I have a '96 F250 PSD. (My first diesel....I'm hooked..) I have owned the truck for 3 1/2 years with very minimal problems. (water pump, cam sensor, monthy truck note..). There is one problem though. Sometimes when I hit a bump (railroad tracks, potholes that are impossible to avoid down our road) the front end will start shaking. Usually it just shakes a little bit, barely noticable. But twice it has been a very violent shake. I had to pull over and stop to get it to stop shaking. After that the shaking is gone until next time it happens. I have had the tires balanced and rotated around March and the front end was aligned about a year ago. I don't know if this has anything to do with it but I work 30 miles away so I put 60 miles on it daily. I also use my truck to pull a 3-horse steel gooseneck trailer. I don't haul it very much, maybe once a month. (the truck hauls like a dream by the way.) Any suggestions will be appreciated.

 

I won't try to write a boring how to on vibration analysis here but maybe I can help a little. If you are not interested in the why skip down to the recommendation below. Most, my experience 80%, vibration is caused by three issues. They are: component imbalance, component misalignment, or mechanical loosness. You could consider them in any combination. In your case the impacting of a pot hole, RR track, or bump is enough force to excite the front end at it's net natural frequency. Like plucking a guitar string resonance typically multiplies the amplitude of the source by a factor of 10 to 50 times. Usually this excitation will degrade quickly if there is no source force to excite the part at NF. In your case there is most likely risidual imbalance in the tires that maintains a source of force to excite the front end at NF. Until you almost stop there is not enough degradation of source force to stop the shaking or re-excitatin of front end NF. I perform vibration analysis for a OEM of industrial equipment and I have had this "shaking" problem on a Jeep, an old 1972 "Commando" model. Seems to be more common on 4X4's probably the stiff damping of axle and spring assemblies. In my case the larger then stock mudder tires did it. The recommendation below fixed it.

Recommendation: rebalance the front tires, important to make sure they are not eliptical. Check front end bushings. Then go run over that bump and let us know how it went.

 

Something is loose in your steering system or front end. Tires just being out of round or out of balance won't cause this kind of problem (the shaking would subside as speed decreases). I had this problem with an old Bronco years ago (oversize tires). It kept eating steering boxes (classic problem with early Bronco's - just install a steering stabilizer and it will stop eating steering boxes). I'd start at the steering wheel and work my way down. Get someone to rock the wheel while you inspect for play. Things to check: the collapsable tube assembly for the steering columb, steering box, loose pitman arm, drag link and tie-rod joints, ball joints (or kingpins). If you don't find anything start looking at the chassis attachment points (for the front-end). Bad bushing are the least likely thing to find (they would have to be REALLY bad). Problems like this are worst when you run oversize tires. If you have play in the steering system you're subject to oscillation. Once the front-end goes into oscillation it's hard to stop it (you can't damp it with the steering wheel because the play is downsteam) and it's very hard on your front-end components.

Bob

 

Bainbob, imbalance can and will in some circustances produce the posters "shaking" symptoms. The phenomomenon of resonance requires only a small source force to maintain a high amplitude once impacted. There is usually, but not always a degredation of the excited energy ( like slowing down ) but not always. If the problem is NF related the steering stabilizer won't help. I have had the same problem, put a stabilizer on and no change. Front tires that are slightly imbalanced, or eliptical, can provide a l0ow amplitude energy source that when multiplied in resonance 10 to 50 times is not low amplitude any longer. That force mulitiplier feeds the excitation regardless of vehicles speed, becaue it is not speed related at that point, once front end NF is excited. You are correct that something loose can also contribute to the "shaking". A loose component can retune the assembly to a point that original engieering did not anticipate. I have successfully done this kind of trouble shooting, for more than 20 years, on machines that cost more than a whole parking lot of Ford PSD's. It is in 80% of vibration cases component imbalance, component misalignment and or mechanical looseness in some combination. Those primary conditions can induce secondary system symptoms that are often misdiagnosed as primary.
Twist wrote " I had to pull over and stop to get it to stop shaking. After that the shaking is gone until next time it happens". That means that it is not a costant condition but is a singular event, like running over the RR track that starts the symptom of "shaking". The problem does not necessarily have to be any more complicated than tires that are imbalanced slightly. If it were mine I would start at the easiest, cheapest to correct and most likely culprit. I would invest a few bucks to rebalance the tires and repair any loose bushings and or ball joints. Then if there were no improvement I would move on to more complicated components downstream of the problem like steering boxes and steering wheel assemblies.

 

Ok, you must know everything. Lets not get into name calling and other stupidities. By your logic any vehicle with tires out of round (or balance) would exhibit these symptoms. I'm sure everyone has had this problem becuase their tires were out of round... So far as the steering stabilizer goes I was NOT advising him to install one (read the post again). I was telling him that on the old Broncos it was needed to keep from eating up steering boxes, especially with oversize tires (increased mass). In fact, Ford started installing them as of 1970 (mine was a 68). On any vehicle with oversize tires the stabilizer is a good idea because the steering components were not designed to handle the added mass.

So far as his shake is concerned, if there were no play in the steering system it he'd be able to damp it with the steering wheel and the forces would be reduced as speed was reduced. If it really is JUST tires out of balance then the problem will get worse as speed increases. If some thing is loose AND the tires are out of balance he'll find speeds where the "shake" gets better and worse as the frequency changes (harmonics of the fundamental). Running with out of round or out of balance tires will cause it to shake, but without something being loose it won't go into oscillation. Hitting the bump just kicks it to start harmonic oscillation, and after that you can't stop it.

No point in arguing about this. Think what you want and so will I. He'll find the problem, and hopefully tell us what it was.

Bob

 

You seem very angry Bob. I was not arguing just presenting a case for a certain diagnosis. It is fine by me that we disagree, but I would like to disagree without being disagreable. I find myself in this position often with vibration problems. It is within my experience to have teams of Engineers that disagree with me. However it is usually when they have called me for a correction tha eluded them. The catch is, if I am wrong we ( my employer ) don't get paid. I get paid they get paid the problems get fixed and sometimes we still don't agree on the cause. Everyone is satisfied with that, I just fix the machines and move on. If you would like an in depth continuance of our debate on the basics of vibration analysis please feel free to contact me at my profile e-mail.

 

No problems here - I'm not angry, but I wasn't appreciative of the "Brainbob" comment. I have already emailed you, and I understood your position and diagnosis of the problem from the start. You could be entirely correct, but then so could I...

I'm curious to see what he really finds in the end. I suspect we may both be right - that his tires may be out-of-round AND he has a "loose" steering system.

Bob

 

Bob, the "brainbob" was a typo. Please accept my sincere apology, it was not intentional.

James-David (Oneof6)

 

<Moderator>
I fixed the unfortunate typo in post #3.

 

There is also a ujoint inside the cab at the bottom of your column, if the bolts have loosened up it can cause a mild shimmy nothing like you descibe though. that sounds like a wore out pivoting point maybe a badly wore ball joint.

dusty

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Why Buy new when you can build and tailor it cheaper?
F-350 4x4 ext cab Short Bed Green Monster, it is now Sporting a 300HP 715lb/ft Dyno rated 5.9L 12v Cummins diesel w/ NV4500 5spd and custom NP205 ford T-case w/ rear spicer 1410 yoke, Converted to Sterling 10.5 gears w/3.73 gears using the 41 spline pinion and 1410 U-Joints, Power-Lok equipt Dana 60 front, 33's on 16x8" Outlaw II's. (Used to have)205000 miles, Mass air 460 w/ performance heads and a E40d to 5spd conversion and Doug Thorleys. Old EFI 460 @10 MPG to the 12 V @ 23.2 MPG @ 75 MPH and loving it.
My other toy is a '69 CJ-5 354 Hemi 4 spd 4.88's 32x12.50 MThompsons w/ power-lok 44 rear.

 

Interesting conversation.

I find it odd that nobody asked the guy if it was 2WD or 4WD. If he had a steering stabilizer. What tire pressures, sizes and type he is running etc. etc.

It is very easy to diagnose based on something that YOU have experienced but I find it easier and most of the time more correct (and responsible) when you have more info to go on.

This should also go in the Super Duty forum, doncha think? He is, after all, happy with his diesel engine...

 

Oooooooohh!! Scathing Monsta, true but scathing.


Sorry, just had to pile on.

Dave