Undrivable suspension

[StephenW]

2015 Yukon XL Denali 54k miles at the shop - told it is undrivable because the only suspension that is working is the tire sidewalls. I’m being told the front mag shocks are locked out and not compressing. I have made no modifications to the shocks.

Has anyone seen this before or dealt with it? I towed a 6900 lb trailer for 30 miles before the a valve spring broke and caused engine/transmission problems. This doesn’t really make sense to me. Right now they are disassembling the front end to determine if one or both shocks are locked out. Full repairs $1600 of both are bad and not covered by service plan.

Not cool.


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[iamdub]

There's a recent thread on a very similar issue and plenty more here and elsewhere. It's probably just hit-or-miss if it'll happen to yours, but it's still a well-known issue.

A valve spring in the engine broke? This shouldn't have damaged the transmission any and definitely shouldn't have caused the shocks to lock up.

 

[StephenW]

There's a recent thread on a very similar issue and plenty more here and elsewhere. It's probably just hit-or-miss if it'll happen to yours, but it's still a well-known issue.

A valve spring in the engine broke? This shouldn't have damaged the transmission any and definitely shouldn't have caused the shocks to lock up.

Yes, I agree a valve spring doesn’t touch the suspension. That was the impetus for being in the shop. I was asked if I made any modifications to the shocks in preps for towing.

I looked through the forum and most of what I saw was leaking shocks, which mine are not, and not failing to compress.


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[iamdub]

Yes, I agree a valve spring doesn’t touch the suspension. That was the impetus for being in the shop. I was asked if I made any modifications to the shocks in preps for towing.

I looked through the forum and most of what I saw was leaking shocks, which mine are not, and not failing to compress.


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I gotcha. I didn't know if you mistyped something or if the shop was trying to take you for a ride. What "modifications to the shocks" could you have possibly done?! It's not like they have any user-adjustable settings.

 

[StephenW]

I gotcha. I didn't know if you mistyped something or if the shop was trying to take you for a ride. What "modifications to the shocks" could you have possibly done?! It's not like they have any user-adjustable settings.

According to the shop guy there is. No clue what it could be. I’m thoroughly confused as to how my shocks ended up this way after towing a travel trailer.


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[iamdub]

According to the shop guy there is. No clue what it could be. I’m thoroughly confused as to how my shocks ended up this way after towing a travel trailer.


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An abbreviated explanation is that the computer senses the position/height of each corner and adjusts the shock's firmness accordingly by varying electrical voltage or pulses or something like that. If they unplugged the connectors from all the shocks and the car rode soft again (would probably ride TOO soft and feel top-heavy), then the problem is within the computer or maybe with any of the sensors that tell the computer it needs to firm up the shocks. Otherwise, something failed in the shocks themselves. Regardless, it's a crappy deal that it happened at 54K miles and while towing 1,000 pounds under what your XL is rated for.

I wonder if the system defaults to a firm setting when the engine is in distress, kinda like a limp mode and locking up the shocks is safer than keeping it adjusting? If they fixed the engine and reset everything, then the shock issue would clear itself.

 

[StephenW]

Just got an update from the shop -

Left front shock magnetic fluid is completely leaked out. Right front halfway leaked out. ~$1600 to replace both.

Not my normal shop, car broke in Maine I live in RI, so I can’t ask him why this wasn’t noticed 4K miles ago when it was last serviced.

Once a leak develops how fast do these things leak out, speaking in orders of magnitude - days? Months?


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[iamdub]

No fluid means the shocks would be uselessly soft. Maybe the computer locked up the other two in effort to compensate? Or maybe the computer locked them up for whatever reason but the extra load of the trailer forced them to blow the seals? Or maybe the computer firmed them up to compensate for the load, but then two prematurely worn (defective?) shocks started leaking so the computer maxed out the firmness of all the shocks and held it cuz it never saw the two leaking ones getting as firm as it was commanding them.

There's no telling how fast it would leak out. I guess it would depend on what happened to start the leak and how bad it was. I don't think there's any way to tell what caused what, but one thing's for sure, such a failure at 54K miles is ridiculous and you don't know if it'll happen again next time you tow.

 

[StephenW]

Yeah the shop said the other way around - no fluid means fully compressed and resting on the spring.

Talked to my normal shop - they saw no indications of a leak. So it all happened in less than 5k miles?

Chatted with GMC about it and called the repair dealership assistant GM (understand it is not on them but I have to pay them) about some help. We will see. Not helping convince to convince my wife that this vehicle was a good purchase.


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[iamdub]

Yeah the shop said the other way around - no fluid means fully compressed and resting on the spring.

Talked to my normal shop - they saw no indications of a leak. So it all happened in less than 5k miles?

Chatted with GMC about it and called the repair dealership assistant GM (understand it is not on them but I have to pay them) about some help. We will see. Not helping convince to convince my wife that this vehicle was a good purchase.


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The coil springs are what holds up the vehicle, so yeah, the frame/body are always resting on the spring. The shocks (technically "struts" in the front) just dampen the spring's up and down movement. On a normal vehicle, you can buy firmer or softer shocks to suit your needs, but the weight of the vehicle is still resting on the springs no matter what. On your car, the shock's firmness is adjustable by the computer. When it failed, it's like you had a really stiff shock on one side in the front and a blown out shock on the other side, and the opposite for the rear. If the shocks were "fully compressed", then the springs would have to have been fully compressed, which means the car would've been way overloaded and squatting with the wheels tucked into the fender wells. I don't think you were towing like that.

If the computer is what commanded the shocks to lock up, then maybe there is a record of what told the computer that this needed to be done, like a freeze frame. When a check engine light/diagnostic trouble code (DTC) is set, the PCM takes a "screen shot" of what all of the sensors were reporting to it at the time the DTC was set. This is called a freeze frame and a technician can access this to help pinpoint what the root issue is. It'd be great if there is a freeze frame for the suspension controller computer that would have a record of why it suddenly commanded the shocks to maximum firmness.