I have a 2001 Ford Taurus that leaks coolant when driving. The car can be idling, and I can rev the engine, but it won't leak coolant then. Only when I'm driving. The coolant hits some hot part of the car causing it to steam very badly. I did a radiator pressure test and it was holding pressure and no coolant was leaking. I replaced the coolant reservoir and water pump but that didn't fix the problem. I don't believe it is the heater core because there is no coolant in the passenger side. Any clues on what this may be?
You say you did a radiator pressure test, but you should test the whole system by pressurizing that.
The system should hold pressure for several hours.
Have seen this type of problem before where one of the hoses has a small pin hole in it and it only leaks when the system is fully hot. The coolant escapes as steam and leaves no trace around the hole - very annoying and difficult to find.