I recently bought a 1998 grand marquis and I noticed the coolant temp on the dash doesnt move and stays on cold. The car is getting warm and heat blows good. I replaced the coolant temp sensor, cylinder head temp sensor, and overflow cap but no change. Would a bad thermostat cause this ? The car is still getting warm so I dont see how it wouldn't just move a little bit. The car also doesnt seem to fully pressurize and in not losing coolant.

 

If the engine is getting warm, why would you suspect the thermostat? You could try grounding the wire off the coolant temperature sending unit, not the one for the computer to see if the gauge moves.. I suspect a bad gauge in your dash.I have dealt with them before.
The lack of pressure is another issue. A bad pressure cap or minor leaks will not allow pressure to build as it should.Check and tighten clamps at hose connections first, then look at the cap or just replace it..

 

Another first time run and hide poster from a troll..

 

Hello, Pingpong, and welcome to the Mercury Forums. Not much action here, so replies usually take a while.
Your problem's discription indicates a "no power" to the dash guage -- or a bad guage.
IF you determine that the guage is busted,you MAY get an Instrument Cluster from an Auto Recycling facility -- or E-bay.
The cluster is an easy swap -- no special tools needed.
Good luck !!
'95 MERC at Flaming Gorge, UTAH