I was chasing a similar "too hot" problem on my 300D and did much of the same tests and replacements as you. Nothing changed. My solution? Buy a 300SDL and give the 300D to my son. Now it does not bother me as much.

Seriously, I think wilth all you have done, as long as there are no other symptons and the car does not actually overheat, the problem could be with the calibration of the guage, or it just runs hotter than some. There has been a lot of discussion about this over the last few years.

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Alan Hamm
'87 300SDL 277K Miles
'89 560 SL 68K Miles