Duramax Diesels Forum Truck of the Week
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Old 07-10-2019, 01:58 PM
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Default mysterious coolant system issue

Truck is a 2012 LML with an H&S tuner, head studs, and a full delete kit. Has a new surge tank and cap. Water pump and thermostats changed when studs were done.

The truck will get up to operating temp and some of the coolant will flow into the fender side of the surge tank. After the truck has been sitting overnight the coolant level will be low enough to give the low coolant message. The coolant on the fender side is still there and not siphoning over to the pressure side when cooling. This happens every day the truck is used. The fender side of the coolant tank continues to fill a little each day and never makes its way back into the pressure side.

We put a pressure tester on the system for an hour @ 20lbs and did not find a leak. I am going to put dye in the system and let him drive it for a day and check for a leak.

I am assuming a leak somewhere is causing this issue, is my understanding of how this coolant system works correct? Thanks.
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Old 07-10-2019, 02:13 PM
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If the cooling system isn't drawing a vacuum when cold (collapsing the upper radiator hose), then it is pulling in air somewhere as it cools. The theory is the only place to let fluid in/out of the system is the expansion tank, which captures excess coolant as it expands, then pulls it back in as it cools. If there is some other way for air to get in, it will pull air in from that leak instead of lifting the coolant back in, as moving the air is easier.

Since the system holds pressure in a pressure test, that implies there are no leaks, so I would be suspicious of the surge tank and cap. Just because it is new doesn't mean it isn't bad.
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Old 07-10-2019, 03:15 PM
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Thanks for the info, I am going to clamp the lower surge tank hose and put a vacuum through the upper hose using my mityvac (with the cap on) and see if I can get to pull over.
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Old 07-11-2019, 06:34 AM
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My guess is head gaskets.
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Old 07-11-2019, 08:55 AM
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Thought about that also, but I want to eliminate everything else before I recommend head gaskets...
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Old 07-11-2019, 09:31 AM
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I had an '11 doing the same thing, and it was head gaskets. That's why I said that. The guy had even put a new coolant tank and cap from GM on it before he brought it to me because that's what he thought the problem was.
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Old 07-11-2019, 10:26 AM
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Would the system build enough pressure to make the radiator hose rock hard? The reason I ask is that I went out and flogged this truck to try and get it to build pressure and nothing.

After he brings the truck back and if the coolant system checks out I'll pull the heads and have them pressure checked... I've seen cracked heads before on 3 Duramax trucks and they have all been on the coolant/intake side causing similar issues, just never an LML that already has studs with only 115k. When I figure it out I'll update. Thanks for the input.
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Old 07-16-2019, 06:59 AM
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Quote:
Originally Posted by JoshH View Post
I had an '11 doing the same thing, and it was head gaskets. That's why I said that. The guy had even put a new coolant tank and cap from GM on it before he brought it to me because that's what he thought the problem was.
Same thing as my '12, ended up being HG's. Used Arp's for the repair.
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Old 07-23-2019, 05:23 PM
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Finally got the truck back, put pressure on the system overnight and no leaks, held 21psi. So now Iím thinking itís going to need head gaskets...again. Then I noticed the new cap was a MotoRad not GM. Put a new GM cap on and all is good! Coolant siphoned over like it should overnight. Here is a pic after sitting overnight. Coolant has dye in it.


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Old 07-24-2019, 08:07 AM
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So I went and drove the truck this morning and it has a radiator leak. Have no clue why it didnít show up when I pressure tested it overnight...


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Old 07-24-2019, 08:41 AM
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Huh, that'll do it. At least it isn't head gaskets!
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Old 07-24-2019, 08:55 AM
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Quote:
Originally Posted by DAVe3283 View Post
Huh, that'll do it. At least it isn't head gaskets!
Agreed!

Still odd it would pressure test fine though.
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Old 07-24-2019, 09:50 AM
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make sure the fan cage sealing flaps didnt fold over. Dustin found alot of 11-12 trucks with the flap folded wrong. it rubs on the radiator and causes them to leak
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