I have an old 1965 Jaguar 3.8s - for all intents and purposes the same engine as the Mk2 or old 70/80s XJ6.
It's developed a worrying appetite for coolant - it gets used irregularly, either the odd short trips around town or long 200/300 mile runs at a time.
After it's been left standing for a week or so I have to top it up (usually 2-3 litres of water) and the heater is constantly airlocking - I have to pull the return hose and rev the engine to clear the air.
However, there's no puddle of water underneath or in the car that I can see, neither is it burning anti-freeze (no smoke or steam), there's no water in the oil or oil/mayo in the water.
It returns good mpg (for a sixties Jag), idles, runs and pulls well but simply loses coolant - but I can't find where it's going.
Any ideas?