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Intermittent vibration at 20-30, 45-55, 65+

518 Views 3 Replies 2 Participants Last post by  FLEx7.3
I have a 2000 4x4 Excursion with a regular, pulsing vibration that occurs at 20-30, 45-55, 65+ mph. The vibration comes and goes every 3 to 4 seconds depending on speed. It is worse under load, but doesn't seem to be particularly torque sensitive.

The truck has a 3.5" Old Man Emu lift, brand new Nitro Charger shocks. The brakes and suspension are all new -- everything below the frame with the exception of the gears and rear bearings are new parts, properly installed, thoroughly checked and re-checked. The driveshaft was balanced with new u-joints and the yoke runout was checked and was within tolerance -- this driveshaft is not a double-cardan, it uses a single u-joint front and back. The tires are 35" BFGs, road force balanced. The wheels are new and were checked for roundness, and were found to be perfectly true.

I had thought that I had a bad injector, but they're all fine. The harmonic balancer is new and installed correctly, and there are no other engine issues I can find. We've determined that it definitely is not TC shudder or flex plate related.

Current suspects are motor mounts, trans mount, or incorrect u-joint angles -- and even possibly that the driveshaft was balanced incorrectly.

Any other ideas where I should look?
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I started with u-joint angles -- the angle at the transmission is 5.2deg and the angle at the pinion is 10.1 -- so ∆4.9 -- which I believe is well out of spec. The pinion will attempt to climb the ring gear under load which should correct some of that, but I doubt it will correct nearly 5 degrees.
Was the X running fine and then all of a sudden started the vibration or was it only noticed after you replaced parts. I’ve had a broken cord in a tire shake the whole truck. I’m sure someone will be along with some helpful advise.
It started when I lifted it. This is the second set of tires - first set (Coopers) had issues. The BFGs on it now are road force balanced on brand new wheels.
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