Hi guys...it's 11PM here and I was just checking a few things on the car that were nagging at me. First thing, I haven't driven the car for a day, yet the system was pressurized! So now I'm thinking I have air trapped in the system. I've had W126's that were gas, and this W123 diesel, never had this happen before...but I remember reading somewhere this is why there was a bolt on top of the thermostat housing....Kent at MB Source once had a thread about removing the upper radiator hose from the thermostat housing and filling it up, and then quickly reattaching it...then squeezing the hose until bubbles at the overflow stopped. So I tried that....system started to pressurize, temp gauge was OK, but no movement of fluid in overflow reservoir, and water remained cool, even though upper hose was getting warm. So now I think I need to do two things...remove and check that thermostat you guys suggested, and pressure test my cooling system cap to see if the vent works to relieve excess pressure. I'll keep you guys posted....