Nissan Repair: Engine Temperature, classic signs, gas mileage

Question
I recently purchased a 1983 Nissan Pick-up.  It has a Z24 four cylinder motor, 5-speed manual transmission, air conditioning, and is two-wheel drive.  My question has to do with the engine temperature which never rises.  The guage will barely creep above the coldest mark and will never heat up to medium operating temperature.  I feel this is definately affecting my gas mileage as well as not providing heat or defrost on those cold mornings.  I did change the thermostat and drain and flush the cooling system.  Any advice you may offer would be greatly appreciated.

Answer
Did you use a Nissan thermostat?  I use nothing but Nissan thermostats since aftermarket ones are not near the quality.  When was the last time you saw a chrome plated thermostat?  Nissan has really good ones.  Now, if the thermostat is working properly you should change the tempearture sensor and that should make the gauge work properly.  But, since yours has all the classic signs of a thermostat that is stuck open I would replace it again with a new one from Nissan and you should be done.  If a change of the thermostat does not do the trick and the tempearture is still low on the gauge I would first chagne the termperature sensor and I would check to see if the heater core is blocked.  Remember that you bought a used vehicle and someone could have used stop leak in it and that tends to plug the heater core.  I ended up replacing a heater core on one car due to it being plugged up with stop leak.  So, take it one step at a time and the thermostat is the cheapest item followed by the temperature sensor and then the heater core.