Hay Guys I'm new to this, and found it trying to find an answer to my problem, which is: I have a 2003 F-150 STX 4X4 and between 40 mph and 45 mph I get a vibration that is felt through out the truck! While keeping it at any speed between 40/45mph and maintaning that speed with the gas you can feel the vibration, but once you let off the gas it quits? or if the speed drops below or goes above the 40/45mph it quits? The vibration feel's like the warning track of a freeway, driveling going out, ect.ect. I've had the truck in the shop allready and they can't find a thing. and say drive it till it breaks or gets worse! Which is not a bad idea, but I don't want it to happen when I don't want it to happen if you know what I mean? I guess what I'm asking here is have any of you ever heard of this problem before or have an idea of what it might be?
FAstTrack

 

I'd guess driveshaft.
you might have better luck in the tranny/driveline forums

 

Hi and welcome to FTE
I would suspect either something in the rearend or possibly a bent axle. There were some problems with axles on some of the earlier models, not sure about the newer ones.
Does it do it on OD and when OD is locked out?
Jimmy

 

I'll have to check that out didn't give a thought about the O/D got to run to the store I'll test it then
TED

 

you might want to do a close inspection of the ujoints ,if they are grease-less type (no zerk fitting) than pull them apart and see if they dry, replace as nessary, also if you have a carrier bearing (longbedsc)check the rubber holding the bearing ,

 

Thank you f-250 ford said they already looked at all of that, and jimmy it does it weather the o/d on or off!
TED

 

What makes it funny to me is the fact that it only does it while giving it the gas

 

looking and close inspection can be two differant things just athought thou

 

I'd start with (in order from cheapest to most expensive):
1) tires correctly balanced and tread pattern is decent
2) striaght and balanced driveshaft
3) u-joints greased
4) bad tranny mount (my money is placed on this one)

 

Could actually be an engine miss.
Also check to make sure that the pinion nut is tight.
Actually, now that I have read the original post again, I would take it back to the dealer and tell them to keep it until they fix it or give you a new truck if they can't!!!!!!!!!!!!
Jimmy

 

You want to be careful here. If the axle in question uses a crush sleeve with the pinion bearings, tightening the pinion nut more could destroy the bearings. The FSM will probably tell you how to go about tightening the pinion nut....and it probably includes checking preload with the tires off the ground.

My money is still on the tranny mount.

 

how many miles on the truck and did it just start doing this?

 

I'd look at the driveline, front axle shafts, u-joints and tires/balancing or mud inside wheels. Sounds like a balance problem at only one specific speed. What size tires are you running? Stock wheels or larger? Driveshaft could be out of balance. Has it been unsplined or u-joint changed lately? U-joints should be aligned per driveshaft or vibration occurs. If you have oversize tires, make sure you've cleaned out inside the wheel (can get over 5 lbs mud there) and check tire pressure. One that is extremely high or low can make a difference. Have you lifted this truck? Short bed or long (wheel base)? I doubt it's the trans but do you have an auto or manual?

 

AS for pinion nut "tightness", if you can shake or move the pinion yoke and it appears to be loose it does need tightened. You should have lock tite on threads and it should not back off but that would depend on who did what to this truck. Since this thread was double posted and I already responded to the other, I'll keep this short. I'd still like to know if this truck is a short wheelbase with lift or oversize tires. Tread pattern will make a difference esp. if tires are cupping already. If u-joints were replaced recently, I'd look at the alignment/balance of the driveshaft. Align at the splines near trans. If you have extra large tires, I'd suspect out of balance/wear (out of round)/tire pressure is off. While it could possibly be axle, diff., or trans, I doubt it on this new of a truck. More than likely it is something previously done (or not done) to this truck. You should know when this started and what modifications were done prior to that. Obviously tire wear will happen anyway over time but look for diagonal bars running across the tread. Check lug nuts for missing or loose, etc.

 

Drop the driveline and check each joint, I'll bet one is bad. Good luck.