my C5 has a little bit of leaky butt. since its been at least 30k since it was changed I want to change the diff fluid, also make sure its full!

I've never done this before although I've changed the trans fluid already, I know this is pretty similar.

the car is a 99 (I know for trans fluid selection BW vs tremec makes a difference) 6 speed with 3.42's not sure if that makes any difference or not in fluid.

I kind of like mobil/mobil1 products. is that a good brand to use? I was hoping someone would do me the favor and take the guess work out of this and tell me exactly what I need to get? if mobil's a good brand for the diff what exact product do I need? does synthetic vs conventional factor in? I'd rather go synthetic if that's compatible with my car.
I saw in another thread about some limited slip additive that sometimes needs to be added but some fluids already have it? what is this? where do I get it? and how do I know if I need it?

changing the fluid is pretty much like the trans correct? pull the drain and empty it, plug the drain and open the fill port, pump the fluid in using a hand pump on the container until it dribbles out of the fill port then plug it back in correct? anything else I need to know or worry about?

I know the car needs to be level to get the fluid level right. my garage is sloped, I'm sure it doesn't have to be absolutely perfect but I put a bubble level on the black plastic piece with the C5 logo in the door sill can I use that to tell if the car is level? if I just put it on ramps on all 4 wheels it won't be because I know my garage floor is sloped to make water run out.

thanks!

 

Not trying to be a smart a$$, but that is the beauty of reading your owners manual on "Recommended Fluids and Lubricants"...........you don't have to guess.

 

I switched to Royal Purple when I swapped my clutch. The trans became very notchy so I'm switching it back to GM recommendations. I can't tell a difference in the rear but I'm changing it back as well...I'd rather loose $130 than replace trans and rear end. Just my honest opinion.

 

quite true, however one think I've found on this forum is that if you just follow GM's recommendations on everything you will end up paying way too much for under performing products. even when my plan is to follow the service manual's procedure on something I tend to ask the forum first when its my first attempt at it, sometimes someone will chime in with something that will save me a lot of time, a lot of money, or with a recommendation which will prevent that thing from breaking again the next time. so I figure...never hurts to ask.

 

Again, not trying to give you a hard time, but this is not rocket science, and the manufacturer spends thousands of dollars testing your car with the recommended fluids.

Can you direct me to a thread where someone trashed their engine, transmission, or differential........using the recommended fluids in the owners manual?

The exception to the owners manual would be extreme conditions i.e. drag racing, auto-xing, etc. For a street car, the recommended fluids work as advertised, and are anything but "under performing".

 

I understand completely thanks for your help! that's what I needed to know, is the stuff specified in the owner's manual the best to use or after 13 years of C5's being out there has the community of owners found that something different tends to work better?

its probably safer with fluids and stuff but if you follow GMs recommendation for a dim CC display for example, they'd say replace it at high cost with another unit which is eventually going to suffer the same problem. even with the solution therein the shop manual take it to the forum and people will tell you that a little solder will fix the problem permanently and cost you nearly nothing. The dealer quoted me $600 to fix my turn signal flasher when it intermittently failed, after a quick question to on the forum I fixed it myself for $25 in under 30 minutes. that's why I ask!

 

now the limited slip additive, manual lists it as GM part 1052358 or equiv. but it doesn't list a GM spec that the equivalent needs to match. is this something that I can find at the big chain autoparts stores or do I need to go by a dealer's parts department to get 118ml of this extra fluid? if I can get it anywhere what do I look for?

 

Did you use the Royal Purple Synthetic ATF or the Syncromax? I contacted Royal Purple and while their synthetic ATF meets GM's requirements they recommend using their Syncromax fluid. So no improvement in the tranny... interesting as I was going to change my tranny fluid and use RP....

 

I have used Mobil 1 synthetic gear lube and the GM posi modifer for 4 years of street and heavy track duty without problem. Get the GM stuff from dealer it is about $8 for the bottle. Change fluid same way as you did in tranny and everything will be fine. Make sure car is level and fill till it runs out. You might get some posi clutch chatter in the first 20 miles or so till the GM fluid gets fully blended with the M1 - not to worry, drive it easy and you will be fine.

 

well got it all done and didn't make too much of a mess either. went with the mobil 1 synthetic gear oil. always happy with their products.
gear oil smells like rotten eggs, I think my garage is going to smell for a while. I said I didn't make much of a mess, not that I didn't make any mess :-)

is it normal for the diff to be under pressure? when I removed the drain even before the fluid started flowing I was hearing a hissing sound until I got the plug all the way out. which doesn't make any sense to me at all since the drain is at the bottom and even if it were under pressure the pressurized gas would be at the top and not the bottom. anyone know what that's all about? if the diff is still up to operating temp I could see the gas content of the chamber being pressurized, but it was room temperature at the time.

 

Well,,if its working correctly, that HISS should have been a VACUUM release. The Vent at the top of the tranny lets gasses OUT but NOT IN. The vacuum happens when the rear cools down and helps prevent leaks at the seals.




BC

 

awesome! thanks bill, I was wondering if it could be vacuum also. that makes sense. I guess that means the pressure relief is working correctly. that's a good sign.

 

When I followed the procedures in the shop manual (which are essentially as you described), I developed a slight leak in the diff. I was concerned that I may have created a problem with my preventative maintenance, because I had not had a leak prior to changing the fluid.

I assumed that I was going to have to replace the driver's side shaft seal, but someone on (another) forum gave me some advice, which cured my leak without replacing the seal. As you, I filled the diff up to the level of the fill plug. I was advised that this may, in fact be an overfill. I was told to drive the car for ~30 minutes, making sure to negotiate some turns, to warm up the differential to normal operating temperature. Then, parking on a level surface, I was told to remove the fill plug, and drain out any fluid. I did, and this stopped my leak.