I'd like to get some assistance with an incomplete case study I have at the moment.
Vehicle: 2002 Ford F-250 V-8 7.3 Powerstroke, Auto 4R100, 4x4 NP273
Problem: Vibration noted to whole cab at speeds of 70 MPH or greater, has a buzzing quality to it. Problem noted after having drive shaft R&R with u joints and center bearing. Customer had drive shaft balanced, went in almost completely balanced, came out perfectly balance. Tires road force balanced as well, came out perfect.
I have run several different NVH test to try and isolate the problem and this is what I've come up with. Sensor mounted on driver seat rail. At 70-75 mph +, noted the buzzing cab sound and vibration, locking out OD reduces sound/vibration is almost unnoticeable levels. I ran a NVH test with the rear axle up on jack stands, found a 19-20 mg imbalance at E4 (at 140 Hz as engine rpm was 2100 or 35 Hz). I found that odd. I removed the drive shaft (along with trying Park / Neutral with drive shaft in) and ran the engine back up to 2100. Vibration and cab buzz noticeable, but not even close to the quality it has with the drive shaft in. NVH test noted the same 19-20 mg imbalance at the same E4.
I mounted the sensor on the transmission cross member for giggles and obtained a 156 mg imbalance at E4 (I did change the sensor from passenger to engine compartment fyi).
These trucks are known to have some pretty crazy vibrations when they are developing a cylinder imbalance due to injector issues. Now my question, would a singular cylinder imbalance cause vibrations at E4, or am I chasing a phantom. My theory currently is that a injector is causing an imbalance, coupled with a direct connection through the transmission (OD and a locked TC) is transmitting the vibrations to the body via the drive shaft.
Anyone have any further analysis/test I can perform? Any insight?