Just fleshing out the details of the problem with inline thermostat valves...
The bypass circuit in the Liberty CRD engine is not an open circuit. It is also controlled by a valve, just like the cooling system circuit running coolant through the radiator. Both valves are controlled by the same wax motor; when one circuit is open, the other circuit is closed. When the valve in one circuit is stroking open, the valve in the other circuit is stroking closed. The following link will show you how this type of valve operates....
https://www.youtube.com/watch?v=_HclvBmwWgQ This is a very good design for directing coolant to where it is required, verses older design cooling systems that have a bypass circuit that is not controlled by a valve and is therefore always open. The problem with a bypass circuit that is always open is that some coolant will always be diverted through the bypass, instead of through the radiator circuit where it is needed in heavy demand situations.
To understand why a PROPER thermostat upgrade for the Liberty CRD had to be developed, you need to know that thermostat valves - at the end of their service life - are designed to fail open at the radiator circuit valve because failing closed would bring about an overheat situation.
The problem with a valve-controlled bypass like the one in the CRD engine is that, when the thermostat valve fails permanently open, the bypass valve remains permanently closed, shutting of the bypass circuit permanently. If someone were to put another thermostat valve in the upper radiator hose in conjunction with a failed O.E. thermostat assembly, then any time the upper radiator thermostat valve is closed because the engine is not hot enough, there is inadequate coolant flowing through the engine and cylinder head because BOTH circuits are blocked off. This can cause hot spots to form in the cylinder head - starting around the exhaust ports - and can lead to the cylinder head warping and/or cracking, along with cylinder head gasket failures.
It is not a coincidence that the Liberty CRD has a disproportionate amount of cylinder head gasket failures, warped and/or cracked cylinder heads, and exhaust valve failures. If CRD owners are opting to try and get their engine operating temperatures up by using a thermostat valve stuffed in the upper radiator hose, this is likely the main culprit of those problems.
Wait and revisit this thread... no doubt there will be several defenders of the inline thermostat option posting counter opinions.
They are entitled to their opinion, of course; in Western Society it is their democratic right to be wrong. They do not understand that cylinder head damage does not occur 100% of the time, and that when damage does occur it happens over a lengthy period of time, and most of the time you can not even see the damage until it is too late. Driving habits and the climate make a difference as well... it is likely that CRD owners who install an inline thermostat valve in warmer climates where engine operating temperatures remain elevated for longer periods of time would not experience as many problems as those who do so in the frigid North.
To these members I continue to say that it is rather arrogant for them to make fundamental changes to how the cooling system operates and then expect to have no problems down the road. The cooling system is designed the way it is because the engineers who designed the R428 engine have the education, the ability and the equipment to determine that this is the best design for this application.