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Discussion Starter #1
Hello everyone, first post here.

I own a 2015.5 LML Denali w/ about 61k on the clock, recently the truck was fully deleted including EGR/cp3 swap/ezlynk etc. Shortly after driving the truck home i received a low coolant light and was told to expect this potentially as it is a common occurrence. I was then told to just top off the coolant and return a call if further assistance was required from the shop.

Now, fast forward 1.5 months later my coolant light came up the other day, I then followed the owners manual procedure for filling/purging the cooling system of air. After completing this procedure to no surprise i was still losing minor amounts of coolant with no signs of leakage from hoses/fittings or anything visible on the ground.

I do not believe the head gaskets to be an issue, as I have a non-stiff upper rad hose condition the following morning post driving the prior night, as well as no remnant pressure in the reservoir. Coolant has started pushing into the overflow side of the tank and is also appearing on the top of the reservoir as of now immediately after driving.

So my question is, could the potential culprit be the radiator cap or the reservoir tank itself? My ECT runs around 176-184 on the CTS2 as well, so i'm not entering an overheated state, and like i mentioned no visible coolant on the ground either.

Thanks for any and all replies, and my apologies for the long entry post. Thanks fellas!
 

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Discussion Starter #3
Just incase anyone was wondering, if you are mysteriously losing coolant and you try the air purge procedure from GM- the issue is more than likely the Reservoir cap, don't buy anything other than OEM because one from advanced auto did not fix my issue but an ACDelco did.

I am now no longer pushing coolant into the overflow/losing coolant and it is holding pressure fine. Also if your upper rad hose is soft/no pressure in the tank in the AM it is not HG's in my circumstance Just FYI!
 
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