2007 Peterbilt 379 Isx coolant boil, code sta15

Have a 2007 379 Pete with isx says sta15 on diagnostic tool. Having a problem with coolant boiling in reservoir. Doesn't show it overheating on gauge but it does spike when idling. Fan is working it kicks on and off on its own and cools it down or when I do it manually . The cap on the tank is from Napa I replaced it when I overheated and looked under and didnt have a cap. Can't remember if that was my fault? No white smoke coming out of stacks besides a little bit when firing from cold here in colorado. No egr we took it off and welded the valve shut where it connects to cooler. Fuel mileage did improve horsepower is about the same. Thought I had a bad turbo or something else when this morning after idling I found what seemed like oil from back of turbo and exhaust pipe mechanic said it was probably fuel and soot turbo is newer. Drove it hard home and didnt see a drop. Is this normal ? Truck has really done nothing but idle to keep batteries up for the last month. Can my replacement cap be defective it's been almost a year ? Coolant level stays full and I will flush it if need be it's been awhile . Probably has different kinds of coolant in it since I was always adding before egr delete.
Thanks in advance for any help .

Jay

 

Thanks for the response Fieldrat. I dont see any steam from the cap but i will double check again tomorrow when I head up north . I parked at the house after bob tailing from getting fuel and when i shut the truck off i could hear the boiling at the reservoir tank. Bubbling or boiling idk but it was loud enough to hear through the hood. It smells hot sometimes if the fan isn't running . If the fan is running it doesn't smell hot at all. The truck still runs strong and no lights flash.

 

Well I didn't see any steam from the cap. The more I listened it sounds like bubbling like was mentioned. So I stood there and it took aboit 15- 20 seconds and the bubbling sound was gone. I stopped on my way this morning about 40 miles from home to check for any leaks and sounds and all I got was a sweet antifreeze smell on passenger side . Wasn't hot on gauge and didnt smell hot at radiator only because I turned the fan on climbing the hill. On the way back I ran mostly with fan off and didnt smell any coolant after stopping a couple times. Got home and shut down and same bubble sound . It did not last long. So I am hoping there is no gasket issues or head or anything but it is definitely not how it used to run as far as staying cool without fan. I will try a new cap and then thermostats . Maybe the bottle test ? Not to sure how that works but seen a few videos of folks trying that.

 

Thanks for the reply guys . Yeah the bottle test seems easy enough. Lots of snow here yesterday maybe tonight so I will probably let the roads clear and have time to run that bottle test . I had the parts store order me a cap I will pick that up and try it . Still defiantly going to do the bottle test and will report back. I'm not sure of the pressure rating on the cap Amodelcat? I talked to the mechanic to order me one and he said it would be in in a day or two. Does a bad cap cause a failed bottle test? Or is that limited to head gasket nd compressor? I will give it a go and go from there.