Coolant loss, no external leak or coolant in oil

[Gokhan]

Try the GM OE ACDelco cooling-system seal tabs (ACDelco 10-108/GM 12378255). It worked in my Corolla.

http://www.acdelco.com/auto-parts/vehicl...-seal-tabs.html

 

[MaximaGuy]

There is a UV dye you can drop into the cooling system that can be observed in the oil if there was a leak from the pump.

I used the dye to determine my car had an leaking pump.