Hey all. I've read a lot of threads on this topic, most of which don't end with a conclusive answer. I'm already pretty sure this is a bad head gasket but I wanted to get some input.
2011 LML, just hit 100k miles. 30hp tow tune, I don't beat on it. EGR is intact but unplugged.
I performed a coolant flush about 4K miles ago & replaced thermostats (not upside down). I've had to fill it up a few times since then, assuming I passed some air through. However, recently I have been getting "Low Coolant" on the DIC. I look and sure enough the level is way below the seam. After sitting all night the upper rad hose is firm, not tennis ball hard but not soft either. I open the cap, pressure rushes out and the coolant rises back to the MAX line on tank. I don't think I'm losing coolant, the levels return to the same once I let all this air out.
I had the system vac flushed in hopes there's just some air pockets stuck somewhere; no change. Combustion fluid test was negative, although now I understand that test isn't accurate for the Duramax. Anytime I drive the truck, coolant level goes down past the seam. Like excess pressure is pushing the level down. I can open the cap, hot or cold, and the level will rise to the max line.
No visible bubbles in surge tank. Replaced surge tank cap. Not puking coolant, no drips on the ground nothing. Truck runs great, has good heat.
So what's the deal with bad HGs on the LML trucks? Some say they aren't prone to failure and others say they have seen a bunch of bad LML head gaskets. Is this something trapped air would cause? Or a bad surge tank? Just want to rule out the small stuff before tearing into this thing