Hi there, hoping someone can help with advice.

'99 F250 4x4 Auto, ext cab, short bed, 4.10 limited slip. 85K miles

I have noticed a vibration, while accelerating from a dead stop. It seems to happen while turning, but not necessarily everytime. The vibration will happen very briefly, during the first ~ 100ft or so, and will cycle through vibrating and not vibrating until i get to up to 15-20mph. (maybe 3 or 4 iterations)

I've noticed its definitely more pronounced when towing, so I suspect it has to be something driveline related. I can hold the brakes, and drive slowly in circles and the vibration is there, so I doubt its brake related.

After searching the forums, I've tried the following;

-Added more limitied slip additive to the rear diff, drove it >150miles and saw no change.

-Pulled the rear driveshaft, replaced the 3 ujoints, and greased the slipjoint. (My truck does not have a carrier bearing on the rear shaft)

-After those didn't work, I drained the rear diff, and refilled it with Mobil1 75W-140 Syn.

...still no affect. What else could it be? Does my rear diff need to be rebuilt? Something in the transfercase? Transmission woes? Is it possible that its something in the front end, even though it seems worse while towing?

Thanks for the help.

 

So by your description, are you "power braking" the truck, meaning the engine is under a load while your driving in circles? What if while you driving in circles you let off the brake, does the "vibration" go away? If your answer is yes, probably time to look at your transmission. Be sure the fluid level is correct, and that the fluid doesn't smell burnt. Sounds like it could be a failing torque converter. Is this the original transmission? How long has it been since it was serviced?

 

Try manual shifting when you take off,about 20k miles before my trans went it would viberate on take off but if I manual shifted it was better

 

Yes,
I can hold 1 foot on the brake and 1 foot on the gas, and the truck shutters/vibrates while driving in a circle at parking lot speeds.

The reason I tried this, was I was thinking maybe I have a problem with the brakes (the brake pads vibrating apart when the calipers are releasing). But because it still happens when I hold the brakes, and it also happens more pronounced while towing, i think it has nothing to do with the brakes, and everything to do with the driveline.

 

I was thinking it was either differential or transmission, but I was trying to be optimistic with the driveshaft. The reason I am thinking differential over the transmission, is that it is definitely worse while taking off & turning. I would think this would probably rule out the transmission, but I am not 100% confident.

Considering that adding more slip modifier to the rear diff didn't seem to affect it one way or the other, lead me to think it might not be the diff either.

Not sure what to try?? I've seen plenty of forums posts talking about similar symptoms, but everything seems to point the finger to ujoints, slip joints, and carrier bearings. (done all that !!)

 

Its the original transmission with 85K miles. The person before me added a HyperTec Trans Pan, so at some point the fluid has been changed.

I pulled the trans dipstick and the color and smell seems normal to me. I also have a trans temp gage on the truck, and it hardly registers any temperature. Very infrequently while towing long distances around town with 4 horses on the trailer (~12K #), it gets near 200 degrees on the gage.

 

Ok, that's good on the temps. Let me point out that more force is required to move your truck from a stop while turning, then moving in a straight line. (a little physics.) Remeber when you were helping your buddies, who ran out of gas, push their car , always easier to push straight, then while the wheels are turned. Your engine sees the same difference in load, so it still most likely is pointing to the torque converter (T/C.) Here is a simple test, accelerate from a stop in a parking lot, or other clear straight area where you can get a feel for the amount of vibration you feel while accelerating, then put the truck into 4-Lo, and do the test again. I'll bet the vibration (or perhaps "shudder" may better describe it) is less. That should give us some better data on your drive line. If it was a "U-Joint", it would still shudder/vibrate the same. Oh and just FYI, don't try the turns in 4x4 on pavement (you probably know this already), but extreme binding will occur possibly breaking expensive stuff.

 

Ok, so if I have 1 foot on the brake & 1 foot on the gas and steer straight, there doesn't seem to be any vibration (2WD). Its only seems to happen when turning, so in my opinion, this limits my list of suspects to either the front end, or something in the rear differential, right?

The part that I am not sure about is;
1) if its in the front end, I wouldn't think it would feel worse under load (even though the weight distribution is different), right?

2) If its in the rear diff, I would think I would see some difference (better or worse) after trying more slip modifier or after changing the oil completely.

One additional note: Even after changing the ujoints and greasing the slipjoint, I do still get a "klunk" sometimes after putting it in drive, which I was hopping to get rid of. I feel an equal amount of slop at the differential yolk and the output flange on the transfercase. ~ maybe about a 3-5 degree rotation on each? Not sure if this is normal.

Thanks for the help.