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Subaru Outback 2005 3.0 H6 LL Bean - Shaking/shudder at certain speeds

I have a 2005 Subaru Outback, 3.0 H6, LL Bean, 5 speed auto. 150,000 miles. No engine codes.

I bought it took two years privately. I didn’t notice during the test drive, but it has a shake/shudder at 18-20mph and at 55-60mph. Only at those speeds and only during acceleration. During light acceleration it isn’t noticeable, but hard acceleration particularly on a hill at 55-60 the car can shake quite violently. Probably 50% of the time the 18-20mph shudder doesn’t occur. It feels like the whole of the front of the car is shaking from side to side.

It doesn’t matter what gear it’s in - auto/auto sport/manual 1st, 2nd, 3rd, 4th, or 5th. So it’s not engine rpm related. I’ve had the transmission fluid flushed. I’ve had two sets of tires. I’ve had the prop shaft replaced. It still does it, and it hasn’t got better or worse. It happens both during straight line and turns.

The engine runs smooth and the gear shifts are crisp and smooth.

Our local dealer doesn’t know what it is (they suggested prop shaft) and our highly recommended local transmission shop doesn’t know what it is. All CV joints have been checked.

I think the big clue here is that it is very vehicle speed dependent; 18-20 and 55-60mph.
Could it be something to do with the transfer clutches? Does this transmission have an electronic control computer that needs an update? (This is a first year model). Is there vehicle speed sensor used by the transmission that could be bad but that wouldn’t trigger an engine code? Unfortunately I can’t disable the rear wheel drive like on the 4 speed transmissions, there is no fuse to do that.

Any ideas would be really helpful. Thanks.

Which one? Sounds like a front half-shaft.

I replaced the main prop shaft between the transfer case and the rear differential.
The problem has existed since I bought the car 2 years and 30,000 miles ago. Could a bad half-shaft give the same symptoms for that length of time?

That vehicle doesn’t have a transfer case.
Are you referring to the Center Viscous Coupler?

Yep, the back of the transmission. Sounds like you know a thing or two about these cars VDCdriver, any thoughts on what it could be?

I can imagine only one prop shaft in the vehicle. Should the vehicle owner call rear output a transfer case, a transfer unit or a center viscous coupler what is the solution?

Because of the two extremely specific speed ranges at which this problem occurs, I think that it has to be transmission-related.
However, if a highly-regarded trans shop has ruled-out the transmission as the source of the problem, then I don’t really know what else to suggest.

I would think so. It interests me that the two mph shudder zones are different by a factor of three…the number of bearings in each inner cv joint.

Yeah, the factor of 3 had occurred to me too. It has quite a significance in the realm of vibration analysis.
As I mentioned, a visual and static check of the CV joints didn’t reveal any problems, and they don’t make any clicking or other noises when driven, but I suppose there could be some sort of manufacturing defect with the half-shafts that could cause this. It seems to be outside of the normal realm of wear/failure.
In regard to the transmission shop’s diagnosis, they did say it might be the clutch pack that controls power to the front and rear, but that they also couldn’t be sure and wouldn’t want to embark on such expensive repairs that might not fix the problem.

My Local Subaru dealer here in Reno hasn’t been very helpful. Is there a Subaru national technical service office with experts who might be able to diagnose this problem?