Gunk in radiator cap

[Diggs84]

Sorry. This is a duplicate post due to spotty internet. There's another with same title.

 

[half k cruiser]

Try putting the cap back on and with the vehicle running open the overflow reservoir and see if bubbles are forming below the tube. You will need to make sure the reservoir is filled and the tube is submerged below the coolant level. Have somebody rev the truck up while your looking.