3.3TT Stinger GT now squirms oddly on hard downshifts after LSD replacement.

[Helo58]

My last trip to the dealership was to have the limited-slip differential replaced under warranty. Since that time, aggressive downshifts now result in the car squatting under power and stepping out to the right in straight line acceleration. When the throttle is rapidly closed (after a full-throttle run), the chassis is also upset with a similar "squirming" coming from the rear-end of the car. The behavior is exaggerated during cornering maneuvers, obviously. Previously, the car was very stable and planted during straight line acceleration and I am trying to understand what has happened to change this. For those that understand RWD suspension setups, I would appreciate your feedback on these points/questions...

1) I assume that removal of the rear differential requires the removal of the entire rear axle. When these components were reassembled with the new differential, is there any adjustment, link, connection, etc. that could be responsible for this feeling if it were re-installed improperly?

2) My car is a 2018 model base GT that was built with a LSD as a factory option. As many remember, not all 2018 GTs got the limited-slip rear differential. With this in mind, it is possible that the dealership could have installed an open differential by mistake? Or possibly the wrong fluid in the new differential? Would an open differential perform the way described above compared with a LSD? What effect might improper fluid result in? Can I distinguish the LSD from the open differential by looking at the differential case?

3) The car is a little over a year old now (since first driven) and has roughly 15,000 miles on it. Is there anything else such as an improper alignment, blown shock, etc. that might account for this behavior?

 

[nhcowboy28]

I’d h

My last trip to the dealership was to have the limited-slip differential replaced under warranty. Since that time, aggressive downshifts now result in the car squatting under power and stepping out to the right in straight line acceleration. When the throttle is rapidly closed (after a full-throttle run), the chassis is also upset with a similar "squirming" coming from the rear-end of the car. The behavior is exaggerated during cornering maneuvers, obviously. Previously, the car was very stable and planted during straight line acceleration and I am trying to understand what has happened to change this. For those that understand RWD suspension setups, I would appreciate your feedback on these points/questions...

1) I assume that removal of the rear differential requires the removal of the entire rear axle. When these components were reassembled with the new differential, is there any adjustment, link, connection, etc. that could be responsible for this feeling if it were re-installed improperly?

2) My car is a 2018 model base GT that was built with a LSD (Limited Slip Differential) as a factory option. As many remember, not all 2018 GTs got the limited-slip rear differential. With this in mind, it is possible that the dealership could have installed an open differential by mistake? Or possibly the wrong fluid in the new differential? Would an open differential perform the way described above compared with a LSD (Limited Slip Differential)? What effect might improper fluid result in? Can I distinguish the LSD (Limited Slip Differential) from the open differential by looking at the differential case?

3) The car is a little over a year old now (since first driven) and has roughly 15,000 miles on it. Is there anything else such as an improper alignment, blown shock, etc. that might account for this behavior?

id make them redo the alignment.

I lived with a shoddy alignment (I paid for after spring install)... it was horrible. Car was twitchy and pulled massively to the left (more than the typical US road crown tweak). Bulbs and rain were seriously unnerving.

had them redo it this last weekend.
So much more planted.

 

[Helo58]

I’d h

id make them redo the alignment.

I lived with a shoddy alignment (I paid for after spring install)... it was horrible. Car was twitchy and pulled massively to the left (more than the typical US road crown tweak). Bulbs and rain were seriously unnerving.

had them redo it this last weekend.
So much more planted.

Thanks for the input. I'm hoping that it will be something that simple. I'll post back once the alignment is done.

 

[Helo58]

Well, here is the latest in the saga. The car basically became un-driveable today. The squirminess under and off throttle that was there before almost put me into oncoming traffic today. The car was uncontrollable at speeds over 55 and it wallowed left and right like I was counter-steering on ice. At speeds no greater than 45mph, I limped the car to the dealer to have it looked at. I took the service technician for a ride and cautioned them to be careful. The technician immediately noticed the detached feeling and returned to the shop to put it on a lift. He noticed the top of the rear left tire was cambered in. Upon lifting the car and inspecting it, the upper arm of the suspension was found to be COMPLETELY DETACHED. As in, no bolt holding it, like 4 inches from where it should be. I am now in the process of trying to determine the cause of the bolt coming out of the hole. If anyone has the shop procedures for the LSD replacement on a RWD Stinger GT, I would love to see it. I need to see if it is conceivable that this was disconnected during the LSD swap or to see if it came from the factory this way.



To anyone having this issue of abnormal chassis movements, some have mentioned the LSD itself, others have mentioned alignment, others have mentioned cords/belts in tires. It may be worth your time to check the tightness of all bolts securing the rear suspension components. I will attempt to post in other locations for those that have reported having this issue.

 

[HerbieTheCar]

Well, here is the latest in the saga. The car basically became un-driveable today. The squirminess under and off throttle that was there before almost put me into oncoming traffic today. The car was uncontrollable at speeds over 55 and it wallowed left and right like I was counter-steering on ice. At speeds no greater than 45mph, I limped the car to the dealer to have it looked at. I took the service technician for a ride and cautioned them to be careful. The technician immediately noticed the detached feeling and returned to the shop to put it on a lift. He noticed the top of the rear left tire was cambered in. Upon lifting the car and inspecting it, the upper arm of the suspension was found to be COMPLETELY DETACHED. As in, no bolt holding it, like 4 inches from where it should be. I am now in the process of trying to determine the cause of the bolt coming out of the hole. If anyone has the shop procedures for the LSD (Limited Slip Differential) replacement on a RWD Stinger GT, I would love to see it. I need to see if it is conceivable that this was disconnected during the LSD (Limited Slip Differential) swap or to see if it came from the factory this way.

View attachment 37023

To anyone having this issue of abnormal chassis movements, some have mentioned the LSD (Limited Slip Differential) itself, others have mentioned alignment, others have mentioned cords/belts in tires. It may be worth your time to check the tightness of all bolts securing the rear suspension components. I will attempt to post in other locations for those that have reported having this issue.

Thanks for your follow up and glad to see that you were unharmed during your scientific experiment. I hope everything gets taken care of!