C15 head gasket issues

If it's not running hot and you replaced cap I would be looking at the head gasket on the air compressor.

 

So when they saw your tail lights your warranty was up?

 

Does your truck have any air controled heater heater valves?

 

You can remove the discharge on the compressor and run truck high idle and see if the overflow slows down. Drain air tanks also.

 

Just because your engine is modded/not stock DOES NOT void cat OPT warranty. If it is determined that the extra horsepower caused the fail, no warranty. However if it is just a parts failure not due to excessive power, all warranty. We once got warranty on a deleted SDP that chucked a rod. Cat knew it was deleted and everything. Find a different dealer that isnt a dick

 

You mean the same thing as feed line? Or I'm dumb and you mean compressor exhaust port? I've never tried that one yet.

 

Not the intake line from head but the output. You are basically taking the load off the compressor it will still pump air but no pressure to the system. Could be your problem and might not but it's an easy and free test anyone can do themselves. That's also why you drain air tanks in case an air operated heater switch is bad it will eliminate that. I don't believe your model has those types switches but once again it's free and easy.