Hi, welcome to Just answer! My name is XXXXX XXXXX i will help with your question
Does your cabin heater blow hot air as normal or do you find its a bit cooler than usual?
The cabin heater works normally. It blows coldish (not properly air conditioned but cooler than outside temperature) when dail in on blue, and warm air when on red. Nothing abnormal there.
Ok its possible the water pump is failing, it seems to be quite a common problem on these engines now where the pump impeller comes loose on the shaft so that's a possibility, If you get the level topped up correctly and start the engine with the cap off, you should see the water circulating, if you get some one to increase the RPM you should see water flowing in the bottle, if its not, its probably the pump. Otherwise the first part to change here would be the thermostat, it sounds like it might be sticking closed, enough to overheat and pressurise around town but keep cool long long runs with a good air flow
Thanks, XXXXX XXXXX check that.
Checked, and the water / coolant flows fine when revving.
Would the thermostat sticking account for the loss of fluid ?
Yes it certainly would, the thermostat will cause the temperature to increase, although it wont over heat necessarily it will cause excessive pressure, the lid on the coolant bottle has a pressure release valve so when the pressure gets to a certain point it will let the steam out, it wont take long for the level to drop. It might be a good idea to get the car into some one with an emissions tester first though because loss of coolant could well be due to a failing head gasket, its not something I would be to worried about at this point but its cheaper in the long run to get this diagnosed correctly. An emissions tester can do a "sniff test" or there is a block test you can do, both do the same thing, they check for exhaust gasses in the coolant. I have known the heads to warp and/or gaskets to fail unfortunately which lets the coolant into the cylinders where its burnt with the fuel. If there is a lot of white gunk on the underside of the oil filler cap or on the dip stick its highly likely its the head gasket, in which case I would recommend a compression test is done and although a thermostat is not all that expensive, half an hour doing a compression test and sniff test will tell you a lot more about what's really going on which ultimately will keep the costs down.