I have a 1996 Ford Explorer XLT, 4x4, 4.0 liter. The problem is this.....I fill the radiator with coolant, drive anywhere between 75-100 miles and arrive home to find the the coolant is 1-2 gallons low! There are no obvious leaks with hoses, thermostat and water pump and no standing water on ground after the car sitting. The car was ran sitting in the driveway and the radiator checks out ok! This makes me believe the head or head gasket is blown. So, a block test was done to check the hydrocarbons in water! The test PASSED! What else could possibly be leaking? Any ideas or suggestions?