Seatback vibration ...

[350bigblock]

So I have this really annoying vibration in the seat backs mainly. You can kind of feel it driving around normal roads but wouldn't notice it if you weren't looking for it. When you get on the highway though ... At around 100kmh (60mph) nothing at all truck feels fine , once I hit 110-120 (75mph) the seat backs will start to shake intermittently , sometimes I'll e cruising along at 120 no problems feels like a Cadillac. Than without changing speeds or anything the seats with Shake for a few seconds maybe 20 at most then it goes back to driving great. Noticed when I let off the gas it seems to go away pretty much instantly but when I shift into neutral on while it's doing it no difference. It's a 1999 silverado 1500 4.8l g80 rear diff 2wd 8ft box single cab. Have replaced front wheel bearings and rear. Tires are road force balanced. Any help would be great thank you

[350bigblock]

Truck has 186 km if that helps at all ...

[Chevy Joe]

In most cases isn't the most likely cause driveshaft or rear dif related? have you ever changed the rear oil?

[350bigblock]

If I knew what the problem was I wouldn't be asking you guys , yes fluid has been replaced recently.. Could It be the wrong fluid ? What do you recommend using

[350bigblock]

I was leaning towards dr

I was leaning towards d shaft myself but thought something like that would cause the vibration/shake all the time no ?

[NARDA1]

Is it a one piece drive shaft or does it have the two piece with a carrier bearing? If it's the one piece i would check the drive shaft for balance. If it's a two piece I would check the carrier bearing and drive shaft balance. Also I believe they used to make a spacer for the carrier bearing to drop it down about an inch or so to change the angle of the shaft.

[350bigblock]

It's just a one piece dr shaft , that was next on my list anyway to have it balanced and replace u joints. Wouldn't that be something that would cause it constantly though ? Not just every once in a while . Putting a new exhaust on tomorrow will let you know how that does , I was just thinking that because the prev owner straight piped from the dual cats back out the side before the axle ... That maybe it's causing a droning vibration ? But again I thought exhaust related would be constant

[Chevy Joe]

If I knew what the problem was I wouldn't be asking you guys

 

My comment was asking others here what they thought too (like a consensus) By the way, it says you liked your own post

[350bigblock]

My bad if I was rude was just frustrated with my truck after working on it all day with no success ? ... This may be a stupid question I'm new to the truck scene this is my first.. But anyway, when shifting from reverse to park with my foot not being let off the brakes at all , the driveshaft is making a decently loud "clunk" at least I hope it's that and not the transmission. U joints ? How much noise is it supposed to make, I'm sure some is normal but it's pretty obvious. Also , sometimes when just cruising along at slow speeds not giving it gas, than I'll give it hard gas I'll hear the same clunk type noise just not as loud. Could it be my problem for my seat back vibration ? It has gotten worse it's almost always there now. I was also thinking maybe my rear shocks since the highways around here are horrible. Let me know what you guys think, I replaced rear diff fluid , transmission fluid and put a new muffler on it which is a lot less "droney" but didn't solve my problem. Thanks.

[350bigblock]

?

[muddkatt]

The clunk may very well be the u-joints and the cause of your vibration. That is where I would start my search. BTW, gear oil weight being off a little will not cause the vibration.

[350bigblock]

Thanks buddy , appreciate a real answer for once instead if a consensus ??.. drive shaft is getting serviced and balanced today , fingers crossed it solves my prob. Only other thing I'm thinkin is tires but they are road forced

[Chevy Joe]

appreciate a real answer for once instead if a consensus

 

You are a real funny guy, Most people know that u-joints would be what you have to "fix" on a driveshaft, unless you bent it some how, So glad it was spelled out clearly for you now!!

[350bigblock]

Except it's not the driveshaft , just had u joints replaced and balanced which is was off. But the vibes still there

[bgbdwlf2500]

almost sounds like what my 5.3 does when afm kicks on and off but as far as i know the 4.8 doesnt have afm