Expansion tank has no pressure

[Odd-job]

Hi
I have 2008 Peugeot 207 1.6hdi sw with a heating issue
So not had the car long and notice yesterday after driving for about 30mins and letting the car get to normal temperature that if I turn the heater blower up to max speed the temp gauge and oil temp gauge drops by if I turn the heater blower down to the lower setting it goes back up so I pulled over to check the coolant level which was fine but didn’t feel any heat coming from the expansion tank so I removed the cap and there was no pressure and the coolant was cold please any advice would be much appreciated

[CitroJim]

Sounds like there might be an airlock... it will be worth going through the bleed procedure just to be sure.

Also, the HDi is a very efficient engine and it may just be, in the cold, the thermostat is remaining closed to keep the engine warm and as a result there's no apparent circulation... if the heater is nice and toasty warm there can't be too much amiss... Does the radiator feel hot all over?

Often, in very cold weather you'll see the temperature gauge fall a little when the heater grabs all the heat when the fan is on full.