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1989 Dodge B250 rumble/vibe

This being the second of no doubt numerous questions re the 1989 Dodge B250 camper I just bought for a summer roadtrip. Engine is the 318 V8 with 120k miles. Tranny was rebuilt in '99 at about 60k.

Problem: beginning about 40 mph, and growing increasingly severe with speed, is a low-frequency rumble/vibration, probably in the 20 Hertz range. Significant enough that I didn’t want to drive home faster than 50-55 mph. Could feel the whole vehicle vibrating, through the drivers seat, NOT through the steering wheel or pedals. Definitely not the engine, because as soon as I let off on the gas, the rumble/ vibe disappears. Get back on the gas and it returns full force. Definitely a drivetrain problem, only under load, that disappears when load it removed. My best guess is it felt something like I’ve heard described as a bad U-joint, something I’ve never experienced in person. The tranny shifts fine and doesn’t feel like it’s the problem, unless maybe it’s something to do with the torque converter at highway speeds?

I’m taking it into the shop next week for diagnosis and repair, but I’d be interested to know in advance what the range of possible causes might be. Will report back after diagnosis with bonus points for correct guesses!

PS, correct me if I’m wrong, but when I pulledoff the highway to check underneath, it looked like this van has 2 U-joints. Is that correct? The van has a rear body addition for the camper bathroom.

Also, the van has been sitting almost unused for almost 2 years, except for an occasional run around the block to keep thr blood flowing. Would that lack of use perhaps. contribute to a U-joint seizing up?

As a mod of a standard chassis it can have additional drivetrain points. A load based rumble maybe u-joints and the age is about right. Do the joints have a clunk noise when shifting from reverse to forward?

As I recall, these vans had an incredibly long one piece tubular drive shaft…You might remove it and have a driveline shop check it out for trueness and balance…Balance all 4 tires first…

This is a REAL longshot, but when they rebuilt the transmission, if they replaced the torque converter, they MAY have installed one for a 360 engine, which is externally balanced by adding a big weight to the converter. If used on a 318, there will be a lot of shaking…It will do this in neutral at a dead stop too…

Euryale, I do remember hearing an occasional clunk, and it could well be when shifting from reverse to fwd as you asked. Is that another reason to suspect the U-joints?

Caddyman, the tranny was rebuilt 10+ years ago and 60k miles ago, and there’s no way the van could have been driven that long with this severe a problem. Plus, the vibe disappears when I lift off the gas (at highway speeds). If the converter was unbalanced/vibrating when I was on the gas, why would it suddenly become balanced/not vibrating when I lifted off the gas?

I’d second the vote for U-joints being bad.

Yes, your van has 2 or more U-joints in the drive line.
I trades off a '96 Dodge B-2500 van last year because I could not find or fix a rumble or vibration that worried me. I replaced tires, brake rotors, shocks, and U-joints. When the muffler started to split I traded it for a Grand Caravan. After the deal, I wondered if the problem had been the muffler all the time.
Check the exhaust system.

…could also be a bad motor or transmission mount and under torque, the driveline is shifting so it is no longer straight.

There is a reasonable possibility of a broken leaf or leaves in the spring pack(s)–not uncommon on an RV. This can cause pinion angle problems which are very common on RVs. However, spalled yokes on the U-joint(s) could be the less expensive answer.