Vibration at 25MPH and under load - need diagnostic suggestions

The car is a 2006 Outback XT, 5 speed manual, 90k miles, stock except Cobb DP and Stage 2 tune. These symptoms started several thousand miles ago and have gotten worse:

 

- Very noticeable shaking of the shifter when driving 20-25 MPH regardless of which gear is selected, can also feel shaking in the steering wheel and seat

- Roughness (higher frequency vibration) when driving 60 MPH

- Roughness when accelerating hard, immediately smooths out when releasing the accelerator (no power loss - car pulls hard).

 

The symptoms are not tied to engine speed, so I figured it was related to a driveline component. The wheels/tires are fine, no misfires or other engine codes. The symptoms are present, but much less noticeable, during the first mile of driving when the car is cold. The transmission mount looks normal (no cracks/tears), the engine doesn't budge when I try rocking it (can't get a good visual on the engine mounts). The driver side front halfshaft is an OEM Subaru shaft with around 30k on it, the passenger side is a rebuilt aftermarket shaft with around 40k on it.

 

Thinking a bad U-joint was the most likely culprit I had the driveshaft replaced with a new OEM shaft. There was no improvement. So now I'm wondering if the passenger side halfshaft is bad, the transfer case/viscous coupling is bad, or there's some other cause. I need to visually inspect the rear diff mounts but I didn't notice them being torn previously.

 

Any reference I've found on the forums with similar symptoms ended up being fixed with a new driveshaft or front halfshaft. I mention the transfer case since that first mile isn't as bad as when the car warms up.

 

Thanks for any suggestions,

Zack

My vote would be the aftermarket half-shaft on the passenger side.

Aftermarket shafts are notorious for vibration issues on Subarus.

Are you lifted or still on stock suspension?

 

 

Have you checked your cv axles for play? You hold the axle rod with your hand and push/pull on the axle. Lots of play?

 

 

No whining sound from the trans area?

Are you lifted or still on stock suspension?

 

 

Have you checked your cv axles for play? You hold the axle rod with your hand and push/pull on the axle. Lots of play?

 

 

No whining sound from the trans area?

 

Still on stock suspension. No whining, clicking, or any other abnormal noises.

 

I checked the front halfshafts and did notice axial play on the passenger side, noticeably more axial movement compared to the driver side (OEM shaft) which barely moves. I did not notice fore/aft movement on either axle.

mmh. You could always do a 'free' test, which is to swap axles from side to side and see how it feels.

mmh. You could always do a 'free' test, which is to swap axles from side to side and see how it feels.

 

You inspired me to get back under the car. This time I checked both rear halfshafts and the left front halfshaft - all OEM Subaru - and they were all rock solid. The aftermarket front passenger side inner CV now has visible radial play - it actually can be made to slightly wobble at the diff without much force. It feels drastically different than the other three halfshafts. I have my original driver side halfshaft which was removed at ~60k due to a torn inner boot without much grease loss. I'm going to reboot it and use it on the passenger side. Pretty confident now that this will solve the issue. I'll report back once it's done.

Yeah definitely do that. I hope that will fix your problem.

Problem SOLVED. I rebooted my original driver side halfshaft which was removed from the car at around 60k miles. Thoroughly cleaned the CVJ, installed a new OEM boot, filled with 3 oz CV joint grease, reassembled and installed on the passenger side. It's like I have a new car - no more violent shifter shake at 25mph, no more buzzing in the steering wheel under load, no more vibration at 60mph. Now I know the crap rebuilt aftermarket axle was responsible for most of the change in feel of the car when it was installed at the same time as new control arm bushings at ~47k miles. The really bad vibration started after the aftermarket halfshaft had ~30k on it.

 

So count this as another piece of evidence to NOT use aftermarket halfshafts in LGTs or OBXTs, they can cause vibration even on manual transmission cars.

 

I inspected the aftermarket shaft after it was removed - the only noticeable difference compared to OEM is that the CVJ seems slightly 'loose' like it takes less force to change the angle. Either it wasn't properly filled with grease or the roller to cup fit is a bit too loose (or both).

Great to hear!