Coolant mysteriously disappears

I am new to the board. Please bear with me while I get the hang of things. I have a 1992 Ford Aerostar XL with the 3.0L engine. It has 101,000 miles on it. Coolant is mysteriously disappearing. I have to add about half a quart every week to two weeks. I had a small leak around the timing cover gasket. A new timing chain and gasket took care of that leak at 90,000 miles. Coolant continues to disappear. The cooling system has been pressure checked with no leaks found by my mechanic. I know that it could be the head gasket, but have not found any antifreeze in the oil, nor does the oil level increase. Has anyone had any experience with this type of problem? Any information would be appreciated.

Thanks,
Tim

Is your van equipped with rear heat? If so, have you also checked for coolant leaks all the way back to the rear core?

foodstamp