This week, I've noticed my TEMP guage dropping to COLD after 10-15 minutes of driving. Morning (cool) or afternoon (hot), around town or on I-95, it makes no difference. This morning, it never moved off COLD during my entire 50 mile drive. No other unusual symptoms during the drive.

Temp sending unit, thermostat, ????

 

Try the heater after driving enough to warm up the engine. If the engine is actually being over-cooled, the heater will not work very well. In that case the thermostat is stuck open; replace it. If you do get plenty of hot air, check the wire on the sender. It is screwed into the head underneath the distributor toward the back of the car. Engine hot, leave the key on and wiggle the wire and see if the gauge goes up and down. Finally, unplug the sender wire and ground it-- the gauge should go all the way to hot. If it doesn't do that, likely the gauge itself is bad.

 

When it happend yesterday, I did try the heater and the air was hot. So, as you said, the more likely cause is the ECT sensore or wiring.

Today, after a 50 mile drive to work mostly at highway speed, I drove in the parking lot with my A/C off, neither fan on (which is normal on my car if A/C is OFF and engine temp not > 199 F), and my TEMP guage reading max. COLD. Upper and lower radiator hoses were hot (upper more so than lower). Good heat inside the car when selected. No other adverse symptons.

 

check the temperature sending unit

 

I replaced both the temp sending unit and thermostat ...problem solved.