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67 289 engine vibration

6934 Views 16 Replies 6 Participants Last post by  Dboxman7
On the freeway most noticeably above 60 mph, when on even flat road if I let off the gas, no problem, but when I give just enough to maintain speed I get a heavy vibration/shaking feel from the engine. Not violent like something is breaking, just too heavy to feel right. Doesn't do it when accelerating or when I let off the gas. I haven't started to try to diagnose yet... maybe vacuum advance? or ignition/timing?

Any thoughts or help appreciated!
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Thanks for the suggestions. The engine is stock, vibrations just became noticeable last week. I have had the car over a year with no problems. Both u-joints are solid and the pcv valve seems fine. Still need to check the torque converter bolts and ignition.
A little better inspection...rear u joint is toast. Not sure if that's the problem, but I will start there.
Rear U Joint

Got the driveshaft off and the U-joint out. One of the 4 bearings was cracked and a piece fell off when it came out. no bearings inside, just dust and the inner part was obviously worn to a taper. The grease port to that bearing was clogged with what I presume was the remains of the needle bearings.
Noticed the vibration still there after a week or so. Driving home from work yesterday it became terrible. Looked this AM and the rear U-joint is hosed on one of the 4 bearings - will need replaced again. Differential causing it to eat u-joints? Appreciate any suggestions.
Thanks for the suggestions, didn't know about the "relaxing" part. I figured if the differential was causing it it would be the pinion bearing. My concern about the pinion is because the old U-joint was fine for a year and a half that I had the car until it started to degrade and vibrate. The one I replaced has only lasted a few months.
Maybe a dumb question, but how much slop is acceptable between the pinion and the ring gear teeth. I haven't pulled the differential, but when the driveshaft was disconnected, I could rotate it a little between the teeth. 1/4" OK but 1/2" too much??? Foreign territory here and just wondering if it needs rebuilt or replaced. 8" stock rear end, 2.79 ratio. My 2004 F-150 has virtually none, but it is a lot newer.
As far as I know it is the original differential. Flange rotation is what I was referring to. Still have some vibration. Don't understand how the driveshaft can just get unbalanced if that is the problem. I'll keep at it. Thanks for the help.
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