The cooling system pressurizes to 20 psi (the release pressure of the cap) and stays there. The pressure drops to 15psi once the engine is stone cold and will hold that pressure for days.

New Radiator
New Cap
New Thermostat

The cooling system never goes above it's normal operating temp as indicated by 2 temp gauges located in completely different areas of the cooling system.

I still cannot detect any combustion gasses in the cooling system. The gas detector has been tested and it works.

There is no oil in the coolant or coolant in the oil.

I still cannot think of any other explanation for the pressure source.

The new head gasket has arrived.

I do not want to R & R the head only to find that was not the problem. If no one else can explain this, then I see no choice.

Please, somebody stop me.

__________________
Greg
2012 S350 BlueTEC 4Matic
2007 ML 320 CDI
2007 Leisure Travel Serenity
2006 Sprinter 432k
2005 E320 CDI
1998 SLK230 (teal)
1998 SLK230 (silver)
1996 E300D 99k, 30k on WVO
Previous:
1983 240D, on WVO
1982 300D, on WVO
1983 300CD, on WVO
1986 300SDL 237k, 25k on WVO (Deerslayer)
1991 350SDL 249k, 56k on WVO - Retired to a car spa in Phoenix
1983 380 SEC w/603 diesel, 8k on WVO
1996 E300D 351k, 177k on WVO