Car is unstable at highway speeds

[Chilezen]

My car has an exciting new problem, because it can never seem to not have a problem. I'm writing this as I sit in the backseat while my friend drives.

The car sways at highway speeds. It's difficult to keep straight. Like I constantly feel the weight shifting from side to side, which feels like it pushes the steering. During long curves, you especially feel the weight transfer, settle, and then the steering "corrects" and you have to adjust the steering to continue your line, causing the weight to shift again and it throws you off again.

This started a couple weeks ago.
What I've checked: put the front of the car up, shook the wheels for bearing play. None noticable. Tie rods seem fine. The steering rack has a little play, but not a concerning amount. LCA bushings seem fine. Sway bar mounts and endlinks were tight.
I put the rear up. No noticable bearing play. Checked all 4 trailing arm bolts, those are tight. Rear sway bar mounts were good, as were endlinks. Checked subframe to chassis nuts: one was loose! Driver's side. So I tightened it.

I thought the problem would go away after that. It hasn't, weirdly.

All 4 shocks seem fine. No leaking oil and they respond evenly at each corner.
Tire pressure is good all around, between 30-32.

But driving the car on the highway is so irritating. Constantly having to correct, fighting the constant weight transfer while going straight.

It doesn't feel like the car is trying to pull you in either direction, it doesn't feel like there is a bias to either side (it doesn't favor pulling to one side). I can let go of the wheel on a straight road and it will track fairly straight, until it doesn't.

I can't see how or why I would suddenly need an alignment. Nothing has happened recently that would throw it off. Last alignment I had was 1.5 years ago.

I have poly bushings in most of the suspension components. I'm wondering if the subframe bushing has failed.. like the nut was tightened but maybe the poly has deteriorated? I thought it would last the life of the vehicle (as advertised). I can't think of any other cause for this.

Maybe you have some idea of what I've missed? Thanks a bunch. We're driving back to LA from visiting a friend in SF and this is awful.

[roguetoaster]

It's either toe front or rear, or a dead shock. Yes, I know you checked the shocks. Eyeballing the toe would tell you if it's so far off, but the shocks can seem fine while being totally dead and it results in a horiffic highway experience. Outside chance of weird power steering pressure.

[Chilezen]

Outside chance of weird power steering pressure.

What's a reliable way to test for a dead shock? All I really did was bounce on the car.

Eyeballing the toe looks alright. I'll go about setting up string to get some basic measurements before taking it anywhere. I have the paperwork detailing what the numbers should be in my files.

About the steering pressure... I did notice one other thing.
Driving in the heat (outside temp 105°) this overall problem was more noticeable. Driving at night, temps around 60-70°, it was less noticeable. There was a point during the peak of the day when I got off the highway, and during low-speed turns, the steering felt heavier. I checked the reservoir and it was fine. We stretched our legs for a bit, then resumed the drive--the steering felt normal again. Maybe I imagined it. In this kind of situation, you get paranoid about everything.

[roguetoaster]

There's not much beyond a jounce test for a front insert, a rear you could easily remove. Normally no more than three bounces from full extension to stop is the metric.

How about a strange tire belt separation or simple out of round wheel?

[Northern]

kind of sounds like my car when I had a lot of front toe out, should be easy to check with a measuring tape in front/behind the tire. Yeah treads have some runout, but it's easy enough to get a ballpark number.

[Chilezen]

Update, found the problem.

In the rear, where I have adjustable camber/toe, one of the toe bolts came loose.
I'm betting that when I first check all those bolts, it was binding in the slot, which made it seem tight, despite not being seated correctly.

I found this out when the bolt snapped during my commute to work. I suppose after shifting around the slot for so long, the bolt finally gave up. I heard a pop, and then the car kinda waddled around the road as I tried to drive straight. It basically felt like I had a flat tire. Luckily, after calling my boss, I was able to drive home and borrow my mom's car for a few days, until I got a replacement bolt.

Dropping the subframe was easier than I expected. So that was a nice surprise. I could lower it enough without disconnecting brake lines or the driveshaft.

Now that a new bolt is in there, I don't think I put it back in the right spot... but at least the car doesn't wander and sway like it used to. So I do need to get an alignment.

[Panici]

Glad you found it.

I have heard of folks tack welding the bolts to the adjusters once they are aligned where you want them.
If I ever go with subframe adjustment that is my plan.

[Chilezen]

I am not a fan of that idea because I like the possibility of adjusting it without having to snake a cutting wheel in those tight spots.

What I recommend for this application is a Nord-lock type of lock washer. Link
Here's a handy video explaining how great it is.

Cutting a tack will absolutely suck. Speaking from experience, a tack will either be to small to have actually melted into the two metals reliably, or it's too big, meaning there's more to cut and you end up cutting more than you wanted.
Please use the appropriate locking hardware instead. As you can see on the product page, they're very affordable, compared to having to replace a broken bolt. I would definitely get some if I felt like dropping the whole subframe again.

[Panici]

What I recommend for this application is a Nord-lock type of lock washer. Link
Here's a handy video explaining how great it is.

Ah, yes.
I use Nord-lock washers on my Miata's turbo exhaust flange.

Not sure how tolerant they are for re-use though once loosened.