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I'm running out of ideas to fix a persistent vibration in my auto H6.

Symptoms are:
1 Occurs at various speeds, most noticeable above 40kph.
2 Putting it in neutral does not seem to help, though perhaps a little.
3 Vibration is reduced when under load or accelerating.
4 Whole car vibrates, but not severely, can't locate source.
5 Unaffected by engine speed.

What I had done:
1 Reconditioned transmission installed a year ago and seems very good. Oil levels good and clean.
2 Tailshaft balanced and U joints replaced.
3. New front and rear wheel bearings.
4. Used rear diff installed.
5 New aftermarket front driveshafts.
6 New front control arm bushings and ball joints.
7 New front sway bar D bushings and new sway bar links
8 New front struts.
9 New front strut top bearings.
10 New rear bushings for rear upper control arms
11 New tyres and 4 wheel alignment.

Any ideas?

Rod

Have a close look at rear driveshafts. I see you've done the front ones. I've seen them where they don't make any noise, but are worn enough to give the inconsistent vibrations that are a nightmare to find.

Perhaps it could be the rear driveshafts, but they seem fine with little slop. I was thinking of making a video of them with a GoPro from underneath while driving to see if they run true.

Also, most of the things were done after the vibration started, in an attempt to alleviate it. Only the front wheel bearings and bushings were done earlier. Changing the front axles had no affect on it. Also, there is no vibration from the engine, or at slow speed, say below 40kph. Revving the engine at speed while in neutral has no affect, nor does using a different gear. Only the usual Subaru vibe at idle in drive.

There is no significant corrosion on the wheel hubs or inside of the wheels. I've cleaned those areas anyway. Changing the wheels around has no affect.

Rod

Its a shame that numbers 6, 7, 8, 9 and 10 would have had little effect

Because it grows with speed it's definitely something that's rotating. The frequency will indicate if it is engine or gearbox related or driveshaft/unsprung related.

If it's high enough frequency to sound 'buzzy', then its tranny/engine. If the frequency changes with gears then its something pretransmission, so engine or torque converter etc. If its dependent on road speed it could be some other part of the transmission or driveshaft. I know this isn't your symptom, but I'm just putting that here to paint the whole picture.

If its a relatively low frequency, and its dependent on road speed, it will be diff/driveshaft/unsprung related, more likely unsprung. Sounds like this is what you're describing.

Because it reduces under load, I am thinking its unsprung. Basically at low drive load all the bushes are unloaded in the longitudinal direction so the vibration can build amplitude and everything can 'flop' around. When you're braking or accelerating everything is getting held to one side so its all firm. Bushes are designed to have this flex/movement.

Driveshafts and CVs are pretty much on the axis of rotation, they need to be pretty farked to cause noticeable vibration. If they were bad, you'd see noticeable movement when you spin the wheel around, or be able to make heaps of movement when you pull on them hard.

Seeing you've changed shafts and diff, and tyres, and bearings, then i would check for a bent or otherwise misaligned hub or dirt etc stuck inside one of the ventilated discs. There has to be some unbalance somewhere, and its up to you to find it. Spin each wheel and make sure they are running true. Check all nooks and crannies for dirt. Seeing as the bearings where done prior to the vibration starting, its possible they were not pressed in fully (not sure if those are pressed ones) or were manufactured badly. This will show up as wheel misalignment when spun. Or play. Same for foreign material between some of the bearing/hub surfaces.

I had a set of tyres once that for some reason wouldn't balance. Like they ballooned out when spinning or something. Could try someone else's wheels to totally eliminate them as a cause, but you should have noticed a difference when you swapped them all around.

Good luck, vibrations suck

Thanks for the ideas. The car was running increasingly rough in the last week or so, which made the vibration much worse. So yesterday I did the arduous task of changing the plugs (and using an injection cleaner). Engine is now silky smooth and the vibration is much reduced, though still there.

Part of it might be the rear bushings of the rear diff. When I replaced the diff, I could see one of the bushings was shot, but I could also see that it would be a major pain to remove and replace them as the carrier seems to be welded to the body. I did some research and ordered the the Whiteline KDT927 bushings that can be pressed into the gaps from the rear. This might help. Should have them in a few days. Agreed that the vibration may be due to slop in the drivetrain mountings as it goes away under load or hard acceleration. It's of the order of a few hundred hertz at a guess, mostly felt in the seat and body, very little in the steering.

It's most obvious on the level at constant speed or going downhill at 30-50 kph, also around 90 kmh, not a simple increase with speed.

I'm confident that the bearings were pressed in properly. I'll spin the wheel by hand to look for play soon.

Rod

Update: the Whiteline bushing arrived today and I put them in with high expectations, but the reverse happened - the vibration seems to be much worse now! The new bushings are pretty stiff, so it seems that the old half broken ones were absorbing vibrations rather than causing them.

I checked that the rear wheels and tyres spin true.

More background: the car first developed vibration at speed about a year and a half ago and I correctly diagnosed the tailshaft. The U joints were worn out so I had them replaced and the shaft balanced. This completely fixed the vibration. The new vibration started about a year later with similar symptoms, so I took the shaft back and they did a very minor rebalance for free, and we checked that the U joints and centre bearing were all ok. However this had no effect on the car, and despite all my fixes, the vibration has steadily gotten worse.

Running out of ideas here...

Rod

If those bushings made it worse then that could be a hint you're in the right area?

I can't be much help with this later gen stuff but I do know the pain of a persistent irritating problem that seems to have no source.

Dude, stop replacing things that don't spin

Rear shafts. They may run true but at speed there is considerable forces acting on the shaft and joints. At cruise is when you'll feels shafts go out of balance, if both are stuffed you'll get an ossillation effect as they balance themselves out then both go out of balance. I had this with my front shafts - cheap replacements that didn't last long.

What brand of shaft did you use on the front?

Have you checked your tyres for defects? Unlikely but worth looking at.

Cheers

Bennie

I don't think it is the rear shafts. I made a video showing parts of the tailshaft and right rear driveshaft as I drove on the freeway, and they ran true at all speeds. I had also inspected them by hand when I did the rear bearings, and they seemed good. They don't get subjected to the angles that the front shafts do of course.

I've since had a shop put it on a hoist and run it and the rear transmission mount was clearly stuffed - it was in two pieces actually. There was also some run out on the left front driveshaft (that I replaced a month ago) as well. Wheels all spun true. I had the transmission mount replaced, and this improved it a bit, then I put in a new third party front driveshaft and this improved it more, especially at speeds below 80 kph.

Still, the vibration is quite noticeable at higher speeds.

If the rear shafts are the originals (I'm guessing they are) take them out recondition them and put them back in ... "dry" factory grease is great for making vibrations.

TOONGA