Changing differential fluid
Thread Starter
Race Director
Joined: Aug 2002
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From: Atlanta GA
Changing differential fluid
I see some owners post about changing their differential fluid very early into ownership. Is that mainly for certain model years of C7's or is it recommended on all C7's to change it early?
Tech Contributor
Joined: Oct 1999
Posts: 41,043
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From: Charlotte, NC (formerly Endicott, NY)
With my car level on the lift I used my little finger to see how full the diff was. It has been serviced several times by the dealer since I have it changed after each 24 hour period of track usage. The fluid was right to the bottom of the fill hole.
When I drained the fluid I got about 2.5 quarts out (manual indicates a Z06 will take ~2.96 quarts with the diff cooler included. I put in about 2.7 quarts of fluid before the fluid started flowing out of the fill hole. The service manual indicates the job isn't complete until the car is run in gear at 10 mph for a period of 1 minute. This is sufficient time for the fluid to be pumped out of the diff and into the diff cooler. After the car is run for that period of time the fluid level needs to be checked and topped off before lowering the car and saying the job is done. In my case, there was no difference in fill level since as soon as I tried to add more fluid it started flowing out of the hole.
Bill
When I drained the fluid I got about 2.5 quarts out (manual indicates a Z06 will take ~2.96 quarts with the diff cooler included. I put in about 2.7 quarts of fluid before the fluid started flowing out of the fill hole. The service manual indicates the job isn't complete until the car is run in gear at 10 mph for a period of 1 minute. This is sufficient time for the fluid to be pumped out of the diff and into the diff cooler. After the car is run for that period of time the fluid level needs to be checked and topped off before lowering the car and saying the job is done. In my case, there was no difference in fill level since as soon as I tried to add more fluid it started flowing out of the hole.
Bill
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Le Mans Master
Joined: Aug 2007
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From: Orange Park FL
With my car level on the lift I used my little finger to see how full the diff was. It has been serviced several times by the dealer since I have it changed after each 24 hour period of track usage. The fluid was right to the bottom of the fill hole.
When I drained the fluid I got about 2.5 quarts out (manual indicates a Z06 will take ~2.96 quarts with the diff cooler included. I put in about 2.7 quarts of fluid before the fluid started flowing out of the fill hole. The service manual indicates the job isn't complete until the car is run in gear at 10 mph for a period of 1 minute. This is sufficient time for the fluid to be pumped out of the diff and into the diff cooler. After the car is run for that period of time the fluid level needs to be checked and topped off before lowering the car and saying the job is done. In my case, there was no difference in fill level since as soon as I tried to add more fluid it started flowing out of the hole.
Bill
When I drained the fluid I got about 2.5 quarts out (manual indicates a Z06 will take ~2.96 quarts with the diff cooler included. I put in about 2.7 quarts of fluid before the fluid started flowing out of the fill hole. The service manual indicates the job isn't complete until the car is run in gear at 10 mph for a period of 1 minute. This is sufficient time for the fluid to be pumped out of the diff and into the diff cooler. After the car is run for that period of time the fluid level needs to be checked and topped off before lowering the car and saying the job is done. In my case, there was no difference in fill level since as soon as I tried to add more fluid it started flowing out of the hole.
Bill
what fluid are you using ?
Pitt
Race Director
Joined: Mar 2001
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From: Bonneville Salt Flats
If you are not "tubby," it's not that hard to change the fluid with it setting on the floor. I did so. Also, it's easier if you have a St'ray that you haven't installed a rear sway-bar on yet. Additionally, at least through my 2018, the factory fill was often up to 8-oz or more low when going by-the-book (even though the manufacturer CEO said they were all okay as shipped--as if it "settles in shipping" 
). 
).
Burning Brakes
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From: Northeast, PA
Tech Contributor
Joined: Oct 1999
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From: Charlotte, NC (formerly Endicott, NY)
Early deliveries of the 2014 cars had issues with insufficient diff fill but those problems were resolved early in the model year run. I don't know of anybody that had an issue after the 2015s started delivery.
Diff fluid smells nasty whether it is new or used. The smell isn't a big issue. If the fluid isn't dirty it isn't doing it's job. The OM tells you when to change the fluid. GM produces cars that are used by all types of people who never pay attention to servicing a vehicle. Somehow, those vehicles can achieve well over 100K miles with minimal maintenance and somewhat regular engine oil changes.
I had the fluid in my 2015 Z changed after the first track event as recommended by GM. Then I followed GM's instructions to change it after every 24 hours of track usage.
To be careful check the fluid level and change based on the OM directions. Over changing won't benefit a thing and will cost money. I just did my change over and the fluid cost me close to $90 so even if you are doing the job yourself it isn't a low cost job. The more often the fluid is changed the more likely you will have a problem. Don't fix things that aren't broken.
Bill
Diff fluid smells nasty whether it is new or used. The smell isn't a big issue. If the fluid isn't dirty it isn't doing it's job. The OM tells you when to change the fluid. GM produces cars that are used by all types of people who never pay attention to servicing a vehicle. Somehow, those vehicles can achieve well over 100K miles with minimal maintenance and somewhat regular engine oil changes.
I had the fluid in my 2015 Z changed after the first track event as recommended by GM. Then I followed GM's instructions to change it after every 24 hours of track usage.
To be careful check the fluid level and change based on the OM directions. Over changing won't benefit a thing and will cost money. I just did my change over and the fluid cost me close to $90 so even if you are doing the job yourself it isn't a low cost job. The more often the fluid is changed the more likely you will have a problem. Don't fix things that aren't broken.
Bill
