2025 GMC Sierra 2500 6.6L Duramax Transmission Leak

Jrod.Milton · October 22

Shortly after returning from a 450 mile (round trip) hunting excursion I took with the truck last week, I noted that the truck was apparently leaving half dollar sized oil spots on my clean driveway wherever it was parked. Have owned this 2025 2500 for less than one month and at 1,180 miles I crawled underneath to see if I could isolate the leak that was staining my otherwise pristine driveway. Indeed, I noted transmission fluid seepage on the driver side rear corner of the transmission pan. I drove it to the GM dealer and the service rep crawled under the truck and also noted fluid around the pan; worse yet, he said he thought he could feel fluid on the top front side of the pan which suggests bigger problems with an eye toward the transmission (seals, whatnot). The truck has been with the dealer for several days and I'm still waiting to receive diagnostics report and a repair recommendation. I have read other relatively current posts here from other model year 2024 GM 2500 owners with fluid leaking from the 10-speed Allison Transmission at very low mileage and this has me a tad worried and annoyed.

Needless to say, this is a huge disappointment in GM quality assurance, not to mention a colossal inconvenience because I cannot otherwise use my new truck. After all, who expects their brand new $90K+ truck to have major mechanical issues? I'll provide an update whenever I hear back from the dealer.

GMCustomerService · October 22

4 minutes ago, Jrod.Milton said:

Shortly after returning from a 450 mile (round trip) hunting excursion I took with the truck last week, I noted that the truck was apparently leaving half dollar sized oil spots on my clean driveway wherever it was parked. Have owned this 2025 2500 for less than one month and at 1,180 miles I crawled underneath to see if I could isolate the leak that was staining my otherwise pristine driveway. Indeed, I noted transmission fluid seepage on the driver side rear corner of the transmission pan. I drove it to the GM dealer and the service rep crawled under the truck and also noted fluid around the pan; worse yet, he said he thought he could feel fluid on the top front side of the pan which suggests bigger problems with an eye toward the transmission (seals, whatnot). The truck has been with the dealer for several days and I'm still waiting to receive diagnostics report and a repair recommendation. I have read other relatively current posts here from other model year 2024 GM 2500 owners with fluid leaking from the 10-speed Allison Transmission at very low mileage and this has me a tad worried and annoyed.

Needless to say, this is a huge disappointment in GM quality assurance, not to mention a colossal inconvenience because I cannot otherwise use my new truck. After all, who expects their brand new $90K+ truck to have major mechanical issues? I'll provide an update whenever I hear back from the dealer.

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Jrod.Milton · October 22

Just heard back from the dealer service rep on the tranny leak. Evidently, the GALLEY PLUGS are leaking at the rear of the Allison Transmission. They are awaiting parts and will remove and reinstall the OEM factory plugs to hopefully solve the problem in a couple of days. Was told it is a fairly non-invasive repair and as long as it is indeed the cause of the problem AND it can be successfully mitgated, I'll be glad to have this rig back on the road again. I realize S(tuff) happens, but I'm increasingly disappointed in GM quality assurance as a result of this. I've owned nearly a dozen GM trucks in the prior 30 year period and never had (or heard of) issues like this occurring (from 5 seconds after I owned the truck). Get your S(tuff) together GM team!!

newdude · October 22

7 minutes ago, Jrod.Milton said:

I realize S(tuff) happens, but I'm increasingly disappointed in GM quality assurance as a result of this. I've owned nearly a dozen GM trucks in the prior 30 year period and never had (or heard of) issues like this occurring (from 5 seconds after I owned the truck). Get your S(tuff) together GM team!!

Sadly its all brands right now, not just GM.

64BAwagon · October 23

Starting to sound like Toledo propulsion is having issues. The latest bulletin about the filter change for debris has me a little nervous. I checked with my contact at the dealer and he said they didnt have any further info on it and that unless my VIN showed an open status on that bulletin they couldnt do mine. I havent been able to find any info on number of affected units other than the bulleting saying it was a phased rollout. I heard an unsubstantiated rumor that it was related to the machining finish on one of the drums shedding material but I dont have any info on that. Just guesswork at this point but any abnormal material in a trans is definitely not a longevity enhancement.

Chevyfan3 · October 29

This 10spd is starting to be very disappointing and I have my doubts about it's longevity. My 23' had a leak as well and the trans had to be pulled to fix it, sad to see this is still occuring in the 25' model year.

Jrod.Milton · November 2

Well, not good news on the saga of this repair. After more than a week on the rack, it turned out it was not the galley plugs. The tranny service tech at the GM dealer replaced them, test drove it, reinspected and there was still transmission fluid seeping out at the rear of the tranny. He did some digging and found a service bulletin issued in 2024 and determined it was time to separate the transfer case from the transmission and have a look. Upon doing so, he determined the seal between the transfer case and the transmission was the culprit. So, this was a tad more invasive. Upon paying a visit to the shop, I talked with the transmission tech directly and he seemed to be competent. Two more days to wait for the required parts to correct this situation. Get this...he said he was working on another 2025 duramax/10sp allison tranny in the shop that was doing the exact same thing. Not good…my bet is that GM will receive more case reports on this and they have a bigger problem stemming from some robotic function that was not set right at the factory at the time these things were built. I learned that this single seal was a fairly new application -- maybe 2024, or maybe starting in 2025.

The wet photo below is from my truck. One seal does two jobs — keeps transfer case fluid where it is supposed to be (inside the transfer case) and transmission fluid in the transmission case. I truly am not happy about this at all -- it is disconcerting, like having a newborn that needs to have open heart surgery!

I did get the truck back and saw photos of the repair (clean as a whistle as you'll see below). It was not leaking after a substantial test drive post-repair, so at least that's good. For now, I'm satisfied with the work done by Bozarth Chevrolet in Lone Tree, Colorado. As for GM, I'm still disgusted with the quality assurance on this one, given my history with them and their solid reputation.

Chevyfan3 · November 2

Glad you got it fixed and sounds like you had a good tech which is half the battle sometimes. Looking at the picture in your thread I had to reread it to realize that wasn't your photo, so maybe they have fixed the front internal cover leak now on these. I have seen the t case leak on the forums, I wonder if it's a bad install like they claimed on the front cover seal as well.