So I check my coolant today when I get home and decide to top off the Degas bottle. I usually only do this when the engine is cold, but it was hot today. When I crack the cap and let the pressure release (slowly) I notice dripping under the truck. Put the cap back on and the dripping slows to a stop. Take the cap off again the drip continues. Put the cap back on it stops again. Not sure this has been a problem before as I have never had any issues with coolant loss. No white residue, no exhaust colors, 4 degree max deltas, etc. My truck runs really well.

The leak appears to be on the back of the motor on the driver's side. I see no source for it but it is dripping off the junction of the tranny and block. There is an aluminum projection there that is a low point. Checked the underside of the Degas -- no leaks. Wiggled hoses in that area to no avail.

So why would it leak without any pressure but never leak in my garage under normal pressurized conditions? Is there some sort of valve back there that would allow coolant out but not when under pressure? I drove it later and there are no problems...

 

Heater hose rubbed through where it contacts the lower rear corner of the FICM?

 

Long time no see.
I was thinking the same.
If you find it to be something else if I were you I would still protect that hose with a short section
of some plastic convolute tubing. Auto parts store will have some and also some zip ties to keep
it where you want it to stay. Both of my trucks have some on that hose.

 

I had a 2005 F350 5.4 with the same problem,it turned out when I was trying to install a #4coilpack I accidently moved a rusted tube almost the same diameter as the Trans tube.Turns out that was a one piece tube that ran from the heater core under the manifold to the back of the water pump into a nipple with two o rings.I had to teplace the cooling tube and the water pump.Took me about 5 hours,yikes.Good luck.

 

Heater hose at FICM is defeinietly a possibility. I put a big zip tie around mine to keep it out of the way.
The other thing is your blue hose at the EGR cooler, if you have one. I chased a leak down for a while on my '05, finally put some dye in it and found it leaking under the egr cooler. You can't see it from the top since the valley is covered on the passenger side of the engine. It would drain down, get on the starter and look like a leak at the back of the engine. Sometimes it would be on the left, sometimes right. It was maddening, to say the least. Only took an hour to change the hose.

 

Why would the leak only occur when the cap is off with no pressure?

 

The exact same reason why coolant would exit the radiator drain petcock at a much quicker rate when the degas bottle cap is removed, than it would if the cap were installed.