2005 Jetta will not warm up to normal operating temperature. Today temperature in the 70s in South Georgia. Replaced thermostat and coolant temp sensor.. Thermostat not opening. No coolant flow in hose from thermostat to radiator. I tested old and new thermostats. Both start to open at about 180 degrees. I am retired diesel engine / truck mechanic. This little 4cyl 1.9 diesel is causing me to pull my hair out and throw dirt in the air. I don't know history on this car. Does TDI have " low flow cooling" system? Could radiator be clogged or wrong radiator installed? All help and suggestions will be greatly appreciated.