I have a 96 Mercury Sable w/ DOHC it keeps overheating even after replacing the thermostat. The coolant in reservoir is not staying level it keeps bubbling to the top and spilling out. Bled the system once to replace thermostat and got the air out but it overheats just idling what can it be?

cracked head or head gasket

Cooling fans not turning on, check to make sure the cooling fans are running when overheating occurs. Not enough coolant in the cooling system, check the coolant reservoir make sure the coolant is at at the maximum level when combustion engine is completely cool,if your coolant is low add coolant to the maximum level. (1)Start combustion engine, let it warm up but not over heat. Turn off combustion engine, when it cools completely the coolant reservoirs level will be lower . Add more coolant and keep repeating (1) until the coolant level stays the same after combustion engine completely cools down.Sensor that tells the cooling fans to turn on defective. Non functioning thermostat. Broken impeller inside Water Pump. Clogged radiator. Clogged water jackets in combustion engine. Broken drive belt. Radiator has something blocking the cooling fins(possibly a plastic bag that went against the cooling fins when driving)

Could be a bad water pump, sounds like the pump has shot being. That's why it turns hard. if the pump doesn't turn then no coolant will flow through the engine. That will cause the engine to overheat.

could have a bad thermostat or it could have been put in upsidedown

Try this:

3.0L DOHC Engine

1. Drain the coolant until the level of the coolant is below the level of the thermostat.
2. Raise the vehicle and support it with jackstands.
3. Disconnect the lower radiator hose from the water inlet connection.
4. Unfasten the two water inlet connection retaining bolts and remove the water inlet connection.
5. Remove the O-ring seal and thermostat from the water inlet connection. Inspect the O-ring for damage and replace as necessary.

To install:

1. Clean the water inlet connection mating surfaces.
2. Install the O-ring, thermostat and the water inlet connection. Refer to the accompanying illustration If you can-t remember how the O-ring and thermostat are positioned.
3. Install the water inlet connection assembly retainers and tighten them to 71-106 inch lbs. (8-12 Nm).
4. Connect the lower radiator hose to the water inlet connection.
5. Fill the engine with coolant, start the engine and check for leaks.
6. Stop the engine and top off the coolant recovery reservoir as necessary.
   
   

   Fig. Exploded view of the water inlet connection and the thermostat assembly-3.0L DOHC engines
   

3.0L OHV Engine

1. Drain the coolant until the level of the coolant is below the level of the thermostat.
2. Disconnect the upper radiator hose from the water hose connection.
3. Unfasten the three water inlet connection retaining bolts and remove the water hose connection.
4. Remove the gasket and thermostat from the water hose connection. Discard the gasket.

To install:

1. Clean the water inlet connection mating surfaces.
   NOTE The jiggle valve on the thermostat must be in the up position.
   
2. Place the thermostat in the water hose connection making sure the thermostat jiggle valve is facing up.
3. Install the new gasket and place the water hose connection assembly into position.
4. Install the water hose connection assembly retainers and tighten them to 89-124 inch lbs. (10-14 Nm).
5. Connect the upper radiator hose to the water hose connection.
6. Fill the engine with coolant, start the engine and check for leaks.
7. Stop the engine and top off the coolant recovery reservoir as necessary.
   
   

   Fig. Exploded view of the water hose connection and the thermostat assembly-3.0L OHV engines
   
   
   

   Fig. Remove the upper radiator hose from the water outlet
   
   
   

   Fig. Remove the water outlet retaining bolts and . . .
   
   
   

   Fig. . . . remove the outlet from the engine
   
   
   

   Fig. Remove the thermostat from the water outlet
   
   
   

   Fig. Inspect the water outlet connection for cracks or damage
   
   
   

   Fig. Thoroughly clean the mating surfaces of the water outlet and . . .
   
   
   

   Fig. . . . the engine before installing a new gasket
   
   
   

   Fig. The jiggle valve must be in the up (12 o-clock) position after installation into the water outlet
   

Assuming the level is proper you probably have a bad sensor or sending unit.
Good luck

Check the thermostat. The temperature sender is to the right of the thermostat and if the thermostat doesn't open the hot coolant from the block won't be flowing to the sender/gauge showing it is overheating. The thermostat is at the end of the top radiator hose where it enters the engine. Check the wires to the sender to make sure they are connected. Flush the coolant and heater core at the firewall connections as it may become clogged.

the thermostat is in the engine. Follow top radiator hose to engine, There is a neck it connects to, That neck has a cpl bolts, Remove and replace thermostat

Here ya go!

3.0L DOHC VIN S Engine
CAUTION NEVER open, service or drain the radiator or cooling system when hot; serious burns can occur from the steam and hot coolant. Also, when draining engine coolant, keep in mind that cats and dogs are attracted to ethylene glycol antifreeze and could drink any that is left in an uncovered container or in puddles on the ground. This will prove fatal in sufficient quantities.Always drain coolant into a sealable container. If necessary, clean coolant maybe reused unless it is contaminated or more than two years old.

1. Drain the coolant until the level of the coolant is below the level of the thermostat.
   
2. Raise the vehicle and support it with jack stands.
   
3. Disconnect the lower radiator hose from the water inlet connection.
   
4. Unfasten the two water inlet connection retaining bolts and remove the water inlet connection.
   
5. Remove the O-ring seal and thermostat from the water inlet connection. Inspect the O-ring for damage and replace as necessary.
   

To install:

1. Clean the water inlet connection mating surfaces.
   
2. Install the O-ring, thermostat and the water inlet connection. Refer to Figure 10 if you can't remember how the O-ring and thermostat are positioned.
   
3. Install the water inlet connection assembly retainers and tighten them to 71-106 inch lbs. (8-12 Nm).
   
4. Connect the lower radiator hose to the water inlet connection.
   
5. Fill the engine with coolant, start the engine and check for leaks.
   
6. Stop the engine and top off the coolant recovery reservoir as necessary.
   

Exploded view of the water inlet connection and the thermostat assembly-3.0L DOHC engines

look on the intake it should be there