Can you see diesel in the degas bottle? If so its more than likely injector cups.
I just had this problem 2 months ago. the injector cups (not the injector itself)can develop hairline cracks allowing diesel into the coolant which ends up in the degas bottle, if it leaking bad enough to cause it to over fill, you have a big leak.
best way to find the cracked one(s) it to pull the injectors, get a lint free rag and wipe the injector bores clean, then and pressurize the cooling system. The one(s) that are cracked or leaking will have coolant down in the injector bore, the rest should be dry.
There are special tools required to remove and install an new cup. A large tap threads down into the cup and the tap itself is threaded down the center to accept the threads of a slide hammer. There is also a special driver that resembles an injector body to drive in the new injector cup.
I did not have those tools and had a hard time finding them around town, so in order to prevent me from doing damage to my own truck I had my truck towed to an injection shop to have the old cup removed and a new one installed.
Here is a write up on how to R&R injectors. I recommend you have a qualified shop do the actual injector cup replacement, everything else can be done in your driveway.
You will also need to change your oil before you fire the truck back up(after you get the injector cup(s) replaced), and do a coolant flush.
Last edited by POWERSTROKER1 : 05-27-2008 at 02:00 PM.