@LB123 @Aleskoda - The radiator muff idea comes from back in the days when the fan was directly coupled to the water pump, and ran at engine speed irrespective of engine temperature. Even back then thermostats used the same "melting wax" technology, and actually opened progressively across a range of engine temperatures.

 

What has actually changed now is that with full engine management, and more sophisticated sensors, we have a better understanding of actual engine temperature.

Just been out for a run in my PD100 and even after 10 miles or so of motorway driving it wasn't quite up to temperature. I think the problem is it's a big engine that isn't necessarily worked that hard pottering around so it struggles to get toasty on all but long journeys with the heaters going.

 

The bottom radiator hose was cold so it's not even that the thermostat. The water pump is a month old from VAG, done with the cam belt too so it's unlikely to be that. 

My tdi is the same, which is why its wearing a winter grill cover. Now warms up quicker and stays there  (4-6 miles) , prolonged periods of idle (5 mins plus) with heater fan on 3-4 will cause a slight drop in temp, so just keep it on 2 and its fine. 

Any feedback? Did you try a radiator cover or change coolant temp sensor or thermostat? Thanks