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This is in an older Dodge B250 Van, with the Chrysler 318/5.2L v8 engine, not fuel injected.

On a recent long drive, I began to notice the van has what I can only describe as a "rhythmic" vibration issue whenever the accelerator is engaged. This is definitely new.

Basically, regardless of the speed/RPM, or amount of throttle applied, the entire van is shaking for about half a second every 3 or 4 seconds in a very repeatable steady rhythmic way. The vibration isn't coming through the pedals or steering wheel, instead it's the entire body of the vehicle where I notice it most, all the mirrors shake significantly, and the there is a low rumbling/vibration sound seemingly coming from all around.

At first I attributed it to the road surface, since the feeling is somewhat like driving on rumble strips at set intervals.

This vibration goes away immediately when I left off the gas, whether coasting at highway speeds (~100km/h) or lower side-road speeds (50-80km/h). Similarly, the moment I touch the gas, however lightly, the vibration comes back.

Braking appears unaffected, there's no change in the braking performance at all and no vibration when braking.

I am trying to figure out how to narrow down the possible cause of this. It seems like it could be attributed to many things:

  • Transmission, Torque Converter, or Flywheel?
  • Wheels/Tires/Driveshaft?
  • Ignition / spark plug / timing issue?
  • Engine Mounts?
  • Loose exhaust component?

To be clear, I'm not asking for details on troubleshooting all the stuff in this list. It seems like there's a lot of potential causes for this, so I'm looking for the general high-probability things I should check first based on these symptoms.

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    Propshaft / driveshaft uj failing under load.
    – Solar Mike
    Nov 5 '20 at 5:26
  • I would add suspension to your list.
    – GdD
    Nov 5 '20 at 9:16
  • @Solar Mike should I expect play in the joint as a symptom in that case, I tried standing on the brakes and shifting reverse to drive and back but don't detect a clunk. Can visual inspection of the U-Joint help me confirm or rule out? Nov 5 '20 at 17:29
  • @GdD I figured if it was suspension I would expect an issue even when coasting, but I'm just guessing at this point, I'll give the suspension a once over. Nov 5 '20 at 17:30
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    Suspensions sit differently when under acceleration, when you let off the pedal weight shifts @CameronRoberts.
    – GdD
    Nov 5 '20 at 17:35
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Based on the top comment by SolarMike I investigated the U-Joints first. The problem was ultimately the front U-Joint that sits between the sliding yoke from the transmission and the drive shaft.

I couldn't detect any "clunk" or play in the joints while they were installed in the van, but after pulling the drive shaft the front U-Joint showed clear resistance and jerky motion when moving on one axis.

Replacing the front u-joint, and greasing the rear (which seemed to be in perfect condition) solved the vibration issue.

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