This is long and complicated (for me), thanks for helping out!

I've had trouble tracing the cause of a seemingly random shudder my truck (94 ram 2500 2wd 5.2 ltr) gets when on the highway, about 55 mph and up. Every couple miles, the truck shudders violently, sometimes accompanied by a clanking sound. When i accelerate it doesn't happen and when i let off the accelerator after speeding up it tends to happen. Also, when i'm towing my trailer I don't notice any shuddering.
The truck has 80k mis on a rebuilt jasper engine, and 217 on the body and transmittion. Here's what i've done so far to figure it out.

1) check for alignment problem: i posted on here recently after experiencing the problem (three months ago) and the consensous was that it had an alignment problem. Makes sense, except that its a random shudder and not a constant, odd vibration. ANyhow i've replaced worn upper and lower ball joints, and had two alignments and switched out tires since then and I can pretty much guarentee that its not an alignment problem.

2) Check engine/transmittion mounts for wear: i'm told this can cause a binding when the engine and transmittion are jolted and aren't secured well. The mounts are doin fine.

3) check for transmittion/ torque converter problems: this seemed reasonable to me (this is my first vehicle) so i took it into a transmittion shop for a free diagnostic. The guy that took the truck out had just quick working with Chrystler after 21 years, so i was in luck. He came back and took me out in it, and dropped it into first gear. Once the rpms reached 16-1700, the truck started backfiring and vibrating. He told me that the backfiring is probably happening on the highway too, and that i just can't hear it or notice it. He also affirmed that the tranny and torque converter are fine.

So, now that I know its backfiring, I started by giving it a tune up. I replaced the sparkplugs, wires, distributor cab, and rotor. I did a compression check and got an initial jump of 90-120 psi and a peak of 160-165. So the engine's fine. I just put a Lucas additive in the gas tank to flush the fuel system out, in case the injectors (its fuel injected) were clogged and were leaking gas into the exhaust stroke. But the problem persists. I'm wondering if i've got a problem with my catalytic converter and if i'm building up back pressure, would that explain why i'm only backfiring at higher rpms?

Kinda stuck. If you've read this far, thanks alot! I've been staying off the highway ever since I bought the truck because I don't want to ruin it. It's very powerull and i like it, but violent shuddering on the highway is scarin me. I'm also not a mechanic, so if you have advice to offer, please make it simple -- also, being new to working on autos i can't easily discern if advice is bad, so please try to offer solid advice. Thanks again! I'm looking forward to getting this thing running well.

-James

 

welcome to df...

is it the front end that is shaking? there is a thing we call the death wobble (DW) where the front axle starts moving back & forth causing some nasty vibrations... with yours being 2wd, that wouldnt apply, however you could have some issues up front. one of the things we do to stop it is put in the maximum amount of caster in the alignment. again, not sure if your issue is front end related but it is worth mentioning.

as for the backfiring, could it be running lean? did the mechanic check the fuel pressure and volume?

 

the shudder and jolt of the engine/body:
i have heard about this somewhere else...
it is a defect in the drive line somewhere.
if i find out more info on this. i will repost and let you know.

the backfiring, do u have stock exhaust?
if u do not, check to see if u have enough backpressure, mine does not have enough,
and will backfire if u stomp it and let go.