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1997 chevy "through seat" vibration

I have a vibration in my 1997 Chevy pick up that comes up through the seat. Before this vibration I had steering wheel only vibration and could tell it was the front left wheel. Called my usual, trustworthy shop. I did not talk to the service manager, he was on vacation. The owner’s son took the order and the next day I ended up with two new u-joints. Now I have the steering wheel vibration AND the “through seat” vibration. Service manager returned from vacation and took over. The left front tire finally got balanced, like I asked for initially. That tire was 4 ounces off. ALL tires got balanced. Steering wheel is OKAY, no steering wheel vibration, but vibration through seat is the same. The shop changed the u-joints with GM brand u-joints…no change. We put new tires on…no change. We put a set of different new tires and a set of wheels off a Tahoe…no change. Balanced the drive shaft…no change. In the midst of all this the rear seal at yoke starting leaking. The shop installed new seal and bearing. The shop did pay for the second set of u-joints, the second set of new tires and used Tahoe (but nice) wheels, the seal and bearing and balancing of the drive shaft. I was politely told that I am now on my own.
I can feel this vibration at 20 mph, kind of subtle, but noticeable. At highway speeds the vibration increases, not violent, but very noticeable. Once in a while (very seldom) the vibration on the highway is subtle like it is at 20 mph. Then I will go into a slow, long curve on the highway and that seems to set it off. Maybe this is a coincidence, maybe it would happen anyway if there were no curve and the highway were straight. 95% of the time the highway vibration is bad and I can see the console arm-rest shaking. The truck does shake a little when idling, but it always has. 1997 Chevy truck is four wheel drive, 350 with 185,000 miles. This is my work and fishing truck. I am up and down the highway to St. Louis, Mo. daily (construction worker) and that relentless “through seat” vibration is taking a painful toll on my back. Starting to panic. Do not what to address next and worrying about spending a lot of cash trying to do away with this vibration. Thought about selling the ‘ol truck, but I never did like handing someone my troubles. At the very least I would no doubt point out the vibration to any potential buyer. All in all I can not really afford to buy another truck. Work has been terribly slow and I need to resolve this issue. Any of you Car Talk - Tom & Ray Brainiacs have some assertive direction for me? Thank you so much, Red.

Have the u joints on the drive shaft checked for wear. The truck certainly is old enough and vibration in the butt is often in the drive train from there back. This is not unusual with older trucks and not a big ticket repair item if it is.

Since the u-joints are new, wear on them can’t be an issue. You didn’t say whether the truck had an alignment when the tires were balanced. That might be it. You might also have worn suspension parts, like bushings. Did the shop check your suspension?

@seabreez

Has anyone checked the motor mounts and the transmission mount?

A collapsed trans mount could cause a seat of your pants vibration . . . seen it

Without truck in hand it’s hard to say but some possibilities other than previously mentioned might be:
Driveshaft center support bearing.
Worn tailshaft bushing in the transmission.
Excessive end play in the rear axle shafts.
Engine miss maybe due to a dropping cylinder.

There are other things it could be and one would hope that the first three I mentioned would have been checked during the U-joint work.

If I remember correctly, these trucks had an unsolvable issue with the center yoke - a combination of driveshaft angle and U joints.

You might try looking at the pinion angle.

Maybe the differential pinion gears are starting to go away. Does it make any noise along with the vibration? Maybe the transfer case? Looseness in the front driveshaft at all?

Ok my buddy at work has this same problem. He has had it in numerous shops and spent a lot of money on parts. Drive train, wheel bearings, hubs, spindles etc. He finally asked me to take it for a spin. I drove around the parking lot and the vibration and harmonics were crazy when turning left. Turning right no sound at all. My first thought was the trans or engine mounts. When in drive with my foot on the brake I opened the driver side door. I was leaning out to see if I could hear noise under the truck. The vibration stopped, so I leaned back into the seat and it started again. We did this 3 or 4 more times. The driver seat bolts were loose and the padding between the seat rails and the floor was completely worn out. Rigged some rubber padding and tightened the bolts back up and problem solved. I know this might sound crazy and very simple, but sometimes its the simple fixes. We always look for the hard stuff. Hope this helps. 97 Silverado 4x4 extended cab all stock.

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I would hope that after 4 years they solved their problem.

Agreed, but still, I like the idea that the solution is something so unexpected and so basic. A doctor once told me that in med school they were taught the theory “When you hear hoof beats, think about horses, not zebras”. I try to keep that in mind. Sometimes the very basic thought is right. Other times the solution is more complex.

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Sorry I would think a zebra sounds like a horse, maybe a better analogy would be when you hear hoof beats think of horses, not tigers

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These tires… do they have any anomalous wear from their previous imbalance? Out-o-balance tires can develop vibration-producing anomalous wear that can only (maybe) be removed by “shaving” the tread, a practice that nobody does anymore except perhaps shops that sponsor race cars.

Try moving the wheels around and see of that changes the current problem. If it does, suspect the old tires. Post the results.

The idea is that hoof beats could be horses or zebras, but horses are a heck of a lot more likely to be running around here in America, so go with the ordinary choice before you look for the more exotic choice.

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Good explanation, but I was looking at a more global aspect at first kick at the cat I guess.

Med school, huh? That would explain why a physician I was arguing with used the phrase. He didn’t seem to understand that sometimes it is zebras - it just depends on where you are.

Yeah, that’s a problem. Sometimes it is zebras. Generally you try to start with the straight forward solution, and work through. When you’re dealing with a doctor you might have to keep them working at it, because, the way lots of insurance works, they get paid the same whether they see you for 15 minutes or an hour.