COOLING CONUNDRUM WHEN TOWING
Anyone any ideas on this?

I have an XC60 D5 Auto 2009.

Driving without towing, there are no problems at all, even if driving briskly.

Hitch up 1500kg (boat or caravan) & after 20/30 miles the Low Coolant Level warning comes up.

Leaving the engine running, under the bonnet both fans are working hard. The coolant level has dropped to 75% of the header tank & the coolant can be seen boiling in it.

Switching off the engine, there is no way that I am going to remove the pressure cap! Leaving the car overnight to cool, the coolant level drops to about 35% of the header tank. Removing the cap releases a lot of pressure & the level recovers to about 65%.

Top it up & drive away (without towing), no problem at all.

Hitch up & the low coolant scenario repeats itself.

There is no sign of coolant escape. Both fans are working (at least when the car is stationary & I can see). No idea what is happening with temperature – this is the first car I have had in 40 years that doesn’t have a temperature gauge! The car drives well, no engine spluttering or misfires.