truck problems -need advise

[Erik the Red]

Okay, so here's what's up; I have a '00 GMC Sonoma 4WD (automatic trans) pickup and recently I've noticed a shake developing. I changed the pads and rotors last month, but this shake only started a week or so ago.

It only shakes whenever the throttle is applied. Seems to shake harder with higher RPMs, but not by a whole lot. As soon as I let off the gas, there's a second's pause (the time it takes to disengage perhaps?), and then zero shake. She coasts as smooth as glass.

Is this something for which I'm going to have to shell out big bucks to fix? I'm a novice shadetree mechanic, and can deal with minor stuff on my own. Does anyone have any suggestions as to what my problem could be?? Thanks in advance.

-Erik

[lparker]

check for bad u joint (binding)

[trbig]

Probably a no brainer... but you'd be surprised. I would check that all your lug nuts were tight to begin with. Then I would jack up one side at a time and tug on the wheels and check the tie-rod ends, as well as that the bearing (hub) nuts were snugged back up after rotor replacement. A cheater bar or pipe under the tire and slightly lifting will check the ball joints while you are there. Since you recently had work done to the front end, that's where I'd start.
Is there any noise you can hear? Does it feel like it's coming from one tire in particular... or even tell wich end it seems to be coming from? (Does it shake the steering wheel or your a@@) If you are accelerating... and it's shimmying... does it go away if you slightly apply the brakes?

Most 4X4's are bad about cupping the front tires out also, but it will usually vibrate all the time when this happens. This could be something major (Like tranny/ transfer case.. etc) to simple like a u-joint or a glob of mud or something stuck the inside of the rim, throwing the balance off. (Or balancing weight falling off). I hope your luck is better than mine and it's something simple.



Tod

[prometheus578]

U joints: truck level, tires blocked, in nuetral. crawl under and grab the drive shaft. can you spin it... do the u joints wiggle? stick a screwdriver through the ujoints and see if there's any play.
Motor mounts: open the hood, start the truck. foot on the brake and put it in gear. give it a little gas... does the engine want to jump out ? a little movement is fine, as the mounts are rubber and steel, but if you can make the motor jump, then the mount's broken.

[CWilliamson]

Don't forget the rack & pinion steering. The bushings can wear out and throw the alignment out of wack.

Clark

[prometheus578]

would seem to be a driveability issue...

It only shakes whenever the throttle is applied. Seems to shake harder with higher RPMs, but not by a whole lot. As soon as I let off the gas, there's a second's pause (the time it takes to disengage perhaps?), and then zero shake.

This would rule out stuff such as wheels, bearings, etc.
Action on accelleration points to engine, tranny, and drive train.
Make sure you're running on all cylinders, (not 7, like my Blazer, which has blow-by so bad that I have the dipstick duct taped to the tube so that it doesn't shoot out and spew oil on the firewall. Broken ring or hole in piston, haven't tore it down yet, but still run it) check the plugs.
Nice blazer... check for broken tranny mounts, which could act the same way. (Been there, had that) Put a floor jack under the tranny and see if you can lift it offa the mounts.
Check that the tranny bell housing is tightly attached to the rear of the engine and hasn't worked it's way loose (been there, had that)
remove the inspection cover on the tranny torque converter and make sure the bolts from the convertor to the flywheel are tight.
When you're putting strain on the drive train, it's acting up.
Make sure the 4WD system is fully engaged or disengaged. (been there, had that)
How's your timing? "Check ignition, and may God's love be with you." "This is Ground Control to Major Tom."
Rear end usually doesn't cause a shake, but a drive shaft will.

[Erik the Red]

Thanks for the ideas guys. I'll try to check tranny and drive shaft tonight after work if I don't get out too late. I hope it's something as simple as a bolt working its way loose or something along those lines.

Does the drive shaft stop spinning if the throttle isn't being applied, or is it a constantly moving piece? I'm assuming that it'd only spin when gas is applied --> makes the engine work --> outputs power to the driveshaft --> makes the wheels turn. Is this correct, or is my understanding flawed?

I'll update when I crawl around underneath some. Thanks again all.

-Erik

[LoserShoes]

Does the shake follow engine rpm? That is does the frequency if the shake change with engine rpm when the tranny shifts or is it more or less the same all the time?

Had similar problem with my Explorer, changed plugs, wires and was about to check start checking injectors, when I crawled underneath to find that just one journal on the rear ujoint had gone bad. Made a vibration that felt like a bad cylinder.

Steve

[trbig]

If your tires are moving, your driveline is spinning. A bad U joint will usually sound like a kid playing with a rusty gate...over and over.... when you take off. But not always!


Tod

[prometheus578]

"I've been shafted!"

Does the drive shaft stop spinning if the throttle isn't being applied, or is it a constantly moving piece?

Drive shaft spins if the truck is moving. Axles are geared with the rear end, which is geared with the drive shaft, which is geared to the tranny. Tranny has a neutral position, which allows the engine to be running, but the truck not moving. When the truck moves, it is because the driveshaft transfers power from the tranny to the rear end.
When U joints go bad: U joints are the same design as the Universal joint that you use on the end of a socket wrench. Is able to tranfer power at an angle, instead of just a straight line. The u joint has cups at the ends with greased needle bearings in them. When joints go bad, it's because for one reason or another, the needle bearing are shot. (grease dried up, shock from slamming it into gear crushed the bearings, tightening the u joint too much and deforming the caps causing premature wear(been there, dumb that)
This damage causes a lot of slack in the drive train at the joint as the bearings disentigrate. Can often be heard squeeking when moving, or a 'tinny' sounding clicking (Vibration magnified by the hollow drive shaft)as the truck is moving. The drive shaft now, due to the slop in the bearings flops around and is no longer nicely in line from the tranny to the rear end, hence the vibration. Shaft now being out of balance. Have you run over anything lately? A dented shaft (they're a hollow tube) is now out of balance and will vibrate.
When throttle is applied, you're putting strain on the system and noise and vibration can appear. When coasting, the rear wheels are driving the shaft, not the engine... no load or force involved, noise and vib disappear.
Of course, your prob may not be u joint related, but if it is, this may be why.
Then again... not running on all cylinder could do the same thing. Truck would seem to be idling nicely(though it really isn't) When load is applied(accellerating), vibration is more pronounced as the engine stumbles to keep up with the demand. (coasting, no demand, no vibration)
Loose tranny mounts cause the rear of the engine/tranny to bounce around under load.
Loose motor mounts cause the engine to bounce around as the torque going to the rear end wants to make the left side of the engine raise up.

[prometheus578]

A bad U joint will usually sound like a kid playing with a rusty gate...over and over.... when you take off.

You can also feel a "clunk" when first putting it in gear as the shaft spins a few degrees by itself before it takes up the slack caused by the destroyed u joint bearings. (gears in the rear end not being properly adjusted and meshed will do the same thing, and put a 'shock' load on the joints and make them fail quickly.)
Occasionally you'll see, parked on the shoulder of the highway, a car with an end of the driveshaft hanging down; the result of ignoring a U Joint problem.

[trbig]

A bad back U-joint cost me a new rear end on my GMC one time.... moving... 3 1/2 hours each way on a Sunday evening... couldn't get it changed.. tried to limp it on anyway. Bye bye bearings and seals! 20 dollar u-joint got expensive then!

Can be a MUCH more dangerous situation if a front u-joint fails. If you're lucky... it only beats the underside to pieces before you stop. But if unlucky..... anyone ever watch pole vaulting??

Tod

[Artic_rider]

Mythbusters busted the pole vaulting myth...at least I think they did...I still had my head in the beer chilling myth...lol

[Erik the Red]

problem solved

There was occationally a clunk shifting from reverse to drive, and I thought I've heard a squeal-ish niose coming from back there, so I decided to look at the U-joint first (after making sure everything was bolted on how it was supposed to be). Here's my story:

So I get home from work last night about 8:30 and decide to replace the loose U-joint. I crawled under, looked, and saw that it was just a couple bolts that needed to come out. "Huh. That shouldn't be too hard to do." (I really should know better than that by now)

I took the yokes (?? whatever the C shaped pieces bolted onto the differential are called) off and figured it should just come out. Nope. Having never even looked closely at a U-joint before (funny, the U-joint is shaped like an X. Well, I got a kick out of it at least) I enlisted the help of my neighbor. Together we were able to figure out the intricacies of U-joint replacement.

The preffered method for removal was the hammer and chisel method. After we dislodged the U-joint from the differential, I then had my driveshaft sitting on the tailgate. Next came the removal from the driveshaft itself. The hammer and chisel method only succeeded in bloodying one of my knuckles.

So after soaking the thing in WD-40 for a while, we got a C-clamp and a socket to try to force it out. When that didn't work, we got a 3' cheater bar to turn the threaded piece. That just bent that little rod used to do the turning as well as the actual threaded rod of the C clamp.

By now it was about 9:30 and I was expecting the cops to show up to tell us to knock off the hammering. We called another friend to see if he had a press we could use. Nope, but he suggested using a propane torch to heat up the driveshaft and then knock it out. This finally worked. Heat and hammering. The smashed finger was just a perk.

So long story not quite as long, it was the U-joint and now she sails smooth as glass. I don't think anybody had ever greased that thing. It was dry as could be. Thanks all for the advice and ideas as well as the explainations of how things actually work. I am continually amazed at what I can learn on this site.

[trbig]

I am continually amazed at what I can learn on this site.

I think several people (Especially myself) are kind of like a guy on a high-fiber diet..... just full of useless crap!! Would be great on Jeapardy, but for real life.... pretty useless! lol.

Just for future reference... my personal way of doing it.. and I'm sure there's several...after you remove the retaining clips from the U-joint, if you have a vise, put the bottom side of the yoke on top of the vise, where it will let the u-joint cap pass by, but still holding the yoke. Then get an old socket (Like your buddy's good Snap-on ) that fits just inside the yoke, against the cap on the top side.... and use a shop hammer on the back of the socket and drive the bottom out between the jaws of the vise. Just make sure you return your buddy's socket when you're done!! : )


Anybody got any tools I can borrow??


Tod