Still pressure in cooling system of my M923A2

[MarcM35A2]

Hi @ all,
I still have a problem with my M923A2.
When my engine cools down over the night, it builds up pressure in the cooling system. I have already done a CO test to check the cilinder head gasket. Result: No exaust gases in the cooling system.
The strange thing is. I don´t have any other problems. Only that the truck has pressure in the system when it coles down. But ist´s not enough ot open the cap valve. But to be sure I´va already changed the cap.
The facts I have checked:
-Truck starts well
-No white smoke during starting or driving
-No oil in the water
-No water in the oil
-No pressure on crankcase
-Temprature ist always O.K.
-The CO test is negative. (No exaust gases in the cooling system)
-The thermostat opens (But I am thinking of changing it)
-The surge tank cap is new
-The radiator is brand new
The only thing is -like I said- when the engine cooles down over the night, I have pressure in the cooling system!
So what is the problem?????
Next week I am going to check how much pressure it is, but the hoses are really hard.
I am thankfull for any ideas.........
Marc

 

[doghead]

As long as the pressure is no greater than the cap can hold, why is there a problem?

Overnight, is a short time. What about 3 weeks later?

 

[MarcM35A2]

It is not enough pressure to open the cap, but to bleed out coolant out of the thermostat housing and under the hose above the housing. It´s not much, but it´s something.
Overnight is short, of cause, but I am thinking that there shouldn´t been any pressure in the system when it is cooled down. Right???

 

[Coffey1]

Marc I would edit my post and remove the f bomb before a mod picks it up.

 

[doghead]

I've opened caps in trucks that have sat for months, and there was pressure.

I would not be concerned.

Leaks, I would fix.