When I replaced the head on my '87 wagon, initially it would leave the upper hose pressurized overnight and it used some coolant. This went on for about a year. After I towed home a trailer full of oak wood flooring (about 2000 lbs or maybe more), which entailed hours and hours on the highway in the heat of the summer with engine gauge temperatures hovering around 110 degrees C or higher, all that stopped. I think that one must get the engine REALLY hot at least once to get the head gasket to completely seal. I used a dealer-sourced MB head gasket.

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Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC

Update: Head gasket done, Fuel heater replaced. Still pressure in rad hose overnight.
Any suggestions?

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91 300GD SWB with transplanted 87 OM603 turbo motor

I couldn't figure out where I was losing transmission fluid from, then I found these hoses up toward the front of the car, they were old and leaking. Bought some new hoses and replaced. They were easy to replace. Somehow or other the car uses trans fluid to cool something and that's what those hoses are for.

Good luck.

All I can think off top of my head:

--windshield res heater coil
--radiator coolant mixing with ATF
--thermostat
--heater core
--water pump ( i know you said brand new by MB, but never hurts to check)

I failed to mention new radiator as well so probably not ATF mixing.

How would thermostat or water pump affect residual coolant pressure?

Thanks Mark

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91 300GD SWB with transplanted 87 OM603 turbo motor

Actually the Car radiator serves a dual purpose. Not only does it cool the engine but within one of the side chambers there is a long narrow transmission cooler which is cooled by the radiator coolant flowing over it. That is what your hoses are attached to. Occasionally (rarely) these internal coolers fail. Causing coolant to enter the transmission (very Bad). I have only seen maybe 1 example of this over the years.
If it were mine, as preventive maintenance If yours is is over 200k I would replace the radiator.

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91 300GD SWB with transplanted 87 OM603 turbo motor

1) Pinhole leak in heater core? Lets air into system. Hotspots around cylinder result and vaporized coolant causes increased coolant system pressure.

Try pressure testing just the heater core in isolation, or temporarily taking the heater core out of the coolant circuit to see if the problem disappears.

2) Porous casting or minute crack in head?

Last year had a similar problem on my 5-speed manual Volvo wagon: coolant loss, no coolant in oil, no visible white smoke, rapid temperature rise at idle.
There was a pinhole leak (now plugged) in the heater core and the core will need to be replaced at some point.

Added kevlar fiber stop leak, and changed to a 71 degree C thermostat.
No improvement in condition. (Other than stopping the heater core leak).
Pressure test at coolant reservoir was negative for leaks in coolant system (1 hour with no pressure drop).
Had to take the car our of service for fear of overheating in stop and go commute traffic.

Pulled cylinder head thinking it was a failed headgasket.
Headgasket was fine.
Ended up being a crack in the cylinder head at the the number one exhaust port allowing coolant to escape undetected at operating temperature.
The other three exhaust ports were sooty but the first one was steam cleaned.
Replacing the head with a used Pick-n-Pull head took care of the problem.

Try pulling the exhaust (and intake?) manifold and inspecting the ports.

Hopefully you don't need a new cylinder head.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

Great thanks for the suggestions

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91 300GD SWB with transplanted 87 OM603 turbo motor

Did you ever figure out the issue? I'm about the purchase a 1995 E300 experiencing similar issues. The fuel thermostat rotted out. The coolant in the car appears to be the wrong type. Engine idles badly, and loses coolant according to the owner. Since the car is cheap, my plan is to just rip the engine apart and see what I can find. My fear is that something is cracked/rotted through.