I had exactly the same problem with my '87 300SDL. Fixed all the things you have fixed and the vehicle kept overheating. Going up a long steep hill with constant boost, it would get to ~ 90 C and hang there and just when I thought things were good, the temperature would shoot straight to 100 C & beyond (if I let it, which I tried to not to allow).

I was reading about what exhaust backpressure is normal (2 Bar or less) when I saw a reference to an "oxidation catalytic converter" in my service manual. At first I thought that this was the dreaded "trap oxidizer" which I knew my vehicle did not have. Then I saw the attached figure. The first of the 3 cans is the oxidation catalytic converter and I did have one of those.

Now I have a nice empty metal shell in that location, having removed it's innards. My car no longer overheats and hangs near 85 C, when under a light to moderate load, instead of hard against 90 C most of the time. I don't know why this is not very frequently mentioned as a source of overheating issues. It should be gutted before even doing a citric acid flush. 2 Bar exhaust backpressure is not a good thing for any engine.

The ceramic honeycomb in the OCC had about 75% blockage. It takes a fair amount of hammering to get it out, but it is satisfying work, with great benefits. The turbo sounds much happier.

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1986 300SDL
1987 300SDL
1997 F-350 Powerstroke Turbo Diesel
1969 32' Luhrs Perkins HT3.54M Turbo Diesel
WVO & biodiesel when I can get it
(there is a de-facto ban on biodiesel in CA
- ask the Water Quality Resources Board why
underground storage isn't allowed for biodiesel -
heck, you can drink the stuff )

How can I test exhaust backpressure? Is it as simple as hooking up a pressure gauge to one of the ports in the manifold?

Jason

Also, are you saying I should be hearing the turbo? Haven't heard that high pitch wine in this particular vehicle yet.