Kerry has the answer for you. When cold the fan clutch is supposed to let the fan slip. Or in other words the blades will rotate with a flip of the hand. This slipping allows the car to get warm quickly. Now once warm, the clutch becomes tighter and the slipping of clutch becomes minimal or in other words it should not rotate much( I mean not even from 12 o clock to 4 o clock). This means that the fan will rotate with engine almost without slippage and provide max cooling.

It sounds like you have a failing clutch. Attend to it when you go home. Get it checked from a good mechanic a lot depends on that clutch. Here in NY I have seen some cabbies using a fixed clutch in summertime to get excellent cooling on their oldmobile or chevy.( but for us its not a good idea).

Also when the engine is running hot, the AC cooling also suffers by a few degrees. Since you did some work on AC recently, make sure all belts are tight enough. If you see any of the belts with major vibration( or flapping), it might be loose.

But again the way you described hot engine and free rotating clutch, it seems like clutch.

Also, when you get the coolant replaced, its good time to flush the radiator . Some people use citric acid for this. I have never needed to do that.

Good luck. Le us know what happens. This is the way we can help other guys in future.( I am typing too fast so sorry for typo and bad spellings)

4 bucks cap idea is good too. Nothing major to loose in that

MVK

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1985 300D Turbo 179,000miles