My XC90 tried to kill me

[jimmy57]

I just got back from towing my tractor after a brief fierce rain storm. My dual rear truck hydroplaned at 65 MPH and did something like you described. The wheel speeds up on the water and when you get to dry (er) pavement on the other side of puddle the inertia of spinning wheel causes the lurch. On front wheel drive it is worse. I bet you will find the angle gear collar is stripped or the DEM (haldex unit) is offline. With the AWD system functioning correctly it should not do this. Check the AWD system. In your defense there is no warning message on that vehicle for it not working. on V8 XC90s the RF tire will wear rapidly if it is not functioning for a time.

[MoVolvos]

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"Trashed interior, food stained and smells."
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Maybe the AI software kicked in and it's kicking back, Revenge of the Abused Volvo?
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[timelesstraveler]

At the risk of sounding as incendiary as your title, saying that this car tried to kill you is like saying you were held up in an alley by a .357 magnum. As surely as people-- and not guns-- kill people, drivers-- making conscious decisions about the roadworthiness of their cars-- kill people. As others as have stated, there is no sane advice to be given other than to refrain from driving this car, other than for diagnostic purposes at low speeds on empty roads. You should not be driving this car.

That said, the tires alone can be the cause of the steering effect based on your description of the condition. Especially if the ABS/traction control is not functioning to mitigate the effect.

per https://en.wikipedia.org/wiki/Aquaplaning, no braking or throttle input is necessary to create the condition. Your situation sounds like it was possible 1 tire was hydroplaning, pulling you to the side. It is possible for a single tire to hydroplane because of the water thickness differences on the road. It is very possible for individual tires to hydroplane at different speeds if they have differential amounts of wear, as you have described. I am not familiar with the ABS system in the XC90; generically speaking many systems when faced with an error will disable themselves, and thus not modulate throttle or braking in a hydroplaning situation.

Additionally, differential tire wear can be the cause of the ABS/traction control MIL. If the circumference of the tires is not within an acceptable limit (typically ~3% but again I am not sure in the XC90), that condition alone can trigger the error. Are the tires the same size?

[GreenMagicMan]

Hey Timelesstraveler:
Your quote:
"At the risk of sounding as incendiary as your title, saying that this car tried to kill you is like saying you were held up in an alley by a .357 magnum. As surely as people-- and not guns-- kill people, drivers-- making conscious decisions about the roadworthiness of their cars-- kill people. As others as have stated, there is no sane advice to be given other than to refrain from driving this car, other than for diagnostic purposes at low speeds on empty roads. You should not be driving this car."

Very accurate, very funny, very well written response. I agree with you. Well said!
Thank you for the additional information about possible skid causes, it's a puzzling mystery to me.
I did check the tires again mid trip - actually just about 70 miles of driving - and now both tires at the rear are skins aka bald.
I drove it 5 hours yesterday in fine weather up to 80 mph on a few occasions and there were no problems driving it whatsover (aside from the old 'wallowing' in the corners particularly the rear feels bouncing on right turns hitting pavement imperfections).
I will check tire sizes this weekend when we have the car back again.

Typically my experience with hydroplaning is hitting a visually noticeable pool of water after pavement that is shedding water correctly and does not have standing water. This has almost always happened with deeper pools of water of course and not very thin shedding layers on pavement. This bad experience driving was in thin water layers and only on the very densely grained freshly paved highway. It was very strange and alarming and dangerous.

MoVolvos:
Yep, you might be right about.