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2015 sierra 6.6 spitting coolant from reservoir valve

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Hi everybody new to the forum so hoping you guys can help. Ive got a 2015 sierra 2500 6.6 fully deleted with 203k miles thats having a terrible time spitting coolant from the reservoir valve. I get a low coolant warning at least 5 times a day and even after i took it to the dealer and had it professionally done twice. The system is run on a 50/50 mix water to antifreeze and has never overheated causing me to think thats why its spraying out. Have used temp gun to verify this to make sure dash cluster isn’t faulty. This problem has been going on roughly 6 months but its my work truck so finding time to get it to the shop is hard but Im going to list some symptoms its showing below and I’m hoping someone can keep me from pulling my hair out. 

Things repaired/done

Flushed & Burped system

New reservoir cap 

New Thermostat

Blown out radiator

New fan clutch

Pressure tested at 18 lbs



decent leak from pressure relief valve in 


Only leaks when towing under load (roughly 10k lbs)



no coolant and oil mix

no extreme pressure in radiator hose when cooled completely

Edited 17 minutes ago by zach sotz
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