Cooling system seems to be getting pressurized. Need diagnosis help please
 

Bone stock 2006 6.0, 145K miles. Coolant flushed every 50K, etc, etc. It started losing a little coolant from what appeared to be the top plastic tank seal on the oem radiator, so I replaced it with a parts store radiator and put in a new OEM thermostat at the same time. Now it seems to be leaking from the bottom (radiator?) somewhere. Also appears to be pushing some coolant out the degas cap (original cap). Deltas are in spec, no overheating. Truck seems to run fine, no smoke, no loss of power, no unusual sounds. There is no pressure in the cooling system while idling or after shutdown when I loosen the degas cap.

Could be:
-Bad replacement radiator (lower tank seal?)
-Original degas cap went bad
-Headgaskets
-Waterpump leaking(???)

Any ideas? Thank you

You need to fix all the leaks first. Checking for pressure is useless with known leaks, that is why there is no detectable pressure when you loosen the degas cap. There actually should be some pressure when you get the engine hot (from liquid expansion). I would suggest:
1. Fix the radiator leak if you prove you have one (ie replace it again if it is leaking). If it isn't the radiator, then find and fix whatever it is that is leaking.
2. Replace the degas bottle cap
3. Install a pressure gauge on the degas bottle so that you can see it while driving

I assume you are using the Ford Gold coolant?

Thank you. I am using ELC coolant, Peak Final Charge to be exact. I used to wrench on race cars so this has got me stumped for sure.

So, a quick question .... why change the ELC every 50k miles?

Can you carefully pressurize the cooling system w/ some compressed air by tieing into the hose on top the radiator and see if you can spot the leak when it is under constant pressure? I would take it to 15 psig and hold it there while looking for the leak. It is supposed to handle that much, but you may need a new cap to get there.

Thank you. The first two flushes were with Ford Gold at 50K, then switched to ELC at 100K. I replaced the cap and tightened the hose clamps, no more leaking...BUT I am getting odd water temp fluctuations on the SG2. It will bounce around between 192-200° in short order, it used to be more gradual fluctuations. Deltas are less than 10°. Water pump maybe??

Impellers cracking and loosing integrity with the shaft have been known to happen. PITA with the stator in place and getting the fan off the water pump shaft, but for the price of a water pump o-ring gasket you can check if there is an issue.

I bought a new water pump w/oring from NAPA this morning. I'm going to change it out this afternoon, might as well since I have 145K on the oem pump. That means the radiator, tstat, cap, and pump would all have been replaced...

If that doesn't work then off to the stealership it goes as we are in the middle of a move and I don't have time to swap hg's myself right now.