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Vibration on the highway

Hi Guys, I’ve got a 1991 Ford F150 Super Cab, 351 Windsor 5.8L V8, 2WD with 177000 miles on it. It rocks, I love this truck. But above 60 miles an hour I start to get a horrible vibration throughout the cab, mostly under the seat. It gets worse the faster I go to the point the whole cab vibrates. The challenge is that it only occurs when I’m on flat interstate, with just enough gas to maintain speed. If I accelerate, it goes away (or subsides quite a bit). If I let off the gas, it goes away completely. I had a transmission diagnosis from a local, reputable shop, and they said the tranny was fine. They said it was something in the engine, but I’m baffled that something in the engine could cause such a vibration all over the cab. Any thoughts on what this might be?

We can eliminate tires as a source, since your vibration changes in response to throttle inout. Wheel end vibrations (tires, wheels, and other things rotating with the tire) would be there all the time between 50 and 70 mph.

Try u-joints or the pinion angle.

Certainly all of the drive joints need to be checked out. Given the relation to speed/throttle position I think that’s the most likely thing.

Other than that, though it can never hurt to do some basic things like check the fuel pressure, plugs & wires, filters…

I’m glad to know by process of elimination it’s not the tires. I have suspected something related to the drive shaft (it’s in 2 pieces, and the previous owner replaced the universal mount, but not the carrier bearing), ujoints, etc since the vibration is felt mostly under the cab, and it’s a “whirling” vibration, like something isn’t balanced. Thanks for your feedback!