Disco 2 Coolant loss.

My 02 TD5 has been losing about 100ml/month until recently when it increased very substantially. Warning was given by a float switch I fitted in the header tank soon after I bought the car 21/2 years ago. During this recent cold weather the car does not warm up properly, even when set to full heat. The temp.guage needle is steady at 3o-clock. The top hose & heater rad. hoses are warm rather than hot. The top hose has been bled. For the last 3 weeks or so the car has been run with the pressure cap loose, so the system is not presssurised. When cold the coolant level settles down to about 5 - 10ml below the seam where the top & bottom of the tank are joined, without further loss. I'm wondering if the thermostat needs replacing? Members thoughts please.

It is difficult to see but your pump may well be on the way out. It is mounted on the back of the power steering pump.
Look for pink run off and staining, in the bracket holding it.
There IS a weep hole but it is feckin difficult to see!
A bit fiddly to replace but not impossible!
http://www.discovery2.co.uk/waterpump.html
(I did it last May, only took a morning!

Equinut said:

For the last 3 weeks or so the car has been run with the pressure cap loose, so the system is not presssurised.

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Coolant is probably evaporating before reaching the desired temp. You should start by sealing the system with a good quality reservoir cap. The temp gauge is not a precision instrument so it will show normal operating temps when they're actually lower.

Equinut said:

For the last 3 weeks or so the car has been run with the pressure cap loose

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Hi. This is the sillyest thing to do cos it can lead to excessive coolant expansion hence more loss beside the initial leak... and +1 for @NPG's reply