Temperature Sensor/Coolant Circulation or Head Gasket?
I have a 2007 Mazda 3i that's giving me fits with the temp sensor and heater. I've owned the car since about 2012, and bought it from an individual dealer.
In May I had a massive coolant leak, lurked you fine people and was tipped to the water outlet. Sure enough, it was cracked and replacing it sorted out my leak.
In June I had to replace the coolant reservoir because it was cracked above the seam on the passenger side. Starting a few weeks ago, I've had my temp gauge spiking at odd intervals, my heater blows mostly cold, and my coolant reservoir levels will drop to low over a series of weeks. As soon as I top it off I'll get -some- heat at highway speed, but none when idling or coasting.
Two days ago I replaced the thermostat and housing. Since then my 15 minute commute with a top speed of 65 mph for 7 minutes has managed to peak the temperature gauge twice each way, if I start coasting when it gets high the gauge level will fall in under 30 seconds. The heater still works when accelerating or maintaining speed, dies off when I coast and is completely cold when at idle.
This morning when I got to work, after watching the gauge dance, the larger hose on the thermostat housing was stone cold. The larger hose on the water outlet (opposite end of the engine block) was hot to the touch. Coolant levels are perfect.