Behind Bars

I've been running it with the A/C off since and have not had any issue of fogging since so I assume the condensate was the problem. When I get a chance I'll climb around under the hood and find the condensate drain myself and see what's going on. Maybe I'll go ahead and replace the fan clutch myself as well as I'm sick of listening to the engine fan run everywhere I go. I think if I have to wait for the warranty to perform I'll be waiting a long time.

The windows still fog. I drove the truck about an hour without trouble, stopped to fill up the sleds with the truck at idle for about 10 min, then hit the road and the windows fogged immediately. The interior was completely dry and all gear/clothes were dry. It fogged up and stayed fogged at the side windows and perimeter of the windshield despite running the defrost on max for 30 min straight, with max fan speed. Finally I put the windows down partially and that helped some. However, at 16 deg outside temp that get tiresome as does driving with the fan on hi constantly. Driving back I turned off the AC while running the defrost and it never fogged up again. I drove up and back again with the same results. My guess is that moisture is building up in the system from condensate perhaps from the heat exchanger on the a/c and being introduced into the hvac system. Maybe it drains but freezes in cold temp, though this problem began during 50 deg temps.

The dealer had my truck for over 3 weeks. They say they located and fixed the issue. Apparently GM required them to tape over the ducts/vents throughout the truck and pressure test the cab for leaks, something they seemed very reluctant to perform. On doing the test they found that some cab vents located under the back seat were mis-aligned and so blocked, I guess from the factory. I understand they are to let air out of the cab, I suppose to have new air in you must let air out. According to my service shop, GM is aware of some issue that is not exclusive to my truck. The engine cooling fan still ran for a short time during really cold temps just after entering the freeway but came back off again and then cycled briefly again in an hour highway drive while empty. But my windows have yet to fog up again. Newdude, can you describe where I can find the hvac evaporator drain or tricks to keep it clean? Sorry, I've not looked nor do I think I've had a vehicle before with a drain location. The sequence you describe makes perfect sense as to what I've seen. As well, could ice form there and block it temporarily only to melt later, causing confusion? Also, there never was coolant smell in the cab, which I told them, but the service rep still claimed that the heater core was replaced and that the old one was leaking. I've constantly been changing the temp in this truck. In the 4 previous duramax trucks I've owned it's been set-and-forget but not this truck. The way you describe it fogging, clearing, and fogging again sounds just like what I have encountered. I don't know anything about the vents under the back seat but moisture build-up in the heater box, as described above, certainly would account for this issue.

Yes, normal or even low. I made a video I provided to the dealer when I could hear it engaging while the truck was still coming up to temp, maybe 180 deg. So I don't think the engine is in danger of over-heating, I think it just has a bad fan clutch. It is probably unrelated to the hvac issues.

I did not confirm that the compressor was running. I was not clear but the fan clutch was engaging before the heater core was replaced--I noticed it a couple of times. I do not believe the fan clutch has any electronics--at least the service advisor said as much. I know the fan clutches I've worked on simply had a spring in them that caused them to engage when hot. I do not know if they fan clutch has any relation at all to the hvac issues but it seems suspicious. It is a strange bunch of issues and I feel like the right mechanic, someone familiar with the hvac system and with problem-solving skills, would know right away what might be causing this. At this point I was hoping that this issue might have occurred elsewhere and been resolved. Thanks for your help. I'm growing concerned that it could be a bad computer component/software that is causing the trouble.

I wonder if anyone else is having these issues. Since new this truck has some gremlin in the hvac. The fan will run full blast for an entire drive, sometimes with outside temps at/near parity with int thermostat setting. After a few minutes you sort of get used to the jet-blast sound until the phone picks up or I shut off the truck, then the silence is deafening. When I turn off the hvac the truck is so quiet I can't believe it! Also, at one point during the heat of the summer the a/c quit working entirely--the dealer said it was over-charged and I never had a problem again after they serviced it. Then when towing this fall my truck entered the next faze--window fogging. I was towing a heavy trailer on a 50 deg dry day when suddenly the windows fogged up. Completely. It probably took 20-30 seconds from clear to entirely fogged over. The truck wasn't hot and there was no smell of antifreeze. After getting it to clear (defrost on hi, windows down), it was touch-and-go from there. If I turned off defrost it fogged over, at least while towing. Since then I haven't towed but have had several more fog-over incidents and have had to drive with the defrost on hi for an hour or more while the edges and side windows remained foggy the entire time. I took it to the dealer who diagnosed it with a bad heater core. It didn't make sense entirely to me but there is probably much I don't understand on these newer trucks. They said they had two more (gas engine) coming in with bad heater cores as well. They had it for 4 days to change the heater core. On pick-up I asked the service adviser if the old heater core was leaking. She acknowledged it was in an off-hand way, so I re-iterated the question and was assured that the old core was leaking. Next day after hunting I drove an hour back with the windows fogged up. Now they've had it for over a week and no news yet. Anyone else experiencing hvac issues like this? There is no smell of antifreeze. The way it initially fogged during a dry day makes me think moisture is being introduced, like condensate is backing up into the system. Here's another twist: the engine cooling fan has started coming on at weird times. It kicked on while coming down a pass in 35 deg temps, empty, going 40-50 mph. It came on the other morning before the truck was even up to temp. I can't figure out how they could be related but it seems peculiar that both of these issues are occurring. There was also a buzzing/vibration at idle ONE time and I discussed this with the service dept--they said there was a service bulletin for a fuel line rubbing on the firewall and that they would fix that.

While in for other issues I complained about my radio function and they did an update about 2 weeks ago. Prior to the update my radio would turn on but be at zero volume. It would also automatically connect to my phone and auto-play whatever book or podcast I may have been listening to. It wasn't earth-shattering but it was annoying. Fixed that.