At the start of this story, I had 2012 6.7, ATS twin-turbo kit, head studs, banks monster ram, deleted exhaust, EGR mounted but blocked off at the outlet, tuned.
About a month ago noticed my thermostat sticking in the morning or going home from work after the truck has 8+ hours to cool down (no heat in the cab either). The alarm would go off at 220, and I would pull over and shut it down. After a few minutes, I started it back up, and the thermostat opened, heat in the cab and coolant temp dropping to normal. Had this happened two times before I had an opportunity to swap in a new thermostat? Installed a Gates 192 degree thermostat from Geno's garage (my original 200 deg. thermostat was installed prior). I took a 2500 mile trip to Montana from California, pulling a 10k trailer during Christmas. Hence, it experienced warm and cold outside temps, worked perfectly, not one issue with the cooling system.
Fast forward a month, and I started experiencing the same thermostat sticking issue that I noticed before after 8hrs of sitting. I figured my aftermarket thermostat was garbage, so I picked up a POS NAPA brand thermostat to test out. The thermostat appeared to function fine, so I drove it home from my shop (10 miles). I parked for two days and had it stick again on the way to work. Hit 240, shut it down, heard the overflow bubble, started it up, thermostat opened, and I had heat in the cab again. Bled and bled and bled, I eventually noticed if I put a very slight amount of pressure on the radiator cap, I would hear bubbles passing by. I replaced the radiator cap with a POS Napa, experienced the same thing if I put some pressure on it (crappy fit). I ordered a factory cap from the dealership.
At this time, I removed my blocked-off EGR, installed block-off plates, and installed a Pusher Intake coolant riser reroute kit. Thoroughly drained the cooling system, used an Airlift vacuum radiator filling tool to test and fill. Vacuumed the cooling system for 20+ minutes holding 25 inches of vacuum. Used the Airlift to fill the system two times. I started it up, was still experiencing a thermostat that wouldn't open.
At this point, I purchased a no-spill funnel filler to install on the radiator neck to allow any air to escape. I performed three cycles of running the engine long enough to pop the thermostat. Shut it down, let it cool for 20+ minutes, remove the funnel, and install the new radiator cap. Let the truck sit for a few minutes while cleaning up, and noticed the coolant temp was stuck around 150-160, no heat in the cab again. I took it for a quick drive and saw fluctuating temps as the cooling system had an obvious air bubble.
I'm now at a loss—no sign of oil in the coolant or coolant in the oil.
I noticed no constant stream of tiny air bubbles as the engine came up to temp; I would only see bubbles after I ran the engine at 2k for about 45 seconds at a time and came to an idle.
I noticed my upper radiator hose is slightly saggy, so I would caress the hose and work air bubbles out into the funnel.
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