Losing Coolant...but no symptoms where and why.

That's where I would start. Oil test will tell you a lot.

Also, check your gear box oil if you have a transmission cooler. It's possible it could be leaking.

 

Get an oil sample. If nothing obvious then run it until it becomes obvious otherwise youlyo be spending a lot on "looking around"

 

Isx and 1 million miles? Well overdue for inframe.

 

Check for air bubbles in your overflow tank, compression. If your getting compression up there and losing coolant big time like that, then maybe yes - could be head gasket or bad head

I think Napa sells a compression test, one that you can do your self. I still vote for a pressure test too. A pressure test with the oil pan off will let you look into at what cylinder it may be leaking, if that’s the case.

But Maybe it’s the egr cooler. I think you can disconnect the cooler on one of the ends while pressure testing the system to show a leak.

 

Sometimes it’s just a clamp on a hose. Had one that only leaked when warm, not hot, not cold just warm. Just happened to find it about twenty mins after starting but before going on the highway.
Was a line for the heater, just tightened the clamp.

 

You need an egr cooler, not an inframe.

 

I had a truck using coolant. Several times after I parked I would walk around the truck and smell coolant at the right front corner. After months of looking I laid on the grass and looked up at the underside of the water pump while the engine was still hot. Coolant was coming as steam. I tightened the clamp and the smell went away.

 

I can't believe I forgot about the trans cooler issue, I just had that happen 4 months ago!

Great advice

 

My truck was having the same problem it was drinking about a gallon of coolant a day it turned out to be the water pump cost me 2300 way cheaper than the 25K for a whole rebuild

07 freightliner century C15 acert 435hp