Hi,
I have a '99 V10. Bought last year. 81k mi. Put on new water pump. Later put dye in. Fixed leak in Hose to thermostat. Later had engine flushed and BG stop leak put in. Still the water dropped eventually below the add mark in reservoir. I rented a block tester and the liquid stayed green. A couple months ago put in some expensive copper like Bars leak. Still it goes down after a few hundred miles.
The reservoir has an overflow tube hanging down. My engine does not overheat even pulling my 10k lb. trailer. Could I just be loosing 3/4 gal. from evaporation out of that tube?
Desperately seeking answer...

 

Must be burning it somehow. Plus you have ruined the cooling system putting that bars leak crap in it.

 

The reservoir itself is not cracked and/or leaking?

 

It takes but a pinhole, a leak so small that it doesn't even drip, to cause coolant loss. The O-ring on the heater bypass tube, the coupler in the heater core return line, a weeping leak in the heater core, et cetera, can leak imperceptibly but for a bit of mineralization as evidence of it. Rarely, a leaking intake manifold gasket will behave the same way for just a little while, but every one I've seen that leaked coolant went from mystery to misery quickly so I'd not recommend taking off the manifold to look for a leak.

It's not at all uncommon to develop overheating problems after using miracle leak fixers. The usual result is a cooling system that works okay when you're not seriously taxing it, and a steamed hood when you do. You should flush the ever-lovin' out of the system like you're going to have to drink from it, and then do it again after a couple or three hundred miles of driving. Or at least reflect upon this thread when a hot day and a long grade conspire to steam your hood, anyway.

 

Dam- After posting a reply and sending I found I was off line and lost the update.
The gist of it is that after putting in Kseal which didn't seem to do anything for a few hundred miles I then plugged the overflow tube and put a gasket on the yellow cap to stop evaporation if any. Shortly after, it stopped loosing water.
What came first Kseal after 1k mi. or plugging it? There is no overheating as I pull my 11k lb trailer up Utah hills.

 

Just wait it will and you will be replacing the radiator and heater core. The idiot dealer i got my truck from did it instead of fixing the leak in the freeze plug. I drained the cooling system as soon as i got it but it was to late. When i finally took my heater core out almost all the tubes where blocked.

 

Update:10/01/16 For some reason I wasn't getting notifications from gmail to outlook my main mail.
Well, so far it's working just fine. Water goes to full then down to near add then when hot goes back to full. Still no overheating at all.