Mk 7 - Heater (A/C) version - no heat, vacuum ok

[metalworker0]

As well as the valve that is controlled by the red and blue dial, i was also talking about the valve that is sometimes part of the thermostat, an extended bit on it that covers and uncovers water passageways to the small hoses coming out of the thermostat housing.-- this is a another diversion valve. these can fail in the open position or closed or bust off.
We are clutching at straws now, as you've tried most things but not everything.. going through all possible that can happen . plus some more.
You have a small hose system / circuit on the van that goes through the cabin heater matrix and oil cooler , we've established it does not have an EGR cooler.
Oil cooler has a thermostat .. don't know how that would effects things .. the manuals don't have a coolant flow diagram .. but a few people here have made some diagrams on how the system is set up certain years and models worth finding those in posts perhaps.

No answer from you as regards boroscoping the air chamber where the flap is to see if it is actually opening and closing.

Boroscope can be had if Temu for under £25... also available as device to plug into mobile phone even cheaper .

what else can happen .. impeller on water pump can disintegrate or come loose ... you'd know about that .. as it will start overheating .... but you'd not know if you were only doing short trips of less than 3 miles.

All the best.mark