Do you ever hear a clunking noise coming from your differential? You will hear it when you either switch from park to drive or drive to reverse. That noise means it’s definitely time to change your differential fluid.
The 2004-2006 Pontiac GTO comes from the factory with a 3.46 gear ratio limited-slip differential. Limited slip means it is not a full true locked Posi rear end. Both rear tires are not always spinning at the same speed. They will both spin at the same time and same speed when enough torque is placed on the gears, such as pulling a holeshot or just stomping on the gas to squeal the tires.
You can download the “Rear Drive Axle” section of the service manual below. The differential is the same for all three years, 2004, 2005, and 2006 Pontiac GTO.
|DOWNLOAD (.pdf format)|
|2004-2006 Pontiac GTO REAR DRIVE AXLE Section of the Service Manual|
The rear ends on these cars are pretty sensitive to which fluid they have in them. I’m going to show which fluid works the best and how to change it.
Torco is by far the best differential fluid for these cars. I’ve tried stock and other synthetic brands, Torco RGO outperforms them all! If you use any brand other than Torco, I cannot guarantee your differential will not still clunk or make noises you don’t want it to. Only use Torco RGO 85w140!
And yes I realize Type F friction modifier is for Ford, Chrysler, and Toyota. But the Pontiac GTO rear end also uses a Type F friction modifier.
|Note: If your differential was rebuilt and has the Eaton TrueTrac Limited Slip Posi differential, you do NOT need to use a friction modifier.
Taken directly from the Eaton website:
What kind of oil should I use for my Detroit Truetrac? Can I use synthetic? Do I need friction additive/modifier?
Visit Eaton FAQ’s for more information here: EATON Detroit TrueTrac FAQ’s
Drive your vehicle for at least 10 minutes to get the differential fluid warm. You want as much of the fluid to come out as possible when you drain it.
Jack up the rear end of your vehicle and place it on jack stands and look at the differential. Use the diagram below:
- Number 1 – Differential Fluid Fill Plug
- Number 2 – Differential Fluid Drain Plug
Remove the drain plug bolt (Number 2) and drain the fluid. Be patient, let as much fluid as possible drain from the differential. You can loosen the Fill Plug (Number 1) bolt to let air into the differential for a faster drain if you wish. After the fluid has completely drained, reinstall the drain plug (Number 2) and tighten it to 20 ft/lbs (foot-pounds) using a Torque Wrench. Now completely remove the fill plug (Number 1) for the next step.
Using a funnel, pour no more than 2 ounces of Torco Type F Friction Modifier into the differential. Do this first.
|SPECIAL NOTE: I will warn you before pouring in the Torco RGO. Unfortunately, Torco RGO does not come with an easy pour spout on its bottles. The RGO is extremely thick and hard to pour. I recommend either finding a screw-on pour spout that fits these bottles or pouring the RGO into an empty differential fluid bottle that does have an easy pour spout.|
Next, pour the Torco RGO differential fluid into the fill hole (Number 1). You will use just under 2 quarts. Fill the differential until it starts to drip out from the fill hole. When the fluid is coming out of the fill hole, the differential is full. Re-install the fill plug (Number 1) and tighten it to 20 ft/lbs (foot-pounds) with a Torque Wrench. Take the car off the jack stands and take it for a drive.
A quiet differential is a happy differential. Hope this service tip has helped. Thanks for reading!
19 CommentsAdd a Comment
Submitted on 11/11/22 at 7:52 am
Thank You for recommending the Torco Oil with Torco Friction modifier. Now my rear end is quiet. My car was purchased new in 2007 and has never had diff fluid changed. I had my Chevrolet dealer here in Scottsdale Arizona do the service with my Torco fluids. Wow, no more clunks or noises. What a product. Thank You for Your knowledge of these vehicles.
Submitted on 09/19/22 at 2:47 pm
Hey mark, I know my diff is supposed to be limited slip but it only one wheel peels under heavy load, will this fix that or will i likely need more work done to my diff
Submitted on 09/22 at 4:50 pm
Servicing the differential with the fluid change might help. I say might because really there are only 2 main parts inside the differential. The ring and pinion gear, and the Limited Slip Posi unit. If the ring and pinion gears were damaged such as a broken tooth, the rear end would make a horrible grinding noise and would get progressively worse. The posi unit itself is what controls the gears to spin. Under light load, it will be in “one wheel peel” mode. Under heavy load, the posi unit should lock giving you a nice 2-wheel contact patch.
Changing the fluid to the thicker torco oil may bring the differential back to life so the LSD Posi can grab better giving you the 2 wheel contact patch we all want. If this does not fix the problem, you may need to rebuild the differential. Upgrading to the Eaton Truetrac would be the way to go.
Submitted on 06/24/22 at 3:21 pm
I have 06″ GTO A4. I purchased Eaton Tru Trac 4:11 gears. My shop is telling me the factory rear diff is non serviceable?
Submitted on 06/26/22 at 11:55 pm
The shop is incorrect on this. The differential can in fact be serviced. I’m not going to say it is easy, because it isn’t. You pull the differential out of the car and gut it to a bare housing. The biggest headache is removing the side adjusters which will pretty much be destroyed once they are removed. You will need to purchase new ones which can be found here:
You should also purchase the Major Gear Installation Kit found here:
If I had to do this again, I would buy a used differential and put it all together, than just take a day to do the differental swap.
I have 05 GTO and I just bought rebuilt 3.46 differential from rockauto. After I got the package outside of my door I instilled it on my car I put 75w-90 lucas as factory recommendation but after I drive the car less the 100 mile I heard clunking noise and metal grinding coming specially when I make U-turn or turn left or right. My question is do you still recommend to change the differential fluid to 85w-140. In addition I have whining sound inside the cabin when I’m driving the car coming from T-56 transmission shifter I have B&M aftermarket shifter and I was wondering if there is any advice to get ready of the whining noise.
Reply to part 1 of your question: The 75w-90 seems a little light in my opinion. I have actually called Torco and they 100% recommend the 85w-140 RGO differential fluid, with type F Friction Modifier. You can use as little as 2 ounces of the friction modifier to as much as the whole bottle which is 4 ounces. The clunking and grinding noise is not a good sign especially while turning. If everything was installed correctly on your end, sounds like whoever rebuilt the differential did not adjust the backlash properly. On the sides of the differential are side adjusters (this is where the axle stubs slide into). If those are set too loose, the gears will move side to side inside the pumpkin more than they should. Too tight and the gears will grind and wear down the ring and pinion teeth. Replace the fluid with the Torco and be sure to use the friction modifier. Look for metal in the fluid after you drain it. If the problem persists I would say you have to send the differential back for a replacement.
Reply to Part 2 of your question: The shifter should not make any difference on the transmission. It shifts the gears, that’s it. The most common shifter complaint is a rattle. I would service the transmission. Have the tranny fluid changed and see if the noise improves or goes away completely. The whine could also be caused by a bad carrier bearing on your stock 2 piece driveshaft, if you still have the stock driveshaft. The rubber bushing may be worn on the carrier bearing causing the driveshaft to spin on the metal carrier bearing assembly. A pretty common problem with the 2 piece driveshaft.
Hope this helps.
Thanks a lot, Mr. Mark for your information you really saved me a lot of money or going to hate the car because of the problem from the back end. after your recommendation, I have changed the fluid with type F modifier all the noises went away I really would love to thank you so much for your knowledge. The last thing I would like to ask you about, I have changed the transmission fluid and I put ACDelco manual transmission fluid but still, the same thing whining. Please if there is any recommendation for the manual transmission fluid let me know appreciate your support.
Hello Mark, Phillip here
I have a 2006 GTO with a diff I built about 2 years ago. It is a 4.11 Motive Gear with an Eaton Tru trac posi, G-force side adjusters, and BMR stubs. It has had gear whine ever since I put it together but I was able to tweak it using the side adjusters the make it as quiet as possible. Not terrible but most pronounced at about 50 to 55 mph. I just recently swapped out all the bushings in the rear with urethane and without thinking it all the way through I purchased Energy suspension (Race Track hard) cradle bushings which improved handling a ton but not my acceptable gear whine is now much more pronounced. I have been running standard Valvoline 85W-140 with no additive. If I change to the Torco RGO will it really make that much of a difference over the Valvoline. I have read about it over the years but never put much thought into it because my noise was tolerable.
That’s a nice set up in the rear end. Well done for going with the GForce side adjusters on the differential. As far as the gear whine noise, I’m no differential expert, but I have heard numerous times from many different people over the years including tuners and speed shop owners that gear whine is very common and near impossible to avoid after switching to a bigger gear in the rear. Seems to be most common with the 3.90 and 4.11 gear ratio made by Motive Gear. Then again not to many people run the available 3.70 ring and pinion, which I have heard is much quieter but not the neck breaking pulling performance most of us speed freaks are looking for.
As far as the Torco Gear Oil and is it worth the switch, I say what the hell and give it a try. Since you have Eaton TruTrac installed you will not need friction modifier. Its easy to change the differential fluid, wont cost alot or eat up a ton of your time, and if you don’t like it or don’t think it made a difference you can always go back to what you like. Also you can make a few calls, call GForce1320 and see what they recommend, and also give Andrew Kollar from Kollar Racing a call, as well as Maverick Man Carbon…..think you will find very similar answers. Made these calls myself a few years back and all roads lead to Torco RGO.
Thank you for the questions and comments, and thank you for visiting the website.
Mark I had a bad rear axle that was only a few months old on my 2004 GTO drivers side, must of been defective due to the car is not abused. Long story short it damaged my axle seal, as I was prepping it for a new seal I decided to take a peek inside. I found the axle bearing had a little damage meaning some of the needle bearings were not sitting properly. The axle stub had some minor scoring around it which I was able to polish out with emery cloth. My question is if I decided to replace the bearing is it a diy project or does it have to be pressed in? If you have a part number that would be great. Thanks in advance for your time and information.
I am by no means an axle expert. I appreciate the question though. If you are talking about the axle shafts themselves, if they are damaged at all in my experience and browsing around the internet, you probably should just replace the whole axle. Maybe you can ask around on the GTO Forums and FaceBook GTO groups to get a better answer. You can visit Kollar Racing for purchasing new axles here:
Im sorry I can not be more help on this subject. I have never had problems personally with the axles on the GTO. I know many owners have though.
Mark, apologies for taking so long to tell you the results. I did exactly as instructed, using the recommended gear oil and friction modifier. I also added four ounces of zMAX micro lubricant, which I have been using in the engine oil (full synthetic 5W30) automatic transmission fluid and power steering fluid. Clunk gone! Differential quieted down. GTO now at 162K miles and runs like a new car. Thank you and Happy New Year!
Thanks Mark. I have ordered the products and will let you know how I made out.
Wise choice my friend, LOL. I threw out the “FR Sports” links because they seem to have the best price, and they ship pretty fast as well. Never had a single problem ordering from there.
Just be careful putting the gear oil in. Stuff is extremely thick! Use an old gear oil container that has a pour tip on it, or buy an adapter to screw on the top of the RGO oil, or you could try a handpump……which I have never heard anyone having any luck using a pump. Pump will break before you get half a quart in.
Mark, my 2004 GTO has 157K miles (it’s my daily driver, purchased new). The car is mostly stock, equipped with the 4L60E automatic, which was rebuilt at 100,000 miles. I’ve changed the driveshaft center carrier bearings and both flex couplings. the differential oil was changed at 125K miles but I don’t know what type of fluid was used, and I know that a friction modifier was not added. I get the clunking noise; it appears after the car has been driven for about 10 minutes. I hear and feel it just before the car comes to a full stop, when the transmission is downshifting, and when I start accelerating slowly. Rapid acceleration produces no clunk, the transmission upshifts through the gears smoothly, and the car is stable at very high speeds (I.e 125 mph bursts on the highway). It seems to me that I need to do what you recommend with the Torco products. Agree? Any other thoughts. Thanks!
Completely agree. I would just take a day to change out the differential fluid. You have a stock differential, so you will need some Friction Modifier. Here’s where I get mine.
Torco RGO: RGO
Torco Type F Friction Modifier: Friction Modifier
You will be happier.
QQ: How many miles between differential changes ?
Hi Rob and thanks for your question.
Your question even had me searching for an answer. Doesn’t seem to be a recommended mileage in the service manual when to change the differential fluid. This is just in my opinion, you should probably change the fluid at the 50K thousand mile mark. Its not hard to do, and the rear ends on these cars are kind of touchy. I have seen posts on various blogs and forums where owners have destroyed the rear end in 20k thousand miles, and I have seen others who have never changed the fluid and still have zero issues of differential noise.
The Torco RGO replacement differential fluid by far is better than the stock General Motors fluid. It will quiet up the rear end from gear noise. Any kind of rear end grinding or clunking noise from the differential is never a good thing. Unless of course you changed the gears to the 3:90 or 4:10 ring and pinion, in which case some gear noise is expected. But the noise made from these gears is more of a “whine” rather than a clunk or grind, which is perfectly normal.
Thanks for the question and I hope this helps.