Possibly a bad / dirty Intake Air Control (IAC) valve. It's easy to replace yourself; costs about $50 and takes less than an hour. See detailed info here...
http://www.autoclinix.com/Ford/Ford_IAC.html
Your 1996 Mercury Grand Marquis may need a new transmission or something as simple as transmission fluid.
According to the 1996 Mercury Grand Marquis Owner Guide ( 5W-30 is the PREFERRED engine oil )
The 1996 Mercury Grand Marquis OBD 2 port is on under driver side dash next to center console
what is the part called where the water pump is connected to on a 1996 mercury grand marquis -------------------------- That would be the engine
where is the starter located on a 1996 mrecury grand marquis this site SUCKS and is very user UNFRIENDLY!!!!!!!!
It is 134.
On a 1996 Mercury Grand Marquis the windshield washer squirter button is on the end of the turn signal lever
According to the 1996 Mercury Grand Marquis Owner Guide , Traction Assist was an option As of January 18 , 2012 you can still view the 1996 Mercury Grand Marquis owners manual online at : www . motorcraft service . com ( no spaces ) Click on Owner Guides
According to the 1996 Mercury Grand Marquis Owner Guide : ( 20.0 U.S. gallons / 75.7 litres )
According to the 1996 Mercury Grand Marquis Owner Guide : ( DOT 3 brake fluid , meeting Ford specification ESA-M6C25-A )
It will, though technically it is illegal to istall on older motor into a newer car for emissions reasons.
In the fuel tank.