Defective radiator caps or tubes
Excessive coolant in the reservoir can be caused by a few factors, such as a faulty radiator cap that fails to maintain proper pressure, leading to overflow. Additionally, a malfunctioning thermostat or a leaking head gasket may cause coolant to leak into the engine, resulting in an overfilled reservoir. Lastly, improper coolant mixture or overfilling during maintenance can also contribute to this issue. Regular checks and maintenance are essential to prevent these problems.
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
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.
Check to coolant level in the coolant recovery tank and radiator when engine is cold
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