I have an 87 603A motor in my 90 300GD. Replaced the N/A with the turbo motor Feb of last year. Car runs flawlessly. Temp remains at 80, no oil in the water and no water in the oil.
The Problem is pressure remaining in the system overnight. Hoses are a little firm. I have experienced some sporadic water loss. It only happens occasionally. When it does I see a small puddle under the car. Level drops below the line on the overflow tank.
I have used the Exhaust in coolant tester (see pic) and it tested normal.
In prepping the new engine for install I updated to the new/larger water pump and housing. And sealed the casing bleed hole as recommended.
I have had some dripping from the flushing T and have sorted that out.
I took it to a radiator shop and they said not to worry about it.
I can go weeks without any fluid loss. I have a new Behr pressure cap. There never appears to be coolant coming from the overflow pipe.

Any help would be appreciated.
Mark

Attached Thumbnails

 

__________________
91 300GD SWB with transplanted 87 OM603 turbo motor