Hi..Sorry for delay..

Ok..The white residue you do see is most likly the dried coolant - so you will need to observe not only the condition of the cap itself but the actually neck of the expanson tank to be sure it doesn't have a hair line crack - one option would to clean up the neck of the tank as much as you can to get rid of the residue and the inner side of the cap and then refit and monitior the level there after - Also too checking whilts the engines has been run any obvious signs of leaking from under the car as some leak can tend to form or show up more one the system is upto the pressure or when the engine is hot when things expand so following all the hoses/rubber and solid / connections are properly tight and haven't cut into the hoses which will let coolant seep out / housing the pipes connect to / radiator / and egr cooler two pipes go to that / oil filter housing cooler aswell / and down round the crank pulley side incase the water pump is leaking slightly - i would also check the floors around the foot wells just to be sure it isn't leaking inside the car..if you don't see any obvious leaks wether engine is hot or cold as far as you can see and the coolant is still dropping then you need to have the system pressure tested to see if it loosing pressure - this is guage that fits over the in place of the cap and monitors wether the system can hold pressure or not if starts to fall then there must be leak either externally or sadly internally ie: possible headgasket failure which too dpending how its gone can cause excess pressure build up in the tank caused by exhaust gas escaping into the coolant system thus the coolant forces its way out of the exapansion tank cap - another test that can verify that is sniff test/chemical block test - so there are a few things that be checked as a starting point or tested anyway to see what exactly is causing the coolant to drop and i'm sure any good garage should be able to carry that out for you..but try cleaning up the cap first "replace cap if required" and then monitor the level again...if its does keep dropping then you needto have the checks above carried out anyway..

Hi..

That could be one possibility although usually it should be hotter in through the top from the thermostat and slightly cooler out the bottom - if stone cold then there is greater chance the rad could be blocked and as suggested above maybe causing and increase of back pressure in the system - they would need to undo both hoses and then run water through the rad itself to see what the flows like or try and back flush - the end result if the rad is found to faulty will only be replacement - one thing they may want to check also aswell as the above is to make sure the thermostat is ok and the water pump is working and the but should can be monitiored using a/the pressure guage