I started loosing coolant sometime ago. seems that everytime they replace something it gets worst. i am up to a gallon every 150 miles. I can smell it like crazy but do not see a leek. the dealer does not get anything when they do a pressure test. i can smell it when driving around but that is about it. idle i do not smell it. Also i do not see anything in the oil. little history on the truck. at 5k miles the oil cooler broke down on the inside. all the coolant lines were replaced and the system flushed to get all the oil out.
Check the connections at the heater core, it's known to weep along the tubes. Any fog on the inside of the windshield when defrost is on ? You may have a pinhole in the heater core. GM flate rate is 5 hrs to replace it under warranty, (allow up to 8 hrs to do it yourself) depending on your interior options (console, ect.)
Mine was doing the same thing. I was not leaving any puddles of coolant anywhere and I could smell it every now and then when driving down the road. Look under your unit after you drive it. Mine was a water pump that was blowing it out the weep hole at high rpm. At an idle it would not loose anything. I never heated up (thank god) but that is what was happening with mine.
Try a search on the overheating thread. I'm pretty sure I read something about this in that section. It may be that the coolant is leaking into your into your motor oil through the headgasket?? Not sure but I believe this will not show up on a pressure test. Check your upper radaitor hose to see if it's hard after you have been driving. I remember this being one of the symptoms noted in that thread.
My LB7 (03) did it all the time I owned it, never did find the problem through 90000 miles. Dealer did all the checks including injector seals, no discovery.
I've got an 05 with 35,000 miles, I'm losing coolant about enough every 1000 miles to put the warning light on maybe 25 ounces. its not going on the ground and dealer cant find a leak. they put dye in and i'm supposed to bring it back in 3 weeks. I think its burning it through the motor via a head lek somewhere. No smells No white smoke
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