Constant loss of coolant in a 2002 Golf TDI, with 170000 miles. New radiator, thermostat and thermo-switch. Car heats up when driven aggressively, otherwise stays cool in traffic, as long there is enough coolant in it. Suspecting a cracked head gasket. No coolant in engine oil. Possibly engine pressure is pushing coolant out of overflow reservoir? Smell and micro-traces of oil in the reservoir. Any thoughts?