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Discussion Starter #1
So another guy just posted a similar issue, although he had a 2015, not a 2012. I've had the truck a couple of weeks, peeked under the hood before I purchased it but did not go over it with a fine tooth comb. Saw coolant in the overflow tank and didn't think much of it. Was driving it yesterday, took the longest trip Ive driven with it at about 30 miles total and right as I was pulling on to my road I got a low coolant alarm on the DIC. Popped the hood when I got home. Pardon my ignorance, but the overflow tank looks like it has two chambers? The inner chamber looked fine, well within parameters, but then looked to be very low in the other chamber towards the outside of the truck. I expected the coolant to be hot and under pressure when I screwed the cap off, however it was ambient temperature. The truck was running at operating temp on the gauge and my mini max said 183. It was probably 45 degrees ambient yesterday afternoon. I have not noticed any leaks, although I park on gravel and a drip might not be noticeable.

I have not tried any diagnostics. I do know that the truck has a new radiator. The previous owner ended up with a hole in it (not sure how highway driving), but claims he stopped before any overheating occurred. I suppose its possible it never got filled to capacity again, or air finally bled out, but I don't have receipts on when the work was done. Drove the truck again last night and no codes or warnings in the DIC. No smoke, no power loss, no obvious issues that I can tell.

Curious of two things. First, I'm just wondering if Im paranoid since this a new truck to me and I'm new to owning diesels. Second, why wouldn't the coolant in the overflow tank be hot or at least warm if I had been driving the truck for some time?
 

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I bet some air working out or just an over time thing. I have a 12 also and had a low coolant a couple years back. Have owned the truck since new and just topped up when I got low coolant light and haven't seen anything since. Just make sure you get the eight stuff, I believe it was DeX cool stock bit just check to much sure you don't mix em as they don't all play well together.

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Discussion Starter #3
I bet some air working out or just an over time thing. I have a 12 also and had a low coolant a couple years back. Have owned the truck since new and just topped up when I got low coolant light and haven't seen anything since. Just make sure you get the eight stuff, I believe it was DeX cool stock bit just check to much sure you don't mix em as they don't all play well together.
Good to know. I think you are right about the DexCool. Going to pick up a jug of pre-mix tonight and top it off.
 

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I had disappearing coolant on my 2012 a few years back. GM installed an updated overflow tank (physically looks different) and I havent had an issue since
 

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Hmm, I wonder if they used the 2013 coolant reservoir as I believed it changed that year?
 

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Well as a minor update, I still haven't gotten to the store to pick up a jug of coolant, but have been driving the truck and there has not been a low coolant warning since. I suppose it could have been a fluke... maybe hitting a round about going faster than I should have and the level shifted just enough in the tank to throw an alarm. Still going to pick up a jug to make sure.

Still curious as to why after driving for more than 30 minutes, the coolant in the overflow tank is still ambient temp. I guess the coolant system anatomy of the duramaxes could be different than gas engines im familiar with and they don't open up to the tank unless things get really warm. My digital gauge says coolant never gets above 183*. I know GM instrument clusters aren't known for their pinpoint accuracy, but usually the needle makes it to just under the 210* and sits there. Almost a 30 degree difference.
 

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Well as a minor update, I still haven't gotten to the store to pick up a jug of coolant, but have been driving the truck and there has not been a low coolant warning since. I suppose it could have been a fluke... maybe hitting a round about going faster than I should have and the level shifted just enough in the tank to throw an alarm. Still going to pick up a jug to make sure.

Still curious as to why after driving for more than 30 minutes, the coolant in the overflow tank is still ambient temp. I guess the coolant system anatomy of the duramaxes could be different than gas engines im familiar with and they don't open up to the tank unless things get really warm. My digital gauge says coolant never gets above 183*. I know GM instrument clusters aren't known for their pinpoint accuracy, but usually the needle makes it to just under the 210* and sits there. Almost a 30 degree difference.
Maybe a bad thermostat.

Replace them both if opening it up.
 

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I just bought an lly. 2005 Chevrolet 2500hd. When it was at the used car dealership it was leaking out of the water pump. They replaced the water pump and its hose. So I bought it AS IS for tax savings and now its leaking out of the coolant overflow cap tube. It only drops about two inches in the reservoir. Its not enough to trip any alarms but I'm pretty sure its not supposed to be doing that. I've been told if it was head gaskets the upper rad hose would be rock hard when the engine has cooled and sat for a while. Its soft. I have heat and my gauges show no signs of overheating. I bought it from these Ukrainian Russian dealers in NE Philadelphia. They get their trucks totaled or damaged and swap parts from other damaged trucks. Could it be that the reservoir is larger than the original? I dont even know if that was even ever replaced. If it was the cap, would it stop dumping coolant two inches down? It seems to hold pressure in the reservoir cause when the water pump was leaking the salesman unscrewed the cap after a test drive and the coolant came shooting up and out like a volcano sending the cap across the parking lot. If the thermostats were stuck would I still have heat?
 

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UPDATE:

Topped off the tank just for good measure. It was still well within parameters. I found out from my Bro In Law that they replace these coolant sensors more often than not due to intermittent faults. Won't be worrying about it anymore, but I'll make sure I bring my jug of pre-mix on my first long haul.

He also explained that it is not uncommon for coolant in the overflow reservoir to not heat up even after extended driving, and OE thermostats are 180* which I was not aware of.
 

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My LML has a leaking block heater :(

that's where my coolant goes, very small leak, but I noticed it when I was under the truck fooling with upgraded steering parts..
 
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