My vehicle is a 1990 W124 2.5 Turbo. I recently replaced the oil cooler lines. In checking to make sure that there were no leaks, I noticed that the lines do not appear to be getting hot. I'm concerned about a blockage or about having installed them incorrectly.

Now, I am in Minnesota and the temperature has been around 20 F, but even after an hour of driving, the fittings on the lines where they enter the oil cooler did not seem much - if at all - above ambient temperature.

Does the thermostat just not open in these conditions, or do I have a problem?

On the older models there is a Thermostat built inside of the Oil Filter Housing. I am guessing your year and model is the same.
Someone with access to the Factory Service Manual will need to give you details on what temp the Thermostat begins to open at and when it is fully open.

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84 300D, 82 Volvo 244Gl Diesel

Send a PM to Jeremy. Last fall he ran a test with a thermocouple in the oil cooler core and compared to an auxiliary oil temp gauge. He noted that it takes a long time for engine oil temperature to reach steady state even with coolant at 82*F, and longer still for oil cooler core temperature to reach steady state. From that point they rise and fall together. This was in ~65*F ambient. There isn't any opportunity to repeat the test at anything below 40*F in our corner of the world.

Sixto
87 300D

Yes, it's inside the Oil Filter Housing.

It'll not open until the OIL reaches 90 C (or so)

OTHERWISE, the Engine WOULD NEVER REACH OPERATING TEMPERATURE.

Running a couple of Sumps of SYNTHETIC will HELP restore the OC Thermo's
function.

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'84 300SD sold
124.128