Add a bottle of dye to the coolant and get a small hand-held blacklight to find the leak, start in all the places mentioned and go very slow.

 

the heater core is in the passenger cabin, behind the firewall and behind the dash. There are two pipes that go through the firewall on the passenger side. They hook up to two hoses that then connect to two metal pipes that then connect to the engine. One of the pipes connects to the intake manifold near the temp sensor sender. That pipe has an o-ring on it where it enters the intake. Mine had cracked and someone had used some silicone to try to seal it. Well it didn't work. Leaked slightly, caused the tale/tail smell and coolant loss. Just one more place to look. Good luck. Phil

 

If urs a 5.0 v8 check ur crossover pipe. It's a common problem on this engine. Unfortunately it requires intake manifold removal. A slightly bigger job then just replacing the water pump

 

5.0??? lol

 

Sorry my bad just realise it's a d2 thread while I was browsing D4.
However dont purely relies on temp huage on early discovery they prone to failure for their temp sensor design locations.