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Discussion Starter #1
I have 2015 lml duramax with 99k on it. I am loosing coolant. I have filled it cold and have also filled it per the gm spec and ran it with no cap to "burp" the system. I bought it from a small dealer and I haven't gone more than 2 hrs run time without the low coolant light coming on. I love the truck. What do I do to fix it or were do I start
 

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On my L5p they did the heater plug recall and didn’t fill the coolant to the height it was supposed to be at . On my truck it’s supposed to be right to the neck of the fill hole. It took about 3-4 fill ups to stay at that level . I still think I have to add a splash.

The low coolant light is pretty high on the tank


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Discussion Starter #3
I filled about 4 times now and when I got back in the truck to start it now it shows low coolant and a check engine light🤷‍♂️
 

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Read the code and let us know what it is. Our trucks are pretty smart, although they don't always speak our language.
 

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Discussion Starter #5
I didn't even get to get it to the dealer to read the code. I tried to start it and it seems to be hydro locked fml
 

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My mistake they way it sounded like it was trying to turn over i thought it was hydro locked. I never had a starter act like that. The dealer is a
Saying it has a bad coolant sensor and a bad starter.tye starter is still under the GM warranty too
 

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So is your truck ACTUALLY loosing coolant or not? Neither a bad coolant sensor nor a bad starter will fix ACTUALLY loosing coolant.
 

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Discussion Starter #9
See i am not sure this dual stage coolant tank had me confused. I would fill it then run it and it would fill the surge side. Then the fill side would be 1/2 way and the truck would say low coolant. But i am being told the truck should only say low coolant if the tank is empty in both side. Does that sound right?
 

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If you are having to add coolant then there is a hose leak, a failing water pump, or a failed head gasket. This is where mechanics that rely on computer codes are worthless at diagnosing the problem. Often an independent garage mechanic is better at troubleshooting.
 

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Discussion Starter #11
I was told that the system was pressure tested and there is no leakes. When I filled the over flow tank it would just move to the surge side and the show low coolant on the dash. But right now they are doing it under warranty so I will try it and let you know how I make out.
 

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Discussion Starter #12
I very much agree with you. So I got it back to the dealer. I made it about 20 min down the road and the low coolant light cam back on. The surge side was full. This is what the sensor is saying low coolant. But I should have too a 2nd picture because when I got home I let it run in the driveway for about an hour and then both side were filled to the top. So i took it for a drive. Then got a low coolant light again when I was driving. So I came back home and parked it. With the truck off and cooling the tank on both sides filled back up to the point it started leaking on to the ground. Could be a bad water pump not moving the coolant so it keeps surging?
1078627
 

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Have you guys tried to test the coolant tank reservoir cap? Sometimes they go bad. I had the same problem where coolant tank would overfill the reservoir then dump out the drain tube. The pressure spring in the cap can fail against the water pressure from the pump at operating temperature. I replaced mine. Seemed to work. At least for me. 🤷🏼‍♂️
 

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That looks and sounds almost exactly like what I was experiencing earlier this year. My head gasket was blown.
When you let it sit overnight, is the upper radiator hose hard?
Also check the inside of the fender right next to the reservoir for any hint of coolant or put a towel or rag there and take it for a drive and see if turned dexcool color.
Technically it could also be the radiator cap. It is easy to check if the radiator cap is the issue by pressure testing the cap.
 

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Discussion Starter #15
Have you guys tried to test the coolant tank reservoir cap? Sometimes they go bad. I had the same problem where coolant tank would overfill the reservoir then dump out the drain tube. The pressure spring in the cap can fail against the water pressure from the pump at operating temperature. I replaced mine. Seemed to work. At least for me. 🤷🏼‍♂️
The garage put a whole new tank and cap in. It's still having a low coolant light. But I can't add any because as soon as I open the cap the coolant comes rushing back into the tank and it shows full again
 

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Mine did that as well its more than likely the head gasket
 

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Mine did that as well its more than likely the head gasket
So today the dealer did some kind of chemical test in the coolant to test for hydrocarbons. This is the story I got to prove that it is a bad head gasket. So Monday it goes in for that warranty work.
 

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So today the dealer did some kind of chemical test in the coolant to test for hydrocarbons. This is the story I got to prove that it is a bad head gasket. So Monday it goes in for that warranty work.
That is a legitimate test. I've used it many times with excellent results.
 

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I have 2015 lml duramax with 99k on it. I am loosing coolant. I have filled it cold and have also filled it per the gm spec and ran it with no cap to "burp" the system. I bought it from a small dealer and I haven't gone more than 2 hrs run time without the low coolant light coming on. I love the truck. What do I do to fix it or were do I start
 
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