I have a 2004 F-150 with the 5.4 V-8 and when I take off from a stop....under moderate throttle...I get 2 thumps from the rear it seems and then a slight shimmy...and then its moving.....Ive already had the rear replaced from the whine at around 50mph.....and I noticed that when I put significant weight in the back...then thumps go away...any help?

 

U bolts loose maybe? That could cause the diff to move around a litttle causing a thump. Might check it if they replaced the diff.

 

If i recall....it did it with the old 1 too...

 

There seem to be several newer threads in "Transmissions" that identify the exact symptoms you are talking about. I am personally trying to solve the exact problem you have on my 1999 F150 5.4 V8.

So far I have changed the tranny/torque converter fluid/filter and replaced the rear axle fluid. No improvement yet. I have heard it may be the slip yoke, leaf springs, or even the spider gears.

One thing I did try on my truck was to hold the brakes and then press the gas. Almost like a brake stand without spinning the tires. The rear end thump was still present even though I had not moved. This leads me to believe that it may not be the leaf springs or the spider gears.

I will continue troubleshooting and post any good news.

 

well with weight in the back and it goes away...it sounds like a suspension problem...something is shifting with weight..bad shock mounts? worn rear bushings?..look around the rear springs for any wear...zap!

 

The clunk you are feeling sounds like it is comming from your rear driveshaft slip yoke. You can remove the driveshaft and lube the slip yoke with teflon grease. I hope this helps.

 

Yes, I'd bet on the yoke. It's a common problem.

 

Thanks for the help guys....apperciate it

 

What is a "slip yoke"?

 

The purpose of the slip yoke is essentially to change the length of the driveshaft to accomodate suspension changes due to load.

 

My 97 150 4x4 has thumped since new. Why would the yoke make tha tnoise and not fail after 8 yrs?

 

Well I believe the "rear end thump/jerking at takeoff" has finally been fixed. Sure enough it was the simple greasing of the slip yoke. I used Mobil 1 synthetic u-joint grease. No thump what so ever at least so far. Simple fix that takes maybe 20 minutes!