Heavy vibration at highway speed under load

[BRetty]

All,

Per the title, my '73 C10 (350 sbc, th350c (but TCC not wired up)) has very recently started to vibrate severely on the highway. It seems to maybe start around 35mph, jumps up around 45 and is shaking/rattling through ~75.

It is only under throttle -- if I let off the throttle it coasts and vibration is gone, press the gas it is back. The freqency feels like RPM-driven. I have a secondary vibration also, the interference makes the sound surge and pulse. (This might actually be the 170 and 5 freeways natural bumpiness frequency.) Whatever it is, it keeps unscrewing and spinning out my air cleaner stud, all the way out.

A week or more ago, I thought I felt a low-freq thump at idle - felt like an out of balance condition, felt it while parked and also in motion. There was a distinct pulse in my exhaust I could both hear and feel at the tailpipe. That low-level thump is still faintly there, but sudden driveline vibration more pressing.

My first thought was U-joints. Sat, I took her to the shop on the corner with the U-Joints I had got months ago, but I had the wrong size. The mechanic, pretty lazy honestly, said the u-joints looked good. He suggested tires/wheels out of balance, although I don't feel it when coasting at 60. He (allegedly) checked the flywheel/TC bolt-up saw no cracks nothing alarming.

Last week I worked on carb/fuel line. I put a vacuum gauge on and it read perfect 19", rock steady. So steady it seemed suspicious. I also swapped out my alternator last week b/c voltage seemed to read low.

So reading other stores of similar vibration, I can think of these possibilities. Any input on probability of these will be appreciated.

-- Harmonic balancer
-- flywheel / torque converter interface
-- engine mounts suddenly crumped
-- transmission has shifted / trans mount
-- driveshaft may have lost a balancing weight
-- U joints are going suddenly
-- DIFFERENTIAL IS FAILING (GM 12-bolt)
-- wheels/tires/alignment/suspension (suspension needs a rebuild and all soft bits replaced, certainly)
-- misfire or preignition under load
-- timing is somehow farked.

Thanks for all the knowledge I have already gained,
BRetty

 

[BRetty]

Following up, the vibration under load/throttle, that stops when coasting, makes me think my differential is failing. I surely have almost 1/2 turn of slack/lash in the driveshaft and often a hard CLUNK going into reverse or forward from parked.

I'm going to pull the plug and check the gear oil and top it off. It was last inspected/filled ~3 years ago.

Hooper's Rear Ends are the best around, they are very up-front about inspecting and recommending service. If it sounds like the diff is likely the problem, I will take it to them tomorrow morning.

Worst case, total rebuild on-vehicle, new gearing, same axle tubes, they quoted $1,200.

 

[Vbb199]

I'd say its rear end if the behaviour changes with decel

Everything else on the truck will persist no matter the load/speed/rpm im afraid
Rear end noises and failure is such an anxiety inducing thing lol

 

[fast 99]

Really sounds like a driveshaft problem, balance, phasing, angles, loose bushing, ect.

 

[SquareRoot]

It's gotta be a wear item if it just started. Bearings or bushings. I had a very similar issue that took me months to figure out. A loose bearing on the output shaft of the t-case. I know you don't have a t-case but I'd surmise the same issue could happen with the output shaft of the transmission or the pinion shaft of the differential. And if you really have 1/2 turn of slop in the driveshaft, that's an issue. If the bushings are severely worn, it's possible the pinion angle is changing under deceleration. Typically the angle is 3 degrees off static and zero's out (or close to it) under load. And yes, it's under load at cruising.

 

[BRetty]

@SquareRoot ,

My understanding was that rotational slack in the driveshaft meant worn ring & pinion gears in the differential.

ETA: also, the condition of noise under load / no noise in coast, incriminates the pinion/ring interface?

When useless local mechanic had her on the lift, I thought the balancing weights cemented on the driveshaft looked less than totally cemented. Could I have thrown a weight?

So maybe the first move is to the transmission shop to inspect the tail and the pinion bearing and shaft? Caesar owes me a favor and I am leaking ATF anyway.

Thx,
BR

 

[Bloodhound1981]

I'm currently battling the same type of thing. 1981 C10, with a new Chevy Performance crate 350, a 1990 Corvette 700R4, and a 12 bolt posi with about 30k miles on it, which sat for about 12 years. Re-balanced drive shaft, new u-joints. No shakes at all until you get to about 67 mph. Then it kinda goes away, and then over 80mph it gets way worse. I re balanced the tires, re aligned the front end, changed the rear diff oil. Same problem. I finally brought it to a dyno shop and they confirmed it was happening on the dyno. Turns out, somehow, my transmission cross member was totally loose. Now it remains to be seen if the loose cross member was causing the vibration, or if the vibration caused the cross member to loosen up somehow. Hope to know soon!