High pressure in coolant system???

How hot has it been running?

 

Are you getting bubbles or just pressure build up?
Do you have a heat control valve under the bunk that is air operated?
I've had injector cups leak exhaust into the coolant.
Is the coolant reservoir tank inside getting any darker?
If you think it's exhaust or compression from the engine, the parts stores sell a test kit for the antifreeze for determining this.

 

the 07 volvo d12 is bad about egr coolers leaking and in some cases pressurizing coolant system, another possibility on the d12 is a air compressor with a bad head gasket will do this as well. napa sells a combustion gas tester, not very expensive ( like flightline mentioned). there is also a water indicating paste you can put on your dipstick and insert it down in the oil and wait a min or two and pull it out and it will tell you if there is any coolant in the oil.... when this trk sits overnight and you first start it do you get a puff of white smoke followed by a sweet smell of burning coolant?. just some suggestions from someone who has been in that situation. first and last swedish junker i will ever buy. sold that 07 volvo a year after i bought it.

 

I have Herd that the expansion tank that sits on top of the radiator, gose bad sometimes, Check On the driver side of the expansion tank, there is a blue cap It's supposed to release the pressure, if it doesn't, It tends to push the coolant out, instead of the air pressure.

 

Isolate the air compressor and see if that helps (block the intake ).... That will tell you if your on the right track .