There is a ton of information on this site, but almost too much! My query into "sdl cracked head" yielded several tens of pages and hundreds of threads. I think it would be a good thing to as much as possible distill that knowledge. At least that way people can get the basic information and know where to hone their searches. Something like what BoostinBenz has done on his web site, where he got all the relevant information about the 123 into one place. FWIW.

BTW, I'll update the group on what I find as I make the suggested repairs. Nothing like real-life to help educate!

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Casey Cooper
"From a long line of Mercedes ownership"

'86 300SDL 250K miles (Gone, but not forgotten); best diesel I have ever driven, too bad about the Achilles heel.

'81 240D 370K miles (Sold to my brother after 9 years and 150K miles of reliable driving!)

[Five other Mercedes in family clipped for less length.]

Replace the simple cheap stuff first. Replacing cooling system parts in a 18 year old car is not a waste of money. You will get a more reliable better cooling system even of the part you replace doesn't fix the problem. As sudjested above replace a few little things and clean the radiator out. Next have your oil tested. If the oil comes back without coolant and the temps stay down I would ignore the pressure in the hose. If the oil comes back with coolant in it then you have to move on to step 2.

For me at least step 2 would be pulling the head and having it checked.

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2016 Corvette Stingray 2LT
1969 280SE
2023 Ram 1500
2007 Tiara 3200

You can't trust the temp gauge when there are severe head cracks. The excess pressure does strange things to coolant flow and the single point of reference is not a good indicator.

Are you sure the AC compressor is installed properly? It might be sitting at an angle putting undue loads on the pulley and clutch. I'd say check the tensioner idler pulley but the power steering pump isolates the AC compressor.

Sixto
95 S420
87 300SDL

While it is odd you haven't had to add coolant, the level going up and down and the hard radiator hose were 2 things I noticed on my SDL when the head crack was developing/worsening. Actually at first I didn't have to add much coolant either, but eventually had to add more and more frequently. I never saw any abnormal temperature indications (at least no different from what I see now with the -17 head). I never had any white smoke or coolant in the oil.

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Keep everything as simple as possible-but no simpler--Albert Einstein