Coolant disappearing...but where is it going?

[Capt. Jim]

My truck keeps loosing coolant but I can't determine where. During highway speeds, the low coolant warning pops up about every 90-100 miles. Coolant temp does not fluxuate or overheat. Resevoir tank does indeed go down, so it is not a faulty sensor. I pressure tested the coolant circuit, 15 psi, and it did not lose pressure. There was no pressure variations after starting it up with the system pressurized. Oil was clean and engine runs normal at speeds up to 85 mph. Truck is a 2004 LLy with 185k on the clock. There are no coolant trails or drips on the engine. Any ideas?

 

[Dkatubi06]

Gonna have to look real hard for a coolant leak. It might be small enough that the coolant itself evaporates before it has a chance to leave any evidence. Also might want to look around the passenger wheel well liner it make be coming from the overflow. If you put a pressure tester on the bottle no pressure will come from said overflow. I should also mention I had a similar issue but it mysteriously cleared itself up

 

[Capt. Jim]

I recently replaced my reservoir tank. Also thought about the overflow. May swap the old one back in and see if problem goes away. The overflow was the only component that the pressure test did not test.

 

[Kuzz]

I had the same exact problem. Getting the truck back monday with new head gaskets, heads, and arp studs not saying thats what it is i would check into it more but sounds like heads to me

 

[NC-smokinlmm]

It's an LLY... Do the HG's and be done with it...

 

[Capt. Jim]

Just did my return trip of 700 miles and truck did not lose any coolant. Glad it self-corrected but wish I knew what was happening.