapsed radiator hose.