Car front wheels shake while jacked up and engine idling. Why?
July 11, 2020 2:56 PM   Subscribe

Car front wheels shake while jacked up and engine idling. Why?

While the front end is jacked up and the engine is idling, the front wheels shake with verve and vigor left/right with no steering input. With some steering input either direction, non-shake places exist.

I've replaced the fluid.
I've replaced the power steering pump.
I've bled the system (without vacuum, turning rack to rack ten times, top up, repeat).
No obvious leaking.
No fluid loss (either old or new steering pump).
No grindy or pump noises.

Note that the car is parked, front end jacked up.
Car is 2011 Impreza STi. Fluid is correct spec ATF/DexronIII.

Is there anything but the rack and pinion assembly it can be?
Do I want to replace the rack and pinion assembly myself, or is the difficulty/frustration much higher?

My Google-fu and youtube-fu fails me.
Asking a car forum makes more sense, yes, but I belong to nothing but MeFi.
posted by lothar to Travel & Transportation (11 answers total)
Can you stop the shaking with your hands at the wheel end? Also, when the car is not running, can you replicate the amount of movement at the wheel by hand? Wiggle it side to side or up and down?

Also, does it shake the steering wheel too?

It could be play in any suspension component, the steering arm ball joints, suspension ball joints etc. It's hard to know how much free play is being seen here, and how much is an actively 'powered wobble'. I'm also wondering if the assist sensing part of it is faulty - like it can't find it's centre and is constantly overshooting by adding assist in each direction.

Does it do it when on the ground?
posted by Brockles at 4:17 PM on July 11, 2020 [2 favorites]

Sounds like maybe engine mount/tranny mounts are done? So like it's awd, front wheels attached to driveshafts attached to tranny attached to engine... engine shakes while idling and shakes the whole chain?
posted by some loser at 5:13 PM on July 11, 2020 [1 favorite]

Video, if possible, would be helpful.
posted by JackFlash at 6:12 PM on July 11, 2020

Best answer: Have you tried jacking back end up? See if rear wheels do the same.

IME this is usually the clutch.
posted by unearthed at 8:30 PM on July 11, 2020 [1 favorite]

Best answer: Take a look at this post and his suggestions. If either of those conditions exist in your vehicle (bad ball joint or slack in the tie rods) I would imagine it could possibly cause the front wheels to shake around while the engine is running & front end jacked up, just because there would be some slack in the system that would allow steering-related things to move around more than they should.

Also both things are easy to check so at least you can rule a couple of possibilities out in a few minutes of work.
posted by flug at 8:51 PM on July 11, 2020

Response by poster: Brockles--
Yes, with the car idling I can stop the shaking by grabbing a wheel, and it remains still after letting go.

With the engine off, I cannot replicate the motion on either of the front wheels, neither the left/right movement nor an up/down. (Even when testing with flug's NASIOC link, by using a crowbar to check for ball joint movement.)

Yes, when the front wheels are shaking there is some minimal steering wheel twitch. Turning the steering wheel 1/8th revolution in either direction kills the shake. Front wheel shake can be found at many locations rack to rack, and can always be cancelled by turning the steering wheel a bit either direction.

I don't find signs of any loose/worn components "allowing" the shake. It looks/feels like a very actively fed powered wobble. But since the car has 166k miles and has been driven on twisty, bumpy mountain roads its whole life, there could be tired ball joints, etc. The Ohlins struts are pretty new.

On the ground idling forward in first gear, some shake can be felt but it is very attenuated.

Unearthed-- With the front on the ground and the back end up, no motion occurs at the rear.

***In fact, engine idling and turning rack to rack with the front wheels scrubbing concrete (with the back still jacked up) the power steering pump only sounds happy (silent) when some turning pressure is put on the steering wheel. Letting go of the steering wheel creates the sound of a slightly unhappy power steering pump. No feeling of shake whatsoever, either.***

I'll post video if this post by me doesn't nail down the problem.

Many thanks for all the diagnostic tests so far. I really appreciate it.
posted by lothar at 3:04 PM on July 12, 2020

Best answer: Yes, with the car idling I can stop the shaking by grabbing a wheel, and it remains still after letting go.

That suggests it is not a powered shake (as in it doesn't start itself by applying force then correcting), but that it is hard for the rack to know it can stop assisting or that it doesn't know where centre is. Holding it there till it gets its shit together seems to quell its tendency to overcorrect maybe?

Turning the steering wheel 1/8th revolution in either direction kills the shake.

So the shake only ever starts when the wheels are straight ahead? Does it ever shake if the wheels are off centre?

I'm starting to think that the load sensor or position sensor around straight ahead is dodgy or broken in some way and it is a fight against no load on the steering. Like it is going "Oh, you need assist! Wait, no you're at straight ahead OH NOW YOU NEED ASSIST THIS WAY, Oh wait straight ahead again. ". The fact it can be stopped by holding it suggests the zero position of the steering is extremely fine and there is some unhelpful feedback loop in there. The noises from the pump suggests the same. It's getting confused when straight ahead/unloaded.

I don't know that rack (and road car PAS racks are a little outside my experience) but either the mechanism for straight ahead/ or side to side load sense is suspect or the rack itself has some wear or leakage around straight ahead that is triggering this. I'd have thought from this that a new rack would fix it, but I don't know for sure if just replacing a valve pack or a sensor would also fix it. Sounds like a 'time/effort versus just replacing the whole thing' calculation needs doing.
posted by Brockles at 3:24 PM on July 12, 2020 [1 favorite]

Looks like a remanufactured rack can be got for around $300. That's pretty damn reasonable, to me. If you have the means I'd be looking at just doing that unless you particularly enjoy tinkering around covered in PAS fluid (which.... does anyone?).
posted by Brockles at 3:26 PM on July 12, 2020 [1 favorite]

Response by poster: The shake can happen any point from full rack left to full rack right, but the steering wheel has to be in a position favorable to the shake. (No the wheels don't have to be straight ahead, yes the shake occurs when the wheels are off centre.)

Yes, "dodgy or bent, fighting against no load" sounds right.

On preview, yeah I'm happy to swap out the rack rather than find out exactly which little bits are causing the symptoms (fwiw, I've been thinking it's one of the O-ring seals on the rack partially failing and letting fluid past, causing negative feedback to correct the motion, which then re-occurs when the pressure is equalized between the sides causing the 5 cycle per second oscillation).

How frustrating/fiddly can swapping out the rack be? Does it entail more swearing? (I'll look this up online, too.)
posted by lothar at 3:40 PM on July 12, 2020

It can be dead easy or a massive ball ache or somewhere along that metric. Hope that helps. ;)

I haven't ever worked on an Impreza, so it mainly depends on whether there is a path straight out the side or down for it. Hopefully it doesn't mean dropping subframes, but it may do.
posted by Brockles at 4:02 PM on July 12, 2020 [1 favorite]

Response by poster: I've been pleasantly surprised over the years fixing Impreza STis. It's almost as if the designers consulted with people who have to repair the things after hard use and yumping. As opposed to some other vehicles I've worked on.

Let's call this question resolved. It's something fiddly in the rack, and swapping for a new rack is a sensible solution.

Thanks for the help everyone. And I'll admit that I almost titled this question "Ask Brockles".
posted by lothar at 4:19 PM on July 12, 2020 [1 favorite]

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