faulty pressure cap or center housing has rusted through to the outer seal ...
Sounds like you may have a bad head gasket.It will leak exhaust into the cooling system and create alot of pressure
The radiator cap controls cooling system pressure.The radiator cap controls cooling system pressure.
Low coolant? Radiator plugged or restricted? Thermostat not opening? Water pump not circulating coolant? Defective radiator cap? Radiator hoses collapsing under pressure? Cooling fan not working? Cooling system air bound?
it is normal, when the coolant is hot and pressure built, the pressure cap allows the coolant to drain to reservoir to reduce pressure in the cooling system But, if it continually does this whereby the reservoir overflows and the vehicle overheats due to a lack of coolant actually in the radiator when cool, there may be an overheating problem or blown head gasket on the engine.
Radiator-- for storing and removing heat from the coolant. Pump-- used to circulate coolant throughout the engine and back to the radiator for cooling. Fan, either electric or mechanical -- used to draw air through the radiator thus removing heat. Thermostat--Used to keep the coolant at a set temperature. Radiator pressure cap--Used to keep the pressure at a set amount in the system. Hoses uses to transfer coolant from the radiator to the engine and back. Recovery tank--Used to catch any coolant that may overflow.
First I eye-ball the radiator and all coolant related parts then if the leak isn't obvious I pump 15 psi of air pressure into the cooling system with a cooling system pressure pump. This usually helps locate the source of the leak.
The thread on cap on the engine coolant reservoir is the pressure cap / radiator cap for the engine cooling system
When vehicle is running: Water pump is circulating coolant and cooling system pressure is building
Low coolant? Radiator plugged or restricted? Water pump not circulating coolant? Thermostat not opening? Cooling fan not working? Hoses collapsing under pressure Defective radiator cap?
sounds like a leaking head gasket have a pressure chek done on cylinders and cooling system
Keep the cooling system at a predetermined rate of pressure. Raise the boiling point. Allow excess pressure to escape. Allow the cooling system to withdraw coolant from the reservoir.
High pressure in a car's cooling system might be caused by exhaust gases from normal engine combustion entering the cooling system. The exhaust gases might be entering the coolant due to a head gasket that is leaking, or perhaps even due to a cracked engine block. Note that under normal operation, cooling systems are pressurized due to the heat expansion of the coolant. However, if a fairly new radiator and/or hose has burst, or if the coolant hoses connecting to the radiator are rigid with pressure, this is abnormally high pressure.