[Porsche944]

Sounds like you hit the nail on the head. The 944 series T-Stat is a dual poppet type.

When the coolant is cold, one poppet is closed, and one is open. The open poppet allows coolant to circulate through the block and heater core.

When the coolant reaces the T-Stas set temperature, the first poppet colses, and the second poppet opens. When this happens coolat circulates throuth the block, heater core and radiator.

If the PO removed the T-Stat, the coolant does not recirc through the block properly during warm-up, and during this time of year in your area the engine will never come up to temperature.

When you get a new T-Stat, be sure to get a snap ring. The T-Stat goes in the suction side of the water pump (flange where the lower radiator hose is connected). When you put the snap ring in, make sure to put the open end of the snap ring UP. With the open end up it is a LOT easier to remove the snap ring later.

BTW: Welcome to the world of "wasser pumpers"!