Sounds like a job for the super-duper-watercop-winterweather-thermostat-optimizer-cabin-heat-enhancer:
Thermostats are designed to allow some coolant to flow even when fully closed - this ensures that the thermostat properly senses actual coolant temperature coming off the engine - if the thermostat closed completely, a "plug" of cold coolant would surround it, preventing it from reading and reacting to engine coolant temp, and an overheat condition could result.
So what, one might ask...well, the TDI is so efficient that it generates relatively little waste heat when lightly loaded, and much of that goes out the tailpipe in the form of heat in the exhaust gas flow. Recall that Diesels flow MUCH more air than gassers at light load because there is no throttle plate to restrict air flow.
The fact that the t-stat bleeds coolant even when cold is easily discernible by the fact that the upper radiator hose warms well before the engine reaches operating temp.
This bleed is enough to delay warmups, especially in cold weather, and cool the engine during conditions of light load in cold weather, as described by earlier posters to this thread. This likely reduces mpg and cabin comfort, not to mention giving rise to concerns of malfunction as by the originator of this thread.
DISCLAIMER - DO NOT TRY THIS AT HOME!
What I have done: Clamp the upper radiator hose using a hose clamp, 3" piece of 5/8" heater hose, and a 1.5" sch 80 PVC conduit connector (or similar short piece of rigid tubing approx 2" in diameter - (serves as an anvil - read on below)
Pass the hose clamp band thru the 3" section of 5/8" ID heater hose. Wrap the hose clamp around both the straight section of the upper radiator hose AND the rigid tubing described above. Tighten the hose clamp, pinching the radiator hose against the tubing anvil. Position the 5/8" heater hose section between the hose clamp band and the radiator hose, ensuring that no part of the radiator hose contacts the steel of the hose clamp to reduce risk of hoase clamp cutting into radiator hose wall.
Experimentation has shown me that stopping coolant flow completely (Upper rad hose downstream of clamp does not warm to touch at all) via the above apparatus substantially quickens warmup. On a typical 40-50 degree winter day in Florida, the temp guage gets to 190 in 2-3 miles rather than 4-5 miles. I have driven many 10 or so mile trips in my gentle high mpg manner with zero coolant flow to the radiator.
BE CAREFUL beyond 10 miles or at high speeds: Obviously, this could overheat the engine in short order!!!!!!! I have learned to watch the temp guage and open the hose clamp a quarter turn or so as needed.
AN IMPORTANT NOTE ON THE TEMP GUAGE - 190 indicated IS NOT 190 actual - the guage is programmed to indicate 190 whenever the actual coolant temp is between around 160 degrees and 220 degrees - refer to Bentley manual for exact details on this.
Once the temp guage moves past 190, meaning an actual coolant temp of above 220, IT MOVES UP FAST! One can prevent this by slowing or stopping completely, opening the clamp slightly, 1/4 turn or so at a time, or running cabin heat at max air temp and fan set at 2. A fan setting of 2 will cool the engine under most conditions with heat control maxxed.
ANOTHER IMPORTANT WARNING - do not "shock cool" the engine by allowing a large amount of cold coolant to suddenly flow, for example by radically loosening the clamp while the engine is hot. Engines don't like to be cooled quickly! - expensive stuff breaks.
Florida is too warm to run the above mentioned amounts of cabin heat, so I can't run this rig too much or long beyond 10 mile slow trips.
I suspect that drivers in subfreezing weather and short commutes could use this setup to some advantage, increasing cabin heat, improving mpg, and reducing emissions due to cold engines. Drivers in very cold areas might be able to go substantial distances since the demand for cabin heat might be ebough to provide what little sngine cooling is needed on very cold days - maybe +10 deg F or lower.
Once again, beware substantial risks of both overheating and shock cooling should you try something like this!!!
