Help with F-250 vibration

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Help with F-250 vibration

Postby _r2h » Mon Oct 19, 2015 2:38 am

Hello all,

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?

Brad.
_r2h
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Re: Help with F-250 vibration

Postby Steve Smith » Mon Oct 19, 2015 11:43 am

Hello Brad and thank you for the post.

This sounds like a challenging one and just how we like them.

Forgive my ignorance here, you refer to drive shaft with centre bearing and “U” joints can I be sure we are talking about Prop-shaft?

Prior to the prop-shaft replacement can you confirm there was no vibration?

If so I would be intrigued to confirm there is no alignment issue between the rear of the transmission (output shaft) and the rear differential pinion flange (Prop-shaft alignment)

Theoretically, if you were to draw a line through the centre of the crankshaft to the rear of the vehicle it should pass straight through the rear differential pinion flange where the prop-shaft should follow the exact same alignment.

In the real world this is challenging and hence why we use double “Universal Joints” and support bearings.

Can I confirm you are happy with the prop-shaft alignment and the angle formed at the centre bearing?

Moving onto the NVH results, my concern is the different test conditions used will most certainly influence NVH and possibly have you chasing your tail.

On the road at 70+ mph we have a cabin vibration and buzzing at what frequency?

Would it be 140 Hz as in your test with the rear axle raised?

Could you post the NVH file here for the road test demonstrating the difference between OD (Over Drive) in and out?

Could you also post the NVH file for the axle raised?

E4 vibration will characteristically be the highest vibration visible for a V8 engine (4 combustion shocks for one crankshaft revolution) V8’s are normally very smooth and 19-20 mg seems excessive.

2100 rpm / 60 = 35Hz x 4 = 140 Hz (E4)

It would appear that drive shaft in or out we still have the same vibration of 19-20 mg but you feel an improvement has been made.

I would rely upon the measured value in this scenario Brad as our perception of vibration is subjective at best. By removing the additional mass of the prop-shaft we may have simply changed the transfer path of the vibration into the cabin.

The 156 mg value obtained is truly high and a real concern but I have still have reservations about the vehicle being raised from what would be its normal driving position (vehicle ride height and posture at 70 mph)

What I would like if possible would be to reassemble the vehicle with an assured prop-shaft alignment.

Road-test at 70 mph and capture the data both in the cabin and transmission cross member. Can you confirm both have high amplitudes at 140 Hz. (E4) Could you post these captured files here?

A combustion related engine running condition on a V8 will most certainly increase E4 with similar effects to E1.

There has to be a clue in the information you have discovered surrounding the OD in/out condition.

With overdrive engaged the output shaft speed/frequency will be higher than the engine.

With the overdrive engaged the load should be reduced on the engine at cruising speed which may point to combustion issues.

Can I ask Brad if this vibration is continual at 70 mph or apparent for a brief period only as you drive through 70 mph? (This could be attributed to resonance of a particular component?)

I hope some of this helps and any feedback would be most welcome.

Take care…….Steve
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