this i an information thread for others to use, it is a a problem and fix. i will try and keep it simple. i am not saying this will always be the case but it is worth checking.
Problem:-
when the hot weather hit i noticed the expansion tank was empty, i checked the radiator and there was no water visible.
the temp gauge on the dash was reading slightly above the normal level by as little as one notch on the gauge.
diagnosing the issue:-
i filled the radiator back up with about 2L of water. and filled the expansion tank.
i checked the radiator and hose system and water pump area for water leaks, or signs of dried coolant stains, there were none visible.
i went for a run out about 5 miles the engine did not over heat but went to the same one knotch above normal temp.
checked water level after trip out, expansion tank looked full, let the car cool down and checked radiator, there was no water to be seen.
topped up the radiator again with 1L of water.
i repeated this a few times and each time coolant was lost, about 1L, but the expansion tank level never got drained as it should do if the radiator needs more coolant top up.
Fix:-
top up the radiator.
Change the radiator cap for a working one (i happen to have one in the shed).
i can not see any physical damage to the old cap and seems to squidge up and down like the new one.
well the good news is the problem has gone away, so all of this is due to a dodge radiator cap.
it must be something to do with maintaining the pressure and seal in the radiator to expansion tank link.
i don't actually know where the coolant has gone each time i filled the radiator up. it must have come out from round the radiator cap, but i did not see any steam or any staining round the cap.
just to note i deliberately filled the expansion tank with just water and now it smells of coolant, so i know the expansion tank is filling and then giving back coolant to the radiator when needed.
i have checked the oil cap and no gunk in there, and there is no steam out the back, so i happy it is not a head gasket issue either.
hope someone find the useful.