Hi Everyone,

For some background, this is on my 2001 Monte Carlo SS which has around 240k miles on the clock. Engine and transmission are original.

This summer I noticed that my Monte was a little low on transmission fluid, so I topped it off and just kept an eye on the level. The car leaks a little oil and I was parking it on gravel so it was hard to notice but transmission fluid was also leaking. I'm not worried about the oil, it's been dropping a little bit for the last ~10 years, it needs a new pan gasket and I just don't have the time/resources to get it done right now. After putting something under the car to catch the fluid, it was pretty clear that the car is leaking more trans fluid than oil (several drops of trans fluid every time I park the car vs. one drop of oil every once and a while. I got under the car and it looked like the transmission pan was damp along the edges, so I decided I'd replace the gasket and change the filter too since it was due for that. After a new pan gasket, it was still leaking, so I did some reading and learned that the original gasket is reusable and the replacements are junk. I threw the original out, so I bought an OEM-spec (metal+rubber) ACDelco replacement gasket and a new OEM pan too. After installing these (and a second round of new fluid in the trans), guess what? It still leaks!

Now I'm convinced that the pan wasn't actually leaking, and I was just seeing trans fluid on the pan that was coming from elsewhere. The problem is that I can't figure out where it's coming from. I replaced the cooler lines when I did the filter, and they're bone dry. The fluid "drips" from the passenger side of the trans, close to the output shaft. The drivers side of the transmission is bone dry. Looking at the passenger side from the wheelwell, the diff cover looks wet which makes me think it might be the axle seal, but when I feel where the axle meets the transmission, it's dry. The subframe right below the axle seal is also pretty dry. I also can't get any fluid to come out of the seal by rotating the axle. I've attached a picture and circled where the trans looks the "wettest" but I just don't know where it's coming from. Is it possible for a transmission this old to develop a small crack that could seep fluid? I've done a lot of cleaning and searching since I first took this picture and I still haven't found anything. I might just replace the axle seal because it's a cheap part, but it's also not a quick job and it's been pretty cold out recently. It's also worth mentioning that this whole time the car runs fine and the trans shifts with no issues, I don't think the fluid loss was ever bad enough to cause any real problems but I'd be really happy if I didn't have to top it off every few weeks.

 

My guess is from the CV axle shaft. There is a seal in there which may go bad especially after many years. Also if you ever replaced the passenger CV axle NONE of the aftermarket ones are a good fit. All leak. Ask me how I know, I tried 5 different vendors and all leaked very slowly even with new seals. The only CV axle shaft that didn't was an OEM one. You leak looks about at the right spot where mine was leaking. I deal with the leak for years and eventually went OEM and that solved it.

 

I replaced both axles about 6 years ago at 175k miles, but it didn't leak for at least 4-5 years after that. I used a reman part (forget the brand) because I didn't want to spend $300/axle for the OEM spec. I actually had new axle seals from when I did the job that I never installed, so I decided to replace the passenger side one this weekend. Unfortunately, after a couple days of sitting and drying the bottom off (I spilled a lot of fluid when I took the axle out) it's still leaking. The same amount is dripping in the same place. I did have an opportunity to clean up the trans case with some parts cleaner while I was down there and I think I know where it's actually coming from. When I was under the car, I dried things up near the diff / extension housing on the passenger side. After staring at it for a few minutes, I could start to see some dampness forming where the diff housing meets the transmission. I'm pretty sure that's the real culprit. I looked through the documentation I could find and it looks like the diff housing is removable and there's a big O-ring type seal (GM part 8651419) inside of the transmission for it. There's no gasket on the outer mating surface. It's a really cheap part to replace but it could be a pretty involved job. Now I have a question or two:

• Can the diff cover housing be removed with the transmission in the car? The oil pressure sensor looks like it might be in the way and I'd have to remove the trans speed sensor too. Is there anything else that makes it difficult to take out other than the 4 bolts holding it to the trans? Will a dozen pieces of transmission fall out when I take the cover off?
• Has anybody ever had this seal fail? I've done some searching and between here and every other forum for cars with the 4T65e, I can't find a single post about a leak here.

Here are some pics I took doing the job, you can see the fluid on the diff housing where it meets the transmission case after reassembling and drying everything up


You can see a little fluid on the diff housing right by the bottom bolt, this is after reassembly and drying everything up

New seal in place


Thanks guys!

 

So here's a very late update:

I finally figured out what this was. It wasn't the axle seal or the pan, it was the extension housing / differential cover seal! It was a pain to change but the car has been dry for a few weeks now. You have to remove the axle, get wiring out of the way for the speed sensor and oil pressure sensor, and then bend your hands in unpleasant ways to get to bolts in the engine block that attach to the passenger side transmission support. Once I had the old seal out, it was very obvious that it was worn out. The new seal was flexible rubber and the old one felt like brittle plastic. If anybody else ever has this problem, the replacement part number is GM 8651419.

I'm glad this leak is fixed, but now I'm worried about the condition of the other seals in the transmission. I guess I'll fix them as they fail...

 

Thanks for the update and the part number, glad you found it.