In my '90 300D 2.5 Turbo, which I purchased in Nov 05 (needed major work), I changed the head gasket, rebuilt the head, and changed little things along the way. (New thermostat, new fan clutch (too much play), new glow plugs and new filters and flushes all around).

With current outside temps of 30 oF - 40 oF (-1 to 5 oC) the engine temp at most gets to 80 oC if its driven hard throughout the city. But most other times it stays at a constant 70 - 75 oC.

Is it safe to say that the NEW Termostat may be stuck open?
Or the temp sensor / needle displaying incorrectly? (Any way of testing this?)

In my '87 300D Turbo, at normal driving, it stays constant between 85 - 90 oC. While in the summer weather w/ A/C it gets easily in city traffic to 100 - 105 oC.

Could this significant current lower running temp be that the '90 has 5 cylinder vs. 6 in the '87?

Feel free to post your running temps and input.