2007 Peterbilt 379 Isx coolant boil, code sta15

Reason I brought up rad cap pressure rating was because when a fluid is under pressure, its boiling point increases. If the Napa rad cap you're using is rated lower than what the manufacturer specified (or is allowing pressure to bleed off due to incorrect fitment) then that could very likely be the reason for the coolant to be boiling.

 

I gotcha guys. I will put the new cap on before I run a bottle test and hope it's a simple fix. I will have all day tomorrow so I will do that and run to pick up an afternoon load (30 miles) that should get hell ##### good and warm . She doesnt overheat bad at all in the cold and never really smell the coolant or hot smell of radiator from front with fan on . But just can't idle if it's slightly warm outside or it climbs . It never used to do that I ran her night and day for a month straight and never overheated or got warm really with fan off. So I know something is a miss. Maybe simple I hope . I don't want to do the thermostats obviously if it's something simple so I'm hoping it's a cap, fingers crossed. So I'll go cap then bottle test then thermostats I suppose about as far as my knowledge goes. Appreciate the feedback guys.