Just throwing my 2 cents out there. It's long but there are pictures! :thumbsup
Ford refers to the line coming from the egr valve, going to the degas bottle, as the "de-aeration/coolant feed". It has a one-way flow, to the degas bottle. If I remember correctly, a few of you guys have actually witnessed this line bubbling in the degas bottle.
The venturi-T is designed so that the line coming from the radiator to the degas bottle isn't back-fed this excess gas (coming from the egr valve's line), overpressurizing / inflating the radiator. So its pretty clear the pressure in the degas bottle is coming from this "deaeration" line.
If you trace that back, through the egr valve, it then goes directly to the coolant outlet on the egr coolers. If the egr coolers are basically "blowing bubbles", all of that exhaust gas will be filtered to the degas bottle through this line. It is designed this way to prevent tons of bubbles from going back into the pump.
In my opinion, this is where the overflow is coming from. As for "WHY", it probably has two different causes.
One may be that the EGR-delete tunes only close the EXIT of the egr system, allowing the coolers to remain pressurized, especially under high-boost conditions. They may not be able to handle the boost levels that the tuned trucks are seeing, especially with no relief as the "exit" (egr valve) is blocked / closed. This will inflate them beyond their critical failure pressure, and cause internal fracturing of solder points.
Second reason is because of the radiator leaks. If it is not caught early enough, the coolant level drops low enough to starve the egr coolers of coolant, overheating them, and causing major internal damage and leaking.
The difference between the two causes may be in the symptoms.
In situation-one, trucks will probably just overflow at the degas because the pressure is leaking into the coolant in one direction. A failure in the cooler that only opens / expands when the cooler is pressurized, since that is how the fracture occurred. A one-way flow of exhaust gas into the coolant.
In the overheat situation, the cooler may be completely melted or deformed inside, causing coolant to leak in EITHER direction, allowing coolant to make it into the exhaust stream, headed for the intake, and possibly exhaust gas into the coolant stream, headed for the degas bottle. This may be why some people (after an overheat) report white exhaust smoke / burning coolant.
If this proves to be correct, I'll be looking forward to a solution that involves a small delete plate at the entrance to the coolers, the closing of the EGR valve, and the bypassing of coolant flow through the coolers by connecting the inlet line going to cooler 1 with the outlet line coming from cooler 2.
It would probably still be a good idea to keep the deaeration line, to prevent the system from recirculating any bubbles that may naturally occur. That would be as simple as extending the coolant hose going to cooler 1 up to the top, to connect to the outlet coolant hose on cooler 2... and keeping it tied in with the deaeration line at the systems high-point (for trapped air to make its way out).
Here are the two images to make sense of this long-***** post! One image is the coolant flow, and the other, the exhaust gas flow, shown in yellow arrows, with EGR flow shown with white arrows.
Sorry for the length.... Did I lose anyone!?