Having some big problems with my N reg 300tdi disco. It keeps over heating BIG time, but only if the expantion cap is done up- when left loose it runs fine. The heater doesn't work either just blows cold. I have drained and flushed the whole system, the water runs through fine. I've put in a new thermo stat and temp sensor. All hoses go solid when engine runs, so the water pump is working. I can leave my disco running for hours and it dont over heat but soon as i drive it heats up well quick. Once over heated, I undo the expantion cap and it sounds like a kettle- once the cap is off it has used some water. I have used a sniffer and it dont look like head gasket plus no smoke, no water in oil and no gunk in oil cap. Need some serious help please