1992 Mercury Sable overflow

Hi jbee1119,

Thank you for the donation.

When coolant is being pushed out to the reservoir, pressure in the system is high which is due to inefficient cooling or a bad head gasket.

You mentioned low on fluid, I take it to mean it is the coolant?

If the radiator coolant level is low and reserve tank overflowing after the engine cools down, the head gasket is bad.

Apart from above, other possible causes are,
1. Bad water pump
2. Clogged coolant passages or radiator
3. Weak or faulty cooling fan.