I have a 1998 Mustang Cobra. I just bought it 3 months ago. Twice the Check Engine light has come on. I found the coolant level low. I filled it up to the correct level. The car ran ok. Then the light came on again. Again, the coolant level was low. I had the coolant system pressure checked, 15 psi for 15 minutes, and found no pressure leaks. The oil looks clean. There is no white foam on the oil fill cap. No white smoke is coming out the exhaust. Where is the coolant going?