New to FTE and need your help. FTE has helped me many, many times in the past but this time I think I need to post a thread to find the help I need. There are a bunch of threads on this sort of issue but I can't find anything on what I am finding with my truck. I have a 2001 F250 7.3 with 175K.

Here is the situation. There is a disturbing clunk when I take my foot off the accelerator and a corresponding clunk when I push back down on the accelerator. This only occurs when the vehicle is not under load. That is, it does not happen when going up hill or on any sort of an incline. It does not happen when the automatic transmission shifts. It does not happen shifting into reverse or drive. It does not happen when starting or stopping. It only happens when I am at 40-60, or so, miles an hour and let off the accelerator and/or press the accelerator again. It appears that I can carefully feather the accelerator to eliminate or, at least, minimize the clunk. I have crawled under the truck and it looks like all the U-joints are tight, I have lubricated the slip-yoke (although when twisted there is, like a mm of play when this is twisted, not much but a very little). The rear-differential pinion appears to be tight (no side-to-side play or up-and-down play, or at least I can't make it move with my hands). However, when the truck is in neutral the entire driveshaft is able to rotate a half inch, or so, all the way up and down the drive train. That is, the transfer case and the rear differential appear to have a little rotational play in them, but I am guessing this is pretty normal. When in park the entire driveline seems solid. I have seen from some of the other posts that this sort of clunking can come from leaf springs, hangers and such but since this clunk seems so isolated to acceleration changes when the vehicle is not under load, I doesn't seem like this would be the issue. No clunk when shifting, turning, bumps etc. Everything seems perfect until I am on a gentle downhill or flat and let off the accelerator. Doesn't appear to clunk when going down a steeper decline. All this gets me to thinking that the clunk is caused whenever the driveshaft rotation gets ahead of or behind the rotation in the rear differential.

Help!

 

Sounds like u-joints to me with it in neutral look for slack in the u-joints before the trans starts to turn.

 

What configuration is the truck? Single, extended or crew cab? It's either the driveshaft slip joint or u joints.

 

Have you got a cv joint in the driveshaft. They can appear tight but not be!

 

Thanks for getting back to me everyone. Truly amazed by how helpful this community is. The truck is a crew cab. My first thought was also the slipjoint or the U-joint. A couple of years ago I had the typical slip joint issue that I have seen in this forum. The one where it sounds like the spare tire is loose and banging around. Since then I have annually regreased the slip joint.

I just went back out and crawled under the truck and put channel locks and a pipe wrench to the drivetrain with the transmission in park and there is NO movement anywhere. Not at the transfer case. Not at the driveshaft U-joints. Not at the slipjoint. Not at the rear differential. Whole driveline is as tight as can be. Granted the truck is not on ramps so I am not able to get a ton of torque lying on my back but I don't see any telltale movement.

In neutral, I didn't have movement in any of the U-joint. The appears to be about 3 mm of back and forth rotational play in the transfer case and in the rear differential when I twist the drive shaft, this really seems to be pretty minimal and I would think that this would be within specs.

This is why I am scratching my head on this one.

 

It could be binding under a load. Sadly the newer years are no better. My 14 is currently back at the dealer for a similar issue. The CV joints can go bad over time, rare but they can fail.

I can actually roll mine about 2-3 niches back
And fourth while in park. If i block the wheels vss take the load off the driveline I can get about 15-20 deg of rotation each way!

You could also have a dry U joint somewhere. Spray them down with trans fluid and let them soak for a little while. Then go for a drive and see if it helped. If it did you have a dry U joint.

 

CV joints do fail but not on a Super Duty. I don't think I've ever even heard of a CV joint failing on a Super Duty, let alone seen one in person.

 

I too agree with u joints as the first culprit. Not easy to see if tight.

How about your motor mounts? They can create havoc as well once gone.

 

And the saga continues. I drowned the U-joints in tranny fluid, let soak in for a good long while and then went for a drive. No change in clunking. Still occurs only when the load is off on a flat or very slight decline. Not from a start. Not when shifting. Not over bumps. Only when I let off the gas or reapply the gas on a flat or very slight decline. Crawled back under and rechecked all the U-joints. There is no movement when I apply a pry bar. I do not think the are the culprit. I looked at the motor mounts. The all look good. Besides, if these were the cause of the clunk I think I would hear it under other conditions as well (e.g., park to drive, into reverse, shifting) but everything appears to operating perfectly except the clunk associated with the gas pedal on a flat or very slight decline. So I am still scratching my head.

But. . .as I was crawling around I realized I have only been looking at the driveline from the transfer case back. This is a 4x4 SD so there is a driveshaft going forward as well. I cranked on these U-joints as well and didn't get any movement on these either. But, it appears that there is another slip-joint here that has never been touched, at least not since I have owned the truck. To be clear, I have not been in 4-wheel drive when experiencing the gas pedal clunk. But could this slip joint be the source of the noise?

Any other thoughts? I am pretty certain the noise is not the slipjoint or U-joints to the rear of the transfer case nor is it the motor mounts. From reading the other posts it sounds like the clunk occurs more often when these three things are the culprits.

In the meantime, I am going to go and grease that slipjoint to the front of the transfer case.

 

Unfortunately you're describing a similar situation I had with mine a while back. Cruising at around 45-50mph I'd let off the throttle, then when step on it again it would clunk....and more metal on metal type of clunk, not a thud. I could feather it back on and it didnt clunk. It did that for about a year actually....before this past November when the center bearing and forward hub in my transmission gave out. I had to do a full rebuild and....no more clunk....

Not saying thats what it is, but its something to look into. Mine is an 02 7.3L and had 175,000 on it when it went.

 

Front shaft will not clunk if 4wheel not engauged. Shaft will not move.

 

I am guessing it is a gas truck? If yes you may be feeling the torque converter locking and unlocking. On a gas truck the torque converter unlocks when coasting after it counts something like 3 rotations of the rear end. I don't remember the exact spec just the fact it unlocks is what I am trying to get across.

 

He said its a 7.3L

 

Forget what I said, don't know how I missed that ......guess I just droog after readin 2 or 3 posts.

 

CV joints can and do go bad. In time they run dry, and there is no real good way to keep them greased. I have had it happen on at least 2 occasions that I can remember off the top of my head. The last time their was significant scoring of the race and centering ball. They can be a real pain, most folks take out the drive shaft and drop it off at a drive shaft shop for a rebuild. I have not been happy with my local shop and opted to do it myself the last time. If indeed it is bad Advance carries them, I am sure that other auto parts stores do as well. You may as well replace the U joints at the same time. If you have a few tools you can change it your self. Remember to mark each component of the drive shaft so that you will know how to put it back in order and maintain the original balance. The hardest part is removing the race. I carefully used a torch to split it because I did not have a die grinder at the time.