2005 V6 Mustang with about 120K miles. Was running fine. Car had a blown lower thermostat housing and leaked coolant all over top of engine. Replaced that and put in new Motorcraft plugs and new lower thermostat housing. That was several months ago. Now it is loosing coolant as seen on reservior with no apparent coolant coming out of thermostat housing and throwing misfire codes on plug #5. Replaced plug which looked OK and still getting MIL and misfire code. Also now overheating. Just replaced thermostat with Motorcraft part. Old thermostat seemed to have the stem go down about 1/4" to 3/8" further at the bottom part which faces the engine. Same part number for new one. Put in new Motorcraft plug wires and cleaned off coil. Started car and did not see any leaks on engine. Went behind car and saw thick white smoke coming from exhaust which lasted for about 2 minutes at idle. Car had not been run overnight so engine was cold. See on here that cause could be warped plastic intake manifold or blown head gasket. How do I determine the cause? Intake manifold would be easier to tackle as I just did the UIM and LIM gaskets on a Ford Duratec. Not sure what it is like on this engine. How can I pinpoint the cause of the white smoke (steam)?