My 1995 Mitsubishi Diamante will not get hot in the winter time.?

Your engine requires the exclusive use of GENUINE Mitsubishi COOLANT. If someone has installed the generic green stuff, you may have some blockages and wearing internal gaskets. There is a REASON they have their own coolant. Have your system flushed out and restored to factory at the Mitsu dealership. Sometimes they run specials on this service. ALSO>>> power wash the space between the radiator and the A/C condenser. This is where bugs, leaves, and road kill collect. You may have to undo a few bolts to gain access. Power wash the engine and KEEP IT CLEAN. Dirt free engines will dissipate heat more evenly and completely. If they were MEANT to be dirty, they would come from the factory that way! In your cooling system there are a variety of metals and plastics used. Generic coolants will simply NOT protect them from damage like the original coolant will!! You already have a sticking thermostat. There is a REASON for this. GOOD LUCK!

Needs a new thermostat. They can get stuck.

The garaage will flush the radiator and heaer radiator and also get rid of any air leaks.

The thermostat is stuck in the open position. Replace the thermostat.