phil97

Hello guys. Let me start by saying thanks to everyone for the great forum you've got here. I've been lurking since I bought my truck and I've picked up a lot of great information. But I've finally run into an issue I need to ask for some advise.

I'm trying to chase a vibration out of my 97 F150 Lariat RWD (83k miles). I'm getting a vibration at highway speeds (60+ mph). I'm open to any suggestions at this point.

Here is what I've done so far.

Replaced upper and lower control arms (loaded with bushings and new ball joints moog brand), coil springs (my passenger side was broken in 3 pieces!), shocks (monroes), tie-rods, u-joints, rotors & pads, calipers, had the wheels balanced twice by two different shops (Tire Barn & Big-O). Both shops said the tires look fine and don't need to be replaced.

I've also drained the tranny and refilled with Mercon V, and drained and refilled the rear end with Valvoline 75w140 w/ Motorcraft LS additive. (8.8" 3.55 LS diff - the diff still works but whines a bit in sharp turns). Also got an alignment by the local goodyear shop after I replaced the front end.

But I'm still shaking on the highway. I don't feel it in the steering wheel or brake pedal. It seems to be coming from the rear end. I can feel it the back of the cab and under the seats the most.

Obviously I'm committed to making the truck work. I'm just out of ideas about what to replace next... Any suggestions?

bryanwheat

Sounds like the driveshaft might be out of balance.

udderbrudder

Unbalanced driveshaft will cause vibration at much lower speed (15-20mph) Vibration at highway speed will come from somewhere in the wheel rotating mass. eg. tire/rim combo, wheel not seated evenly on hub, bent or damaged axle. Try to support rear diff safely and allow wheels to rotate in drive and watch for wheel hop or lateral runout and go from there

me1234

I'm watching this thread because I am horribly curious what the solution is. So far my 2000 with the 4.6L, 3.55 LS, 8.8 rearend with currently 72,000 miles(had this issue since I got the truck with 43,000) has gotten new u joints, new tires, multiple rotations and balancing. I used the same differential fluid as you but used mobil 1 and added the friction modifier, did multiple figure 8's afterwards. This didn't solve the issue either. I got the truck checked out at Curry's automotive. They did their check, but i told them not to open the rear end up since i replaced the fluid 2 days earlier. After a day in the shop, they told me the issue is likely from odd wear in the clutch packs in the limited slip. They said the only way to fix that would be to change the clutch packs out or hope they wear even and the shake goes away.

I'm planning on putting a detroit trutrac helical limited slip differential in the rear in a few months. I want this to help reduce my 2wd's chances of getting stuck, as well as fix this shaking issue. The helical design uses gears instead of clutches so it's supposed to be maintenance free. I'll report back when I do it and say if it solved the issue or not.

phil97

Thanks Udder, I'll give it a shot. Both shops told me the tires balanced fine and they look like they're still in good shape, but I know tires may still balance fine even with lumps/divots in the tread surface.

How fast would I need to get the tires spinning before I notice any problems? I don't feel the vibration below 60mph, and I wouldn't feel very comfortable running the truck that fast with the rear end off the ground.

This is interesting. I've been wondering if maybe the problem is in the differential. I drive my truck 30 miles one way (all interstate) to work. So I'm running between 65-75 the whole way, non-stop. I've noticed that when the truck is still cold in the mornings the vibration isn't nearly as bad. But after about 5-10 miles on the interstate and the truck warms up the vibration seems to get worse. I've also noticed it gets worse going up hills and slightly better going down hills. Not sure if that is related.

Do you notice the vibration gets better or worse under certain conditions?

TomB275

I can't imagine that the small clutches would have enough mass to vibrate the truck, but who knows.
I would be focusing on elimination of the wheels/tires as the source. Jacked up at the rear rotate the tires and check for run radial runout: is a tire oval. Use a piece of wood to just touch the tire tread and slowly rotate the wheel. Tires can be shaved to be perfectly round at a good tire shop if needed. Wheels ok and not cracked or bent? If ok, then is there something inside the tire? Valve stem tool dropped in perhaps?
With the rear axle secure up on jack stands running it up to 70 mph is not a big deal. A good tire shop has a machine that can spin the tire and measure vibration at the axle. Used to do this years ago to dynamically balance wheels/tires.

me1234

You know, I'm not really sure. I usually try to avoid 60mph because of the shake. The roads i drive on are usually max 55mph speed limits so I just drive 58. Although I have noticed the shake being stronger some days then others when I do occasionally hit 60.

Jbrew

It's probably the slip yoke/output shaft. Once in awhile you slide it apart, clean it very well brake cleaner (the splines) or even Acetone (medium evapor), whatever you have on hand. Then apply a very thin coat of good grease. The output shaft may have a little fluid on there. That's okay, just clean everything and make sure it's dry before thin coating it.

Jbrew

Ah crap.. Changed my mind, after reading your description again...

Sounds exactly like the Transmission Mount is out. I believe they're only about 40 bucks.

What happens is, at some point after 100,000 miles, the bushing (damper) wears down to nothing and then your metal on metal. You get a vibration right were you pointed out.

It's an easy fix.

Jbrew

Trans mount is the middle of this pick (best pic I have of one). Header installs always got a new mount so they would line up on the heads perfectly. That way you didn't bust manifold studs in the future. Yea, when that mount goes out, you'll eventually bust the rear manifold studs at the heads. Anyway, that's a new one, - notice the bushing material that separates the top plate from the bushing housing, -



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Here's one that wore out. Vibrated so bad it cracked the exhaust damper plate that's sandwiched in between lol. You can see the crack in the pick and I think this was from my 98. Anyway, welded the plate, replaced the bushing, - no problems (vibrations) on the e-way .