Either the thermostat was not the problem, or you put it in backwards.
The thermostat is located on the front of the engine under the elbow at the end of the radiator hose
The thermostat will not open properly when installed backwards. The coolant will not flow and the engine could overheat.
Because the plastic housing is cracked. That is what happened to mine.
Check the engine end of the upper radiator hose
On a 1999 Mercury Grand Marquis : The Original Equipment engine cooling thermostat would have been ( around 192 * Fahrenheit )
On a 1999 Mercury Grand Marquis , 4.6 liter V8 engine : The Original Equipment engine cooling thermostat would be : ( around 192 * Fahrenheit )
Mercury Marquis was created in 1967.
the one possibility that i can think of is that your thermostat is stuck in the closed position not allowing the water to circulate threw the engine
replace keypad Mercury Marquis?
at the end of the upper radiator hoseremove hose then the part the hose attaches toits inside that part.
The thermostat gasket for a 1990 Mercury Grand Marquis is the standard "paper" type gasket, and a good RTV silicone is recommended. Permatex makes a sealant well-suited for the purpose - it's called "Water Pump and Thermostat Housing Sealant". You can also use similar sealants, such as Permatex Ultra Grey, Ultra Blue, or Ultra Black.