transmission clunking

[bronco2blue]

I was driving up a hill yesterday when I felt a change and my truck suddenly started to vibrate. Ever since this happened the truck hasn't stopped vibrating and a clunking noise is present, especially when accelerating from a stop. At first I thought it was a bad spark plug. I tried to trace the noise and it doesn't sound like it's coming from the motor or exhaust. The vibrations through the shifter makes me think something is loose in the transmission. I have a spare trans I can put in the truck, I just wanted to see if anyone had other opinions on what it could be before I remove the trans. Thanks.

 

[Slammin Sam]

I'd look for either a broken trans or motor mount, or a failing driveshaft u-joint.

 

[bronco2blue]

The trans and motor mounts look good. U-joints seem OK. There was some gear oil leaking from around the flange between the driveshaft and rear differential. If I get under the truck while it's running in neutral, there's a rattling noise that sounds like it's coming from the trans. If I accelerate in the truck, the clunking noise sounds like it's coming from the trans also.

 

[Spott]

I second the high likelihood of a bad U-joint. It's far and away the most likely cause for the symptoms you describe. One characteristic is a distinct clunk when changing from acceleration to coasting, or from stopping to driving, or from forward to reverse.

Sometimes they're hard to diagnose by hand. Try jacking one rear wheel off the ground, and turning that wheel forward and back. See if the U-joints demonstrate any slop when you do that.

 

[bronco2blue]

I'll give the u joints a better inspection. I'm not noticing a clunk from acceleration to coast. It almost sounds like bad spark plugs but the truck vibrates like crazy. It's more like a sputter than a thump

 

[bronco2blue]

Changed plugs, oil, cleaned maf and same thing is happening. Kind of stumped but I'm leaning towards burnt valves. That's my best guess

 

[Big Jim M]

Burnt valves are apparent just sitting at idle in the driveway! Is this true or does it only happen while moving?
Big Jim

 

[bronco2blue]

Yes even when the truck is sitting there at idle you can hear that it's not running properly. When you go to press the accelerater the noise and vibrations become worse. At top speeds it's not as noticeable but my floor shifter vibrates like crazy.

 

[Big Jim M]

Ok then do a compression check! For it to happen so suddenly I'd suspect wires or even a plug went away before valves.
The compression will tell you what you now need to know.
Big Jim

 

[bronco2blue]

I've aleady replaced the plugs, I should probably try the wires now. It was a very sudden change in performance that I could Notice while driving.