Where is my engine coolant going?

What is your question?

2006 equinox awd 3.4 engine, the antifreeze is disappearing we need to fill the reservoir about every week and a half about a liter. The other day in construction after sitting for about 25 min, the engine overheated and all antifreeze in reservoir disappeared. had to refill. I have had it pressure tested there are no leaks.