Chevy & GMC Vans

I am having problems sourcing out an odd driveline vibration. It acts like a u-joint problem, but I cant find any bad u-joints in my (97 AWD) driveline.

The vibration shows up at the speeds of 55-70, only when there is some load on the driveline. If I am driving at a constant speed, no vib. If I lightly accelerate, without loading the engine, there's not much evidence of the vib., but if I push the pedal a little more and load the engine, without downshifting I get the vib. Also vibration when I let off the pedal. This is only noticeable at the speeds of 55-70.

This leads me to believe a u-joint prob, but I cant find any loose, or worn u-joints. There is a little play in the drive shafts at the transfercase, but not anymore than any other 4x4 I've ever owned. I am wondering if the front or rear gearsets are the culprit. Or maybe the transfercase has bearings starting to wear or some other problem. :-k

Any input or ideas will help.

Thanks

the best way to check the driveshaft u-joints is to remove it.It is sometimes very hard to detect with the shaft in place.
Your driveshaft my be a little out of balance. Check it visually to make sure it hasn't been hit,dented or thrown a weight.
That is where I would start before going into tail shaft bushing etc.

Does the vibration occur when you drop the van into 3rd at those speeds? Does it still do it if you accelerate hard at those speeds? if not, I would start by looking at the engine. It sounds like you may be starting to show signs of a misfire under light loads. If the vibration is there all the time, now it is time to look at the u-joints very carefully. You must remove the drive shafts in order to check the joints. If the joint is seized, you won't be able to tell just by looking and checking for play.

If they all check out good, replace the transfer case tail shaft bushing and seal. I chased an annoying vibration for 2 years before swapping those out. Once I did the vibration was gone 100%. It was also in the 55-65 mph range.

I had two broken teeth in my pinion gear and it only vibrated at 55 MPH. The guy that replaced the gear said the harmonics come together at 55 and that's why I noticed the vibration at that speed. Hope yours isn't as serious.

LiftedAWDAstro wrote:Does the vibration occur when you drop the van into 3rd at those speeds? Does it still do it if you accelerate hard at those speeds? if not, I would start by looking at the engine. It sounds like you may be starting to show signs of a misfire under light loads. If the vibration is there all the time, now it is time to look at the u-joints very carefully. You must remove the drive shafts in order to check the joints. If the joint is seized, you won't be able to tell just by looking and checking for play.

If they all check out good, replace the transfer case tail shaft bushing and seal. I chased an annoying vibration for 2 years before swapping those out. Once I did the vibration was gone 100%. It was also in the 55-65 mph range.

Do you think the engine misfire could still be the culprit, even though I get the vibration when decelerating at those speeds.

Probably not the cause. Did you remove the drive shafts to check the u-joints?

I just bought an Astro, and did my first 200 mile trip today, and notice a similiar vibration. Mine seems to also be affected by the road surface, it kind of 'magnifies' certain bumps in the road. On a smooth surface the van is quiet, on pavement with more wear (and semi truck grooves) I would get an almost 'bouncing' feel in the front. I have brand new Nokian tires, new rear end, Im going to look closer at my shocks...and something I suspected while driving was tranny or motor mounts...very hard to describe the vibration, I will post my findings later.

v

See if the idler arms are wore out. They throw a deceptive vibe...