Defective radiator caps or tubes
The radiator cap could be the cause of coolant reservoir overflow or backup. Have the cap checked to see if it is bad.
It sounds like the tank has a leak.
I had this problem ....Head Gasket or radiator cap
Check to coolant level in the coolant recovery tank and radiator when engine is cold
There are a couple of things that can cause oil to get into your coolant reservoir. A blown head gasket or a broken piston ring are the most common causes.
if there is oil in the coolant system it could be a crack in the intake, a ruined intake gasket, a cracked cylinder head
Sometimes the reservoir tank is broken, where the water pump is located. Im sorry, I was confussed with the winshield tank. But, the coolant reservoir tank sometimes leak in small holes in the base or near the cap.
the reason why the coolant is spilling up over the reservoir is because the thermostat needs to be replaced because when the engine needs water the thermostat heats up from the engine and causes the springs to expant which opens a seal to allow the water throughout the engine i just realpaced the thermostat in my volkswagen
When the low coolant warning light comes on and the car has enough coolant, it usually means there is a problem with the coolant level sensor system. The sensor may need to be replaced.
pressure test the coolant system you will find a blown intake or head gasket
Normally a leak in the sytem, either internal or external. More details would be required to expand on the answers to be given
It sounds like you have a blown head gasket. Combustion gases can enter the cooling system through either a blown head gasket or cracked cylinder head. The gases (under pressure) force coolant out of the reservoir. This also causes a void in the upper part of the cooling system , leaving no coolant at the thermostat and the coolant temperature sending unit. The result is a erroneous temperature Reading and will even prevent the thermostat from opening.