vibration 1990 chevy van

[Steve Demeter]

Yes that is my tow vehicle, 1990 G20 chevy van, 350, Th700 or whatever the 4 speed automatic of the time was.

So we were on the way to MO yesterday (lucky only about 15 miles form home) going around a ramp at about 45 and the vibration started like we were going over cobblestones. I drive that way to work every morning so I knew the road was not that bad. Immediately started checking mirrors to see which tire was shredding. None. Buddy was driving and got off the throttle and vibration immediately went away. Got back on and vibration immediately returned. Not wanting to get stuck roadside on !70 in the nutty traffic for the holiday weekend we stopped while there was still some merge lane left. Crawled under van and it looked like the front U joint was near coming apart. Called AAA. Called buddy's buddies and found an available truck to get trailer home.

Jacked van up today and checked U joints by trying to find the play. Using pry bars there is very little to none. Tailshaft bearing on tranny has a bit of radial wiggle. Put it down and took it for a spin

On surface streets up to 40 no vibration at all. On highway up to 60 no vibration at all. Put brakes on and open throttle in an attempt to simulate towing trailer, vibration is back big time.

Any guesses as to what to check next??

[Dee]

check tie rod and steering rod heims and steering box tightness
Dee

[J Leonard]

fan blade or converter flex plate

[Dee]

You might check for a warped front rotor

[Offcamber1]

Then go from there, but eliminate that variable first. They don't always show play, but I guess that if you drop the driveshaft you will find one that does not feel normal.

My Grand Cherokee did a similar symptom--the vibe was not there through the full range of speed but when the u-joints were pulled apart, one was very obviously junk.

The flex plate is another good guess, but harder to get to.

[ExKartracer]

Where do you feel the vibration? Steering wheel shaking? Brake pedal shaking? Seat/floor shaking? Steering wheel can shake violently when the front rotors warp due to heat and then settle down when cool for example. Trans/engine mount shifting under heavy throttle also possible. Driveshaft is usually felt thru the seat/floor/tunnel.

[Bardahl77]

Does that have a two-piece driveshaft with a carrier bearing? Might check the bearing if so.

[BeerBudgetRacing]

When you lift the van, there may be different tension placed on the driveshaft and joints than when it's on the ground with a tow load.
Can you drive it up ramps to get under and recheck the joints?
Had that with an old Toyota. Jack it up and everything seemed solid. On the ground ... slop.

[roadracer]

Bets are on for a front wheel bearing. Sometimes it is OK, sometimes not, but gets violent when out of shape. Good luck with this and let us know how it turns out.

[Steve Demeter]

This initially happened when easing back on power after coasting down to go onto an exit ramp onto another interstate, towing the trailer. Immediately after backing off the vibration stopped.. Back on power and it immediately happened again. There was absolutely no warning no early signs or anything like that

Felt in steering, floor and the van felt like a tire was shredding or we were driving on cobblestones.

One piece driveshaft. No center u joint. But it is kind of funny: The ends ( only the yokes at each end) look like cast aluminum and the center ( main tube ) is some sort of black anodized metal. Never seen that construction on anything else.

After checking u joints the next day:

Without the trailer, I could accelerate up to 60 from a stop pretty fast , much faster than normal with no hint at all of vibration.

Braking hard from speed without trailer and off throttle like normal, produced no vibration.


Used brakes and power together to try to simulate the load of pulling the trailer


Flex plate is one thing I had not thought of.

[Charlie Schmidt]

Just for grins, check your left side exhaust pipe at the flange to the manifold. My Suburban fractured the pipe just below the flange, and under heavy load it opened up a crack. Sounded and felt like the motor was coming apart....but could not be duplicated at home, required having the trailer on and yanking a lot of throttle to torque the engine over enough to open it.

[Daryl DeArman]

I would check the rear leaf spring shackles and u-bolt mounts at spring perches on axle. My guess is under load/off load the pinion angle is changing significantly introducing a driveline vibration.

[Bardahl77]

Got a GoPro you can mount underneath and see if anything is doing bad things at bad times?

[BeerBudgetRacing]

Did someone mention motor mounts yet? Under load things could be shifting.

[Ralph Z.]

Motor mount. You'de be surprised what this could cause.

[Steve Demeter]

Thought of spring attachments to frame and rear axle housing and motor mounts. Have not checked yet.. Keep em coming and Thanks Again!!!!

[DaveW]

I would check the rear leaf spring shackles and u-bolt mounts at spring perches on axle. My guess is under load/off load the pinion angle is changing significantly introducing a driveline vibration.

That would be my guess, since it also sounds to me like the pinion angle is changing with load - broken spring, shackle, bushing, etc.

[Magic Motorsport]

I had something similar on my '98 GMC Minnie Winnie. Changed a heap of stuff including the shocks all round. It is a twin rear wheel and it was one of the inner wheels had lost all its air from a faulty valve. Only on certain surfaces, certain speeds, usually down hill on a trailing throttle and always on a Sunday night late with another 150 miles to go after a bad day at the track.. Hope that helps. Nick.