My expansion tank?? (the one that comes from the radiator) keeps emptying
Possible leak? Pump issue? Any ideas?

Thanks in advance

 

If you are not getting any puddles under the car id dare to say the engine is burning it off. IE cylinder head gasket may be failing.

 

When have you been monitoring it? And have had the coolant level in the cap to max and filled up the expansion tank.

The expansion tank should be between min and max when stone cold.

Is the rad cap and thermostat OK? Check under oil filler cap for mayonnaise type substance.

 

Coolant level in rad full, tank between min and max and no mayonnaise

No obvious puddles below car

 

How do I check the thermostat? Saying that my in car heating could possibly have been tempermental

 

possible air lock??? Other than that do a compression test to rule out the head gasket as your best off catching it early if its failing.

 

you only get mayonaise when oil mixes with the water so in that case the level would probably not drop it would just turn a nasty brown,

any steam coming from the rear of the car when the engine is warm? if so the head gasket could of failed on the coolant to bore side and its burning the coolant off, in most cases the car will look like a kettle at boiling point driving down the road if its a bad gasket failure, if only a slight failure can be hard to tell with a stock exhaust,

get your local garage to do a compression and sniff test, that will clear head gasket from the options, then time to start looking for leaks.

 

remove it and drop it in a bucket/cup of boiled water to see if its oppening,

if the level is dropping then i cant see it being a thermostat problem personally, thermostat failure would generally be shown by symptoms of the car over heating or if its stuck open taking a long time to warm up, if its coming upto running temp (3 bars on dash) in a reasonable time and not then rising any further i would say the thermostat is fine