2016 F-450 Secondary Cooling System Overflow

[Fordracer]

I'm working on a 2016 F-450 6.7 with 190,000 miles that is blowing coolant out the overflow hose while towing. He is a hot shot driver and has a 48 foot goose neck and he keeps the weight within the legal limits so he is not overloading it and it just started doing it. The only time he drives it without the trailer is when he brings it to me to work on it so we don't know if it will do it empty. There are no codes in the PCM to indicate the CAC, trans, or fuel are getting hot. I drove it 20 miles and the coolant temp never got above 130 degrees and didn't blow any coolant out. I put a new cap on it and it still does it. I can command the fan on and it works, the fins in both radiators are clean. The only thing I can think of is to replace the thermostats. I've never seen any problems with the secondary system except for a couple radiator leaks. Has anybody seen this happening?

[forddieseldoctor]

What about the possibility of a leaking cac? Boost leaking into the cooling system. 

[Fordracer]

I haven't thought about that. Very possible. I guess I could try to pull the cac tubes and pull a vacuum on it.

[JoeR]

Or put some boost in it.  Make or buy a set of adapters and use an air pressure regulator to pressurize it.  If you want an official way to do it, get a high pressure smoke machine.  They will even tell you if you have a leak or not.  

[Keith Browning]

I have replaced CAC's for coolant loss so it stands to reason boost could be entering the system.

[cbriggs]

I had one doing exactly as you describe. Bring it up to high boost and it would blow coolant out of the secondary cap. It was mysteriously loosing coolant out of the secondary side for a while, then it progressed to that. New cac fixed it.

[Fordracer]

I finally got the customer to bring his truck in for the CAC cooler replacement and that fixed it. He made a 2,500 mile run with 15,000 pounds and no more coolant loss. Thanks for the help.