Oldsmobile/Buick Repair: 99 chevy Suburban vibration, overdrive test, cv axles

Question
Hi van,
  We will see if you can figure this out cause no one else has a clue. Not even GM, trust me I called them.
 I have a 99 chevy suburban K1500, 5.7L, 4wd, with 118,500 miles.  It has a vibration that starts at 65mph and smooths back out at 80mph.  I can feel it in the steering wheel and the vibration changes if I move the wheel. Also it is much worse if I have a full gas tank (42 gallons). (Note the gas tank is behind the rear wheels not side saddle or center.) Also I have done the rpm test.  If I run the truck in overdrive when it is vibrating then shift to 3rd the rpm rises but does not affect the vibration. Also shifting into 4wd does not change the vibration either.
   So far I have put new tires on, had them balanced twice. and rotated. Replaced brake rotors and brakes, replaced CV axles, Replaced U-joints at all positions in the rear. I used a pry bar to check steering components, Serviced the tranny, checked all diff fluids and transfer case fluid, cleaned the throttlebody.
   If you have any Ideas I would love to hear them.

steven

Answer
Hello Steven,
I assume you have eliminated belt driven accessories by running the engine at about the same RPM, while not moving.
Actually, the Overdrive/non overdrive test you did kinda eliminates that, and narrows it to a MPH symptom, rather than Engine.
You could get going in the 70 MPH range, on a straight highway, and shift to neutral, and let the engine idle also. I'd bet the noise and vibration doesn't stop then either.
So, that leads me to five things I can think of.

1. I would completely remove the front driveshaft, and go for a ride. Take all the little U joint straps and bolts off the front yoke. Cause even though you are not in 4 wheel drive, that driveshaft is probably rotating, and may be bent, out of balance, or have a bad U joint.

2.If there is any way you can get it up on jackstands, or a shop hoist, and stick it in gear and run it, while watching the rear driveshaft to see if it wobbles. (Bent, or out of balance). Watch the rear wheels also.

3. A bent wheel. I know you had the tires replaced and balanced, but a tech can get them to show in balance, even if bent. But a bent one will vibrate. (PS...if there is a bent one, a good tire spin balancer operator should have caught it, but...)

4. If all those things check out and don't fix it, there is a remote possibility something in the front differential is causing it. There is a set of spyder gears, and a ring gear and pinion in there that rotate, or work all the time while driving, driven by the left front axle shaft, which is directly connected. Your shifting to four wheel drive should have eliminated the spyder gears, but wouldn't necessarily eliminate the front driveshaft or differential.

5. The chain and sprockets in the transfer case could be loose and vibrating.

That MUCH worse with a full tank of fuel also throws the possibility of rear wheel bearings in there.


Van