As I'm sure you've seen me post for 27 years, I've had vibration issues with my car. The last solution I tried was inserting plastic poly diff bushings, though I could only insert the lower pair.

For a quick reference here is what the car is doing


The following video is before, and after poly diff mount bushings were installed.


I finally capitulated and seeked expert council. Replaced the flex couplers\guibos as they have started to crack, installed the upper half of the poly differential mount, tightened the slip adjuster nut a small amount and applied grease as one of the joints (I forget which) had dried out. We also made sure the driveshaft angle is within spec with the correct shimming, and it is. The result? Another massive improvement. Holy ****. I've never felt the car drive like this. All of the improvements from the lower diff bushing mounts have been multiplied by 3, and then some. It's incredible.

And it leaves one, final isolated problem. It appears the driveshaft is out of balance. We think it's the root cause. I've found someone who finally understands a) this car, and b) the correct diagnosis. We are getting phased, almost drone or "I left one window open" wubwubwubwub vibrations at 30, 60, 90mph. It's almost sub-audible, you feel it more than you hear it. I suspect it might even be present in small amounts at all speeds, almost contributing to a huge portion of my concerns by just this one item. I suspect it may be varying in phase and resonance as the suspension moves. I don't have video of this, I will try and get some, but you are able to see vibration in the driveshaft with your own eyes. Clear as day. And we think it got worse after we replaced the hardware mentioned above.

Here are one of the tests that was run, with a very nifty tool, measuring elevated propshaft vibration coming during the specific speed ranges we observed. P1 is the item of concern:



Once money allows I will be addressing this next. We feel extremely confident this is the root cause. Sadly, the driveshaft is NLA. The only option is to rebalance it, replace the carrier bearing, etc. The bearing has a small amount of flex, which (should be?) normal, but we'll be replacing that as well.

I'm gonna edit my other threads and point them to this - The car rode so horribly beforehand I can't believe the mechanics I've had look at it in the past (including Lexus themselves) said the car was kosher. I simply do not understand it. I drive sports cars on 30 profile tires that ride smoother on brand new blacktop pavement than my 400 used to. Unaccecptable.

 

It’s got an independent rear suspension system, so the rear differential is mounted in its position and the wheels move up and down. The driveshaft shouldn’t be out of balance, if it is bent or damaged or anything then that could be your problem, you would be able to inspect the driveshaft and find the damage. Usually the carrier bearing replacement will solve your issues, the driveline is easy to identify as your culprit because it’s going to change as the speed goes up and down, it’s not like it is lower like engine rpm. Your problem is most likely a bad bushing somewhere, there is probably 50 bushings in the back half the chassis,they made of rubber, and get hard and brittle and reward you with vibration

 

The pursuit of a perfectly smooth running old LS.. I feel your pain. I am sure when these cars were brand new they drove as if they were gliding over the road.

A first order propshaft vibration is what the vibration analyzer has picked up as the main culprit in that chart you shared. For each revolution of the driveshaft there is one disturbance or shake. This means anything that spins the same speed as the driveshaft could be the culprit (transmission flange/rear extension housing bearing, center support bearing, differential flange, diff carrier bearing, or the driveshaft itself is out of balance or misaligned). The center support bearing and misalignment is probably the simplest thing to check/replace. Hopefully when the guibos were replaced they bolted the driveshaft to the exact same holes in the diff and transmission flanges.

This video below from toyota is very informative. If you go to the 28 minute mark they describe driveshaft related vibrations and how they recommend solving it by using weighted bolts.

 

The center support bearing was one of the points of inspection, but there was no excessive slop found when they inspected it. That was a point of concern for them as well, but there was "normal" deflection as there should in theory be some sort of give. How much is normal, and how much was observed I'm not sure. At this point honestly I might just remove the exhaust cover and stick a gopro down there and go for a drive to see what she's doing.

One interesting point of difference I've noticed now is that shifting between reverse and drive can change the characteristics of what I feel. If I go from neutral to drive, I get one set of vibrations and a smooth engagement of the driveline. If I go from reverse, with torque on the driveline, to drive, I get a slightly different set of characteristic. Additionally, if the car sits for extended periods of time that can change too.

I'm still concerned about the front differential mounts causing too much flex. My theory (correct me if I'm wrong) is that If the driveshaft has slop in any of those bearings or mounting points, then as the differential moves with the suspension while you're traveling down the road, that could cause "oscillating phase" vibration as the angle subtly changing will have different frequencies imparted to the car. The rear diff mounts are suspect, and I've stabilized the front mounts as far as I can tell - but I've only got eyes on them while the car is stationary. I don't know how this system reacts while the car is in motion.

I've replaced a substantial amount of rubber back there with OE components. 3 lower control arms, the upper A-arms, all parts of the shock assemblies, the guibo couplers on the drive chain and finally the front differential mounts (poly). The main components which remain original are the pressed-in bushings on the knuckle, as you need to replace the whole knuckle to get them (or press-in poly replacements, but they aren't that bad yet). The other pieces that are original are any internal bearings on the transmission or differential housing themselves and the subframe mounts (I highly doubt these are a problem, what is this, a BMW? lol).

I am getting a click and creak at 0 mph when torque is first applied to the wheels from the axles\cx. One click, that's it. It sounds like it's coming from the cv spilne, above the rear brakes.

 

If you’re noticing a difference between shifting gears then that’s a bad trans mount. Pop the hood and have someone watch the engine then put it in reverse and press hard on the brake pedal and rev the engine, if it’s bucking up and down then you have a bad torque strut or motor mount or something. You can purchase all 3 for about $100, and they aren’t difficult to replace but if you’re noticing a difference while changing from forward to reverse it’s almost certainly the trans mount
I guess it could be the rear differential mount or subframe mount, you’ll find it using the method described above. When you rev it up in reverse whatever is bad, will be really sloppy

 

Trans and motor mounts were replaced 5 years ago, made one hell of a difference when I did. Thanks, I'll take a closer look at them, it would be the first OE part to fail but at this point I'm grasping at straws.

On that topic the engine "stumbles" when it's dropping through the rev range. It's not a misfire, but it imparts this weird rotational energy to the car that almost feels like one. In such an otherwise perfect car, it's a strange behavior to feel. You rev the motor up in neutral to idk, 2500, and as it's passing through about 1500 revs it gets this little tumble and shimmy that you feel. It's strange.

 

that’s how you would describe a car felt that had bad U-Joints. Something that is almost never used in civilian vehicles anymore. Since these trans and diff are static, they don’t need any kind of joint on the drive shaft, I think there is something called a guido joint or something like that, I can’t remember, but maybe yours is worn out? When you inspected the undercarriage, you didn’t notice any signs that something was wearing out? Usually you when any kind of joint or bushing or anything that spins will usually make a big mess, evidence that something has disintegrated.

 

When doing the "rev in neutral" test, is the vehicle moving or is it stationary?

 

Doesn't the LS400 have a two piece driveshaft? Wouldn't we call that a u-joint in the middle along with a slip ring and the support bearing?





I don't know what I'm looking for when it comes to driveshaft health to be honest. The local expert here said they thought it visually inspected fine, though they could see the rotational wobble I alluded to in the first post. They also found a very slight amount of play in the center support bearing, but they didn't suspect the bearing to be the root cause. They aren't sure if "this" bearing was supposed to have a very slight amount of play, but the shop foreman (who did this work, older guy) said he didn't suspect it to be the root cause and only mentioned it because of how meticulously we're going over everything.

We replaced the guibos\flex couplers, they were original and on their way out but they weren't the root cause either. We didn't discuss anything else that seemed amiss, other than the differential mounts I've been concerned about. I still haven't gotten around to getting a new video of them after the work we've done.

Stationary. Park or neutral.

 

This would point me in the direction of engine/transmission mounts, and not rotating assemblies (prop shaft/drive axles, wheels, etc.). But since the mounts were inspected/changed not long ago, I'd look at rotating items at the transmission and engine.

Two things that you may want to look at:

1. The crankshaft balancer; the inner pulley and outer weight ring may have slipped with respect to each other. You can check this quickly with a timing light. However, disabling the ECU controlled ignition timing at the DLC1 connector on the engine might prove challenging if the connector has no terminals in it (like my LS400).
2. The torque converter mounting fasteners. Has the engine/transmission ever been separated?

I don't know what the significance of the black bolt is. My guess is, it's an alignment thing. Strange how there is no instruction to mark the converter/ring gear/ black bolt location during removal.

 

It's good that you've replaced the eng & tranny mounts 5 yrs ago but that's where I would start, don't overthink it. I have 400k miles on my .98 LS, just replaced my eng mounts and it's silky smooth, I even have to listen closely when I turn the key to start it now, otherwise I wouldn't know the eng is running. GL.

 

when doing this test the vehicle is stationary, your left foot planted hard on the brake and your right foot revving the gas, put it in reverse and when you rev it up, a bad mount will be obvious, the engine and trans can start bucking and making clunking noises etc.
the all modern cars have switched to CV joints, constant velocity joints on the driveshaft and half shafts, they are like a big bearing and not U-Joints. They can certainly go bad. U can usually identify a problem with these parts if you just grab the drive shaft in a few different places and shake it, if there is a bad joint or bearing anywhere it’ll start wobbling. Usually a good visual inspection of these parts will help you identify the problem. Any signs of damage, or lubricants covering an area that they shouldn’t, torn boots or signs that something has been run over, hopefully you can diagnose the problem without throwing a bunch of parts at it, or racking up a big shop bill. These cars were pretty well built, after 35 years, there shouldn’t be any surprise problems popping up.

 

He said PRETTY WELL BUILT...🚅