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OK I’m baffled...
06 LBZ, Stock,185K miles. Got a low coolant notice on the dash, level was just below the seam on overflow tank. Let it cool for 2 - 3 hours, topped off to fill line. Drove it about 100 miles next day, no problems. Checked when I got home, coolant level was 1/2” above seam on tank. Checked it next morning, level was same, opened top of tank an a small bit of pressure bled off and level went to where I had filled it to. Drove to work the next morning then home again (about a 15 mile round trip), when I checked it again the level was at the seam. This morning I checked it, level was back to where I had filled it, when I got to work it was down to 1/2” over the seam. Drove it about 4 miles then checked, level was back up to where I had filled it.
Truck has not overheated (200 max), coolant level never below middle of over flow tank, upper radiator hose not hard when running at temp, no coolant in oil I can see, no smell.
I’m baffled, any ideas?
 

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My guess is water pump. I’ve been chasing this for awhile and finally saw a leak and it’s from the weep hole, hard to find with no huge puddles in the driveway coolants in the truck and Friday is the day. Just ticked because I just put idlers, belt tensioner, belt on last January. I should have bit the bullet then
 

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I had coolant loss that took me a bit to figure out, no puddles or drips, turned out to be the EGR cooler cracked internally. Luckily my EGR fell off, so I haven't had to worry about that since.
 

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I had a leak similar to yours, i could not figure it out for the longest time. Turned out my leak was from the heater hose connection off the firewall, it was leaking while it was running just a few drips at a time. Its about a $6 part and a good 15 minutes to change, havent had any issues again so far.
 

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Just saw a video on a leak I hadn't seen before. Folks were towing with their LBZ and the hose burst that goes to the turbo. He had something like 250k on the odometer. Just food for thought.
 

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Guess I'm not alone.

My question is, are leaks common on trucks that sit and don't get driven? Story below.

I have almost 200K miles on it. Had it in a shop last year to repair the EGR up pipe that cracked. My bud that owns the shop was supposed to have replaced my water pump at the same time he had the cab off because of a leak. A few months later another leak. Took it back in and his mechanic said it was an O-Ring on the pipe. 6 months later I still get an occasional leak. So a few days ago I top it off drove it up to temp and tried to look for a leak, and nothing. Let it set for a few days and kept a check and drive way is dry. Then I notice yesterday a wet spot after setting for 5 days. Obviously it's not a daily driver. My friend that had the shop closed it a few months back. We were talking and he said for trucks that aren't driven often, like mine, it's common for them to leak since the O-Rings and seals aren't staying lubricated. Any truth to that?

I was able to take some pictures of where I see coolant.
The weep hole (Arrow) looks dry but there is a drip on one of the bolts a couple of drips on the oil pan and a small line on the lower hose.
https://photos.app.goo.gl/ZZgGmZTEt4mnsKnJ8
 

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I think your friend closed shop because he was not doing excellent work.

I’ve replaced thermostats, water pump, o rings, and just yesterday I did the oil cooler gaskets. Things wear out. Yeah he might be right if a truck sits for a LONG time it MIGHT leak but not after stuff was replaced.

Edit: it might be the gasket on the water pump. Pressure test it and look for leaks
 

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To all the Duramax guys check the heater line connectors at firewall. Blew mine going out to Palm Springs dumped all coolant in short order and blew head gaskets
 

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I had a leak similar to yours, i could not figure it out for the longest time. Turned out my leak was from the heater hose connection off the firewall, it was leaking while it was running just a few drips at a time. Its about a $6 part and a good 15 minutes to change, havent had any issues again so far.
I just found the exact same leak in my truck today when wiring some sensors. It's on my list lol.
 

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Mine were leaking too just a few drops every hour. Replacing them is an easy and cheap fix and did it in my driveway in about 20 minutes.
 

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Found out I have blown head gaskets. Stock truck with A Kory Willis two tune. Sometimes those leaks are from other things. Yes replacing THIS wasn’t bad. It spilled some coolant and was cheap. Buy the preformed hose because it’s better than trying to make it work with bulk hose.
 

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The picture appears to be the bypass tube to oil cooler leaking.
 
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