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Discussion Starter · #1 ·
1999.5 Ford Super Duty 7.3L, 4x4, 353,000 miles, auto transmission.

At low(er) RPM's I'm getting a bad vibration or shudder. I cannot isolate the problem to either driveline or engine. The vibration is most notable at low RPM while cruising - in other words, if I'm accelerating the vibration is not as noticeable, or if I turn overdrive OFF while under 70MPH. If overdrive is ON I could mix drinks at 55MPH.

The vibration is not as bad if I'm towing a heavy load, but there is still a vibration.

Under hard acceleration there is no vibration/shudder.

The vibration, or shudder, could be likened to a bad u-joint. Obviously my u-joints are fine, but if you drive the truck at cruising speeds your first thoughts would be driveshaft.

I am not showing any codes.

With the truck in Park, I can still feel a slight "miss" with light throttle.

I cannot determine if this is an injector problem or something in the transmission/torque converter/transfer case.

The transmission was replaced with a brand new Ford unit around 200,000 miles. A new torque converter, from Ford, was also installed. The transfer case has been replaced twice since new. I'm not sure what mileage the transfer case was replaced. Both the transmission and transfer case were replaced by the trucks original owner. I'm the second owner.

As far as I know the injectors have never been touched.

Thoughts? Ideas?
 

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353k it could very well be the driveshaft even with brand new u-joints.

I'd remove the driveshaft and go for a ride in 4x4 letting the front axle pull the truck along. If the vibration is gone then it's the driveshaft.

It could also be the T/C causing the shudder since you say it goes away if you unlock it. This stuff works great for TC shudder Dr. Tranny Instant Shudder Fixx
 

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It may be worth checking your UVCH for continuity, you may actually be running 1 cylinder down. I had a similar problem after a rebulid and even though I replaced the gaskets with new pig tails one of the harness had a bad connection at the injector. When driving it was quite a rough shutter but better at higher speeds. It s not the most fun job but who knows maybe that's it. Also look for bare wires or wires that may be grounding out on the block.
 

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Discussion Starter · #4 ·
I'm still stumped. I'd like to think this is an engine related problem, but even a Ford master tech friend can't figure this one without bringing it into the shop for some computer time.
 
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