Mustang GT S550 diff issues

[ikocijan]

Hey all,

I have 2021 GT, PP1 with LSD. The car has only 20 000 miles. EDIT: This is EU version of the car so it has 3.55 gear ratio and it has LSD, but not torsen.

It appears that I have an issue with my diff. It started when I bought the car 6 months ago. From time to time when driving slowly and turning left or right I would hear a slight grinding noise from the back. It sounded like tyre rubbing. Since it happened rarely and was not that loud, I wasn't able to pint point when it's happening.

Now I lowered the car with steeda min drop springs and it's happening constantly when turning at low speed and it's louder.

The Ford dealership checked the car and said that there isn't anything wrong with the suspension. They say that for some reason the diff blocks the wheels, but they don't know why. They called in a guy who only works on diffs to see if he can figure anything out. They said they don't want to pull out the diff until they are sure there is something wrong with it. The guy says that the problem could be the oil and the additive which is added to it. He said it's common on these kind of diffs that either there isn't enough oil or the additive is missing so it grinds. He proposed to change the oil and add additive as the first step.

I've attached a video. You can hear the grinding when I'm turning left at the beginning and end of video.

Does anyone know anything about these diffs? Could this really be the issue because of the oil? I will change the oil, but if that doesn't fix the issue I want to know what else I could check before disassembling the diff because it's hard to believe that it's dead after only 20 000 miles. The car was regularly serviced once a year at Ford and it literally still has original tyres from 2021 on it. So I doubt that the previous owner trashed the car.

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[MidwayJ]

When my '19 GT had gear failure in the diff it made a very different sound. It was more of a high pitched whining or ringing sound. It was not a PP car, though, so it didn't have the Torsen. Also, there were metal shavings in the fluid that seemed easy to identify without disassembly.

 

[Brokestang]

Maybe someone changed the differential fluid and didn't put friction modifier back in. I changed the rear differential fluid in my ford expedition & used valvoline gear oil with friction modifier already in the lube & the rear made a similar sound when turning. Especially when accelerating in the turn. Was told to put the ford motorcraft friction modifier in to fix it. After putting it in no more noise.

 

[ikocijan]

When my '19 GT had gear failure in the diff it made a very different sound. It was more of a high pitched whining or ringing sound. It was not a PP car, though, so it didn't have the Torsen. Also, there were metal shavings in the fluid that seemed easy to identify without disassembly.

Ah, I'm sorry about that. Did they figure out why it failed?

I forgot to write it down so I edited the post: I don't haven Torsen. My car is EU spec and although it's PP1 it has 3.55 gear ratio and normal LSD, not Torsen.

 

[ikocijan]

Maybe someone changed the differential fluid and didn't put friction modifier back in. I changed the rear differential fluid in my ford expedition & used valvoline gear oil with friction modifier already in the lube & the rear made a similar sound when turning. Especially when accelerating in the turn. Was told to put the ford motorcraft friction modifier in to fix it. After putting it in no more noise.

This is a good point. I have some old invoices from it's service so will check and I will ask the Ford dealership which sold me the car if they know anything about changing diff oil.

 

[MidwayJ]

Ah, I'm sorry about that. Did they figure out why it failed?

No but fortunately, it happened at about 4.5 years and 57k miles so it was covered under the powertrain warranty.

 

[WD Pro]

@ikocijan - Have a read of this :

https://www.mustang6g.com/forums/threads/euro-cars-diff-oil-change-advice-experiences.186902/

WD

 

[pop smoke]

I just picked up a 2017 V6 with 50K miles on the clock. The diff was groaning when cold and turns at slow speeds. I changed the fluid to Royal Purple Max Gear 75W90 that contains "built in friction modifiers" for LSDs. It still groaned. Yesterday I put in 2 oz of Lubegard Limited Slip Supplement. The groan got better on the first drive and a dozen figure 8's. I'll reassess later today and may end up putting 2 more ounces in.

I've been in and out of Mustangs for 30 years, and the one thing that apparently has not changed is Ford's noisy LSDs.

 

[ikocijan]

@ikocijan - Have a read of this :

https://www.mustang6g.com/forums/threads/euro-cars-diff-oil-change-advice-experiences.186902/

WD

For fucks sake! This is exactly what is happening to me. I've lowered the car and added IRS stop the hop pack. Now I understand what is happening. The diff is now stiffer and it can't detach the plates inside it, but not because the oil or aditive is bad, it's because it doesn't have aditive in the oil.

So the guy who fixes differentials was right, but he thought that oil/aditive is bad.

Thanks for this, it now all makes sense!

What doesn't make sense is why ford put different oil in the diff.

 

[WD Pro]

Be warned, the Ford additive smells ‘really good’ …

WD

 

[TomOz]

Be warned, the Ford additive smells ‘really good’ …

Man, I hear ya. I put that unopened additive in four bags, and I could *still* smell it.

 

[ikocijan]

Hey @WD Pro ,

I've put the new oil and added friction modifier. It's 99% good now. However, I still hear some grinding, especially when stationary and start going left or right with more than half of the steering wheel.

Do you still hear the grinding or is it completly silent for you now?

 

[WD Pro]

Hey @WD Pro ,

I've put the new oil and added friction modifier. It's 99% good now. However, I still hear some grinding, especially when stationary and start going left or right with more than half of the steering wheel.

Do you still hear the grinding or is it completely silent for you now?

For me it is gone, and my diff gets a really good test every morning from cold - full lock to get me off the driveway at slow walking pace.

If it makes a difference :

• I have the Steeda red bushes in both the front and back of the diff.
• The oil in mine is the standard Ford fill (with the Ford FM lol).
• If I recall correctly, my diff had somewhere between 2k and 2.5k on it when I had / fixed the issue i.e. basically new.
• From me first getting the noise, to me fixing the noise was a really short interval - maybe 50 miles absolute max i.e. very little use (= very little damage ?) whilst it did the grinding thing.

Quick question which may / may not make a difference - have you given it a really good run / got it hot / made the diff work (both in maximum slip and maximum lock) ?

I did read about doing multiple slow figure of 8's (using full lock for every turn) to really get the FM where it needs to be - I didn't have to do that, but maybe it would help you ?

Hope you get that last 1% ...

WD

 

[ikocijan]

I didn't push the car, I only drove it home from Ford service so 9 miles. I also didn't drive it once I heard this. I drove it 10-15 miles and left it back to Ford.

They have put the Ford oil, but the friction modifier is from Chrysler because the service could not order Ford friction modifier in Croatia. However, it's the same specification and also 100 ml as stated in the user manual.

I have nothing agains doing figure 8's, but I'm worried if I'll break something

 

[TomOz]

I have nothing agains doing figure 8's, but I'm worried if I'll break something

You won't break anything.
The idea of doing the figure 8's in both direction is to get the friction modifier all throughout the plates in the diff. It would get there eventually, but I'd rather get it done and dusted.
I had to do the same when I had the rear diff oil changed, but they didn't put in the friction modifier. I added that later and after a few figure 8's it all went away.

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