I tested what you said and turned on the heater. Within about 10 seconds it dropped from 100C to ~90C. Does this indicate anything specific?

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k

If it were my car, I'd be suspicious of the radiator.

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles

I know you just did your head gasket due to oil-in-coolant failure. As you know the oil just puddles at first as it won't mix with the coolant but after a while it tends to break down into a frothy, foamy light brown goo. This oil residue tends to stick to all the surfaces in the system, particularly the rubber hoses and has great affinity to stick to anything plastic such as the coolant tank walls and plastic radiator tanks. As a matter of fact, I found the best way to extricate it from the coolant tank was to dip a strip of plastic in the tank. When you pull it out the coolant naturally runs off the plastic but the gunk sticks to it like magic. You wipe it off and repeat until all the floating gunk is gone, then do it again next day. It's really hard to get it all out once it bonds to everything even after the problem has been fixed.

So what I'm taking a long time to suggest is that perhaps your radiator (and heater core) still has some of this gunk slicked to it partially restricting the smaller tubes, reducing the cooling capacity by several percent.

I just finished the head gasket on my 91 300E 2.6 about two weeks or so ago and while it's fine on the road it still tends to get abnormally hot when it sits (no overheat, just hotter than it used to get).
I had driven it for about 9 months with oil seeping into the coolant. I would siphon off as much oil and goo out of the recovery tank as I could every day or so. It ran beautiful, only slightly hotter than normal during this whole period (I was really amazed and impressed with this reliability, and a BIG reason why I decided to invest the couple thousand in restoring the top end of this 200K+ car). Anyway, one day it got very hot and just overflowed a quart of this oily gooey milkshake. That's when I parked it and did the job (a year later).

It runs just fantastic now, but even after several flushes (Shout laundry detergent, Dawn dishwashing liquid, MB citric acid flush), it's still getting lots of oily gunk making its way back to the recovery tank. It's less and less each week but still amazing how much was in there. I have a new radiator, hoses and coolant tank, but I don't want put them in until it all clears out. I even went to the junk yard and got another tank and radiator just to use as interim parts because I feel my old parts are just too fouled to clean out properly and just recirculate the gunk.

Are you still getting any residue in your system?