RodA

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 don't think it is the rear shafts. 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 50 mph. Still, the vibration is quite noticeable at higher speeds.

Thanks for the idea of a hose clamp on the driveshaft. I'll try that tomorrow. Don't think the the front of the driveshaft is worn - looked fine a few months ago when I had it out. I can't put the car in FWD only. There is no fuse location for that - early H6 models sold in Australia didn't have that option. I have long thought of putting the car on blocks and running it to observe the vibration, but it seems risky as the car would need to be supported by the suspension arms so driveshaft angles are normal, and I wouldn't dare get under the running car in such a situation. Maybe a GoPro video makes more sense. There is no clear relationship between the frequency of vibration and road speed. It seems fairly constant, though it may go down a bit at low speeds.

The replacement differential was definitely from an H6. It didn't improve the vibration, but did fix a slight gear whine (60k vs 240k miles). The shop that fixed the tailshaft was the only one in town that could do staked U joints, and they have a long history of doing such. As mentioned, it completely fixed the original vibration when the U joints were obviously bad, and when I took it out a year later, the joints seemed like new. They dynamically rebalanced it by tack welding a small washer on the tube, so it was very minor. So it seems possible but unlikely to be the problem.

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. No real difference with turning or braking. No noise either other than the shaking making the cabin trim vibrate. 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 20-40 mph, also around 60 mph, not a simple increase with speed. Update: the Whiteline bushings 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 tires 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

I should have made it clear that almost all the things I have done 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 30mph. 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. This is a Mediterranean climate here, so rust is not much of a problem. Changing the wheels around has no affect. Perhaps it could be the rear driveshafts, but they seem fine with little slop. I was thinking of making a video of them from underneath while driving to see if they run true. Rod

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

I'll do the test on the engine mounts shortly. I thought the FWD mode might affect if it is the center mount as the high starting torque on the shaft might cause it to flex sideways, whereas when just driven by the rear wheels turning, there would be no torque to speak of. Car has done 180k miles. Wouldn't a bad U joint cause vibration and perhaps noise all the time? The shudder sounds like a rotating member lightly hitting something - and fresh wear points as noted. I'l post a photo later. Thought of mounting my GoPro underneath to see if it records anything.

My 2001 H6 auto Outback has developed a mild shaking/noise when taking off from a start. Once underway there is no issue. There is no apparent bearing whine. It seems to come from the propshaft or rear or the transmission. Does not happen with gentle acceleration. I looked under the car and could see the rubber bushing for the center bearing seems quite soft and the shaft sits towards the bottom, not centralized. I was wondering if it's just a matter of replacing that rubber support? There appears to be fresh scraping on the shaft where it might be touching the heat shield on take off (could not see clearly as I didn't have it on jackstands, just crawled underneath). This model does not have a FWD fuse, so I can't test it with no rear power. Any ideas? Rod

Thanks for the replies. The car has VDC and I tried putting in a fuse to put it in FWD, but nothing changed. Should the VDC light have come on? I wasn't convinced that the fuse made contact as there is a raised green plastic strip in the middle of it, preventing the fuse from being pushed home, and running below the center of the adjacent relays. The fuse barely went in at all. So I pushed a bit a wire firmly in instead, still no difference. I recently changed the fluid to synthetic and replaced the filter. I pumped a fair amount out of the transmission with the engine as I filled it until it was mostly clear. It improved it quite a bit, engaging into drive much faster then before. I was jumping to false conclusions about the cause of the jerk on slowing, so thanks for the suggestions. I just did a test drive, and there is no jerk in 1st or 2nd (or neutral as I mentioned), but there is a consistent jerk in 3rd and 4th. Does that suggest a downshift or the torque converter clutch is not unlocking at the proper time? I'm guessing that in 3rd, the transmission does in fact shift down when coming to a stop.

My car has done 150k. No obvious slop in the universals.No CEL or flashing ATF lights on startup. I found the same issue as that video with the rear wheels off the ground (note, that's not my video).

Does anyone have experience with apparent looseness in the H6 center diiff/transfer clutch? I have the slop shown in this video (though this is not my car): The effect is a noticeable jerk when slowing down, just before the car stops. If I shift it into neutral before slowing, there is no jerk. From the photos I've seen, it looks like a sealed unit, until the multiplate units on other models. Is it possible to rebuilt it? thanks Rod

Thanks, I'll try that. I think all the oil drained out of the second hand one, but I can try measuring the amount of the one in the car.

The A/C compressor on my 97 OB 2.5 is bad and I got another from the wreckers. Anyone know how much refrigerant oil should be added to it? Do I need to purge the part first? Rod