Just purchased an 03 blowing alot of smoke from the by pass tube and exhaust truck has about 300000 miles on it. Checked vacuum lines and pump have 20 hg should i be looking at rings? Took oil cap off and set it on the opening it blows off please help

 

If there is enough pressure to blow the oil fill cap of with it sitting loose, then yes, I think you will be looking at tearing it down.