1992 Nissan Pickup - Temp issues

My Nissan Pickup will not come up to temperature. In the summer the temp gauge makes it to the half way point, but during the winter it struggles to get to the first line on the gauge. I have changed the thermostat. Any other ideas?

The thermostat is either faulty or installed backwards.

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Or possibly wrong thermostat

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I have seen some thermostats that have a rubber O ring under them and a new O ring doesn’t come with the thermostat.
Don’t know if your truck is one of those. I seem to recall some Ford trucks that had 2 thermostats.

I had an older Datsun pickup, it used the heater core as the thermostat bypass, it had such an efficient heater core in it, the engine never got up to temp in the winter when I had the heater fan blowing at max speed.

Make sure the fan clutch is working right, and isn’t locked up.