On my 1980 300SD i just replaced the thermo and coolant. Put the car on ramps and ran the engine until the temp got to 90Degrees plus. Put the cap back on the overflow but the thermo is still not opening. I checked that it was a 80 degree thermo before it opens. The side of the housing closest to the block registers 80 plus but the bottom part of the housing is only going to 45 degrees. These temps are taken using a thermo gun. Any suggestions or do I puu the thermo out and see what is keeping it to open. I am pretty sure I installed the thermo correctly.