I have an issue with my 2001 Ford F-150 2x4 truck that I can't seem to figure out.

The Problem

When I back my truck up (hot or cold doesn't matter) out of my garage or a parking spot, stop, shift to drive, and then accelerate away, it feels as though I encounter a momentary slippage in the driveline. I also encounter the same condition when accelerating from a stop in traffic, 'sometimes' (It doesn't do it all the time).

Additional Info

• The truck shifts perfect through all the gears (hot or cold). There is no jerkiness or late shifting out of the tranny, and shifts appear solid.
• Hard acceleration once moving (say over 20mph) doesn't seem to result in any slippage at all.
• Transmisison and rear end fluids were changed with Amsoil synthetic products maybe 30k miles ago, after which this problem did not exist. Problem is a somewhat recent phenomenon.
• Truck has thrown no DTC's per a scan I did today.
• Rear end is a Ford limited slip unit.

Anyone have any thoughts as to what might be occurring here? Its beginning to annoy me that I can't seem to figure it out.

quote:

Originally posted by bigdeal:
I have an issue with my 2001 Ford F-150 2x4 truck that I can't seem to figure out.

The Problem

When I back my truck up (hot or cold doesn't matter) out of my garage or a parking spot, stop, shift to drive, and then accelerate away, it feels as though I encounter a momentary slippage in the driveline. I also encounter the same condition when accelerating from a stop in traffic, 'sometimes' (It doesn't do it all the time).

Additional Info

• The truck shifts perfect through all the gears (hot or cold). There is no jerkiness or late shifting out of the tranny, and shifts appear solid.
• Hard acceleration once moving (say over 20mph) doesn't seem to result in any slippage at all.
• Transmisison and rear end fluids were changed with Amsoil synthetic products maybe 30k miles ago, after which this problem did not exist. Problem is a somewhat recent phenomenon.
• Rear end is a Ford limited slip unit.

Anyone have any thoughts as to what might be occurring here? Its beginning to annoy me that I can't seem to figure it out.

The first, and easiest thing to do, is to check the fluid level in the transmission. If it is too low, it will cause what you are describing.

quote:

Originally posted by GT-40DOC:

quote:

Originally posted by bigdeal:
I have an issue with my 2001 Ford F-150 2x4 truck that I can't seem to figure out.

The Problem

When I back my truck up (hot or cold doesn't matter) out of my garage or a parking spot, stop, shift to drive, and then accelerate away, it feels as though I encounter a momentary slippage in the driveline. I also encounter the same condition when accelerating from a stop in traffic, 'sometimes' (It doesn't do it all the time).

Additional Info

• The truck shifts perfect through all the gears (hot or cold). There is no jerkiness or late shifting out of the tranny, and shifts appear solid.
• Hard acceleration once moving (say over 20mph) doesn't seem to result in any slippage at all.
• Transmisison and rear end fluids were changed with Amsoil synthetic products maybe 30k miles ago, after which this problem did not exist. Problem is a somewhat recent phenomenon.
• Rear end is a Ford limited slip unit.

Anyone have any thoughts as to what might be occurring here? Its beginning to annoy me that I can't seem to figure it out.

The first, and easiest thing to do, is to check the fluid level in the transmission. If it is too low, it will cause what you are describing.

Oops, should have noted that in my initial post. That was the first thing I checked and the fluid level is right where it should be.

Possible u-joint.

Did you add friction modifier when you changed the rear differential fluid? You gotta have it in a Ford LS rear diff.

A transmission filter change might be a good idea. Sounds like a low fluid pressure when at low engine speed.

While Ford isn't really known for this (GM trucks are, to the point of their having a bulletin), it sounds like a sticking slip yoke in the drive shaft. This is most likely in the tailshaft housing of the transmission, similar to this.



It could be in the drive shaft itself, in which case it will have a bellows boot, but I don't think the F-series uses this type. A drive shaft has to telescope in and out (change length) as the rear axle moves. Normally, the transmission fluid lubricates the splines internally, but sometimes they need a little help. Pull the drive shaft and put a grease containing moly or Teflon (so it is extra slick) on the inside of the shaft yoke splines. You need a good even coating for a couple or three inches inside the shaft, but don't gob it on either. Check the universal joints while the drive shaft is out. You might encounter difficulty with the rear flange bolts (4 x 12mm, 12-point) if they haven't been out before.

quote:

Originally posted by PowerSurge:
Did you add friction modifier when you changed the rear differential fluid? You gotta have it in a Ford LS rear diff.

This is where I have a big inconsistency. I used Amsoil synthetic which according to Amsoil, does 'not' require a friction modifier be added to it. However, all other gear lubes seem to require it. So I'm not sure whether that is the problem or not.

quote:

Originally posted by Shifferbrains:
A transmission filter change might be a good idea. Sounds like a low fluid pressure when at low engine speed.

The filter was changed with the fluid just less than 30k miles ago, so I don't think that's the problem.

Amsoil is wrong. I use Mobil 1 gear lube in my Lightning. I added 1 4 ounce bottle of Ford friction modifier as/per Ford and it has no slippage. The symptom you’re describing sounds exactly like a rear diff with none or not enough friction modifier. In fact, Ford had a TSB for some 2005 f150’s back when they were new because the axle manufacturer didn’t add enough when they were built new. They would slip when first taking off from a dead stop but would perform normally after that.

Sounds like the 1st band servo is slow to engage
Two things, the servo seal is worn or the bore is worn

quote:

Originally posted by PowerSurge:
Amsoil is wrong. I use Mobil 1 gear lube in my Lightning. I added 1 4 ounce bottle of Ford friction modifier as/per Ford and it has no slippage. The symptom you’re describing sounds exactly like a rear diff with none or not enough friction modifier. In fact, Ford had a TSB for some 2005 f150’s back when they were new because the axle manufacturer didn’t add enough when they were built new. They would slip when first taking off from a dead stop but would perform normally after that.

Sounds reasonable. Love this quote from the Amsoil website..."AMSOIL SEVERE GEAR is compatible with most limited-slip differentials. For applications that require additional limited-slip friction modifier, add AMSOIL Slip Lock®." Lot of words there that tell me exactly nothing.

Guess it can't hurt to add some of their 'Slip Lock' and see if that helps.

I wouldn’t use Amsoil’s friction modifier. I would use Ford’s or GM synchromesh.

https://www.amazon.com/Ford-XL...refix=ford+fr&sr=8-1

I forget what years, but some F150s were known to have issues with the transmission shift linkage.

Could it be that it's resting between Neutral and Drive and "slipping in" as you accelerate?

I understand it doesn't explain it happening from a stop though...

Wow. Sticking driveshaft slip joint. Bet you it’s worse after a more aggressive stop !!

Allow a formulator of axle lubricants to offer some guidance.

A limited slip mechanism does NOT engage when moving forward in a straight path. It only becomes operational when you have a reasonably curved path and you accelerate in forward or reverse direction.

For the 2001 Ford, the limited slip clutch packs are interleaving steel plates, on which half of which are glued thin sheets of a paper friction material. These friction surfaces, over time, become smoother and smoother, and also less porous. This degradation is irreversible and fluid changes do not improve upon worn parts.

The typical failure mode for 2001 Ford limited slip axles is a moaning sound, not a sudden clunk or jerk. In some cases, a mild chatter can be felt and/or heard. Again, accelerating from a stop, in either forward or reverse direction, in a reasonably straight line, will NOT engage the limited slip mechanism. Only if the vehicle is turned rather sharply to the left or right is this noted.

If in your maneuver of backing up, stopping, and then moving forward you are also turning the wheel hard left or right, you are performing a maneuver that will cause limited slip noise, vibration, and harshness (NVH). If this occurs particularly in the first drive of the day, you are certainly in the realm of limited slip NVH. A cold axle lubricant is more likely to cause NVH.

You mentioned changing the lubricant about 30K miles ago. It is possible that in this time, the chemistry within the lubricant has degraded, sufficient to cause limited slip NVH.

A simple check to see if it is your axle lubricant is to add some limited slip friction modifier. The Ford Motorcraft XL-7 product is sold in a small bottle. You can best insert this "top treat" by using a disposable plastic syringe, and depressing the plunger very slowly. The product WILL cause skin and eye irritation, and you should wear appropriate PPE when dispensing it. If your axle is very nearly full, removing about four ounces of the lubricant first is warranted. Then, the four ounces of top treat will not cause an overflow.

If the clutches are degraded, this additional FM may or may not help. Most often, it does not. However, it is a relatively inexpensive check to try.

If the clunk is when you move in a reasonably straight line, it is NOT the limited slip.

Have you figured anything out yet?

Ford TSB #12-7-13

Part# for the repair kit incase anyone with the issue is out of warranty 9L3Z-4W602-A.