The volvos I'm familiar with (most all through '88) have a buffer circuit in the instrument panel that keeps the temp gauge from actually following the temperature as long as it's in the normal range. Seems some drivers get nervous if they see the needle move too fast. IPD sells a replacement for the (plug-in) card that makes the gauge follow the actual temperature. The same thing can be achieved by soldering a jumper wire on the buffer card.
This may account for different operation of temp gages on different cars, if someone has wired out the buffer.
The D24 cooling system is different from any I have seen before, and I've been a professional mechanic for 50 years, on both diesel and gas equipment. The coolant enters the engine through the thermostat and leaves through the upper hose. Still, I haven't noticed any issues in the way it works (till it blew the bottom hose and cracked the head before I noticed overheating).
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