Resurrecting an old thread, can you do this in situ without removing the exhaust manifold?
My D2 started taking a long time to get to the mid position on the temp gauge, so I decided to swap the thermostat as this was the most likely culprit.
Sadly there was some oil in the coolant and the thermostat was oil contaminated - which probably caused it to fail. There is no coolant in the oil thankfully and the engine runs as sweetly as it always has, so the O rings in the oil cooler are probably shagged.
The job itself seems straight forward, but after a bad experience with my son's D90, I would much rather not touch the exhaust manifold if I can avoid it.