c15 acert pushing coolant out radiator

I have yet another problem with my 2007 t600. It started pushing coolant out of the overflow tube yesterday. Its never done this before. I have no other issues with how it runs, no overheating, rough running, etc.. I added some to it about 2 weeks ago and i might have overfilled it, but i have added it before and its never pushed it out. I drove the truck for about 5 miles home yesterday and went to take the pressure cap off and it blew coolant out so i closed it quick. About an hour later it still had a lot of pressure in it. Its never had that kind of pressure before. I could take off the cap when i would be done for the day with barely a poof of pressure. I have no bubbles in the tank except for some very minor ones about every 30 seconds which seems normal. Before everyone condemns my head gasket is there anything else that would cause this? Air compressor, bad cap, etc...? Truck has 767k miles on it. Also what about the water diverter valve that i have read about? Any help would be greatly appreciated!

 

Ok, well i will see if cat can look at it tomorrow. Would that precooler cause an increase in pressure?

 

Ok, yeah i also have no water in oil or any other symptoms like that. I watched the water level rise up quick with the cap off and engine cold. It was under the filler neck then shot up to the middle of the neck quick like. Never went any higher with the cap off.

 

No i know your not trying to. I just got started as an o/op and do NOT have the money for a head gasket or inframe now. This truck has been nickle and diming me to death so far. I didn't want to hang it up this quick. I like what i am doing. How much is the average rate for a head gasket?