Mystery Vibration

[PSL184]

I need some collective thoughts please......
The pick up has developed a vibration. It comes in at 50mph and goes at 60mph. At first I suspected propshaft so I replaced it with a known good one - but - no difference. I have ordered a new set of engine and gearbox mounts to see if this helps but other than that has anyone any other ideas?
The vibration is road speed related, not engine revs related. I can get to 50mph in 3rd and if I change to 4th it is still there. Also, it is much worse if I lift off the accelerator and seems less bad if I drive hard through the 50 - 60mph range....
So, over to you guys

[andrew.searston]

could be tracking, tyres, suspension ect
just a matter of ticking the list off

[PSL184]

Tracking is OK and I've tried 2 sets of wheels with no change. Which part of suspension would you look at in particular Andy?

[kittyfell]

Axle not sitting true on the springs? The gearbox and diff input centrelines must be parallel. The prop shaft should have equal angle at top and bottom joints.

[Alec]

Hello PSL184,

I take it that it developed without any change to anything prior, so it is a deterioration of some component?

Changing the propshaft should eliminate that as long as your are certain that the replacement is also sound. Did you look at the joints of the original shaft for a failed U\J? You also say you swapped the wheels, all four?
I don't think the engine mountings would have much effect on a vibration you describe. The usual culprits are wheel balance\eccentricity or propshaft joint failure. Does it seem more from the rear than the front, as you suspected the propshaft?

Alec

[PSL184]

Yes, it is deterioration or failure of something. The old prop I removed had a siezed rear joint. The "new" prop has new joints fitted. I feel the vibration is coming from the front end although it is very difficut to tell exactly. I can feel a slight buzz through the gearlever but it feels much worse though the floor....
All 4 wheels and tyres changed for different ones.....
I will check rear axle alignment but It looks OK visually.......

[Stig]

I had a vibration at 50mph which was much worse if I lifted off the throttle. It wasn't the propshaft but the front nut on the diff coming loose. Not something you'd expect as it's supposed to be done up to 140ftlb or something.

[Alec]

Hello PSL184,

as the first propshaft had a seized joint then it is likely to be the cause of the original vibration. If the replacement didn't cure it I would check it carefully. Sometimes in overhaul the flange gets burred over preventing it mating up true to the differential's flange. Also, if the joints were rotated relative to the shaft on reassembling the universal joints then it's possible it is now out of balance. And, more unusually as Stig says check the nut securing the differential flange?

Alec

[MarkyB]

While you have the front jacked up I'd spin the wheels while holding something just touching the rim to check you don't have a bent rim, or something between the wheel and hub putting the wheel out of true.

[PSL184]

Thanks - some more things to check. I might just get the old one rebuilt and balanced and then I know for sure it ain't prop. It does still seem like it though. Now you come to mention it Alec, I wasn't too happy with the flange on the prop to diff end...

[Mogwai]

One other thing to check if the other suggestions don't reveal anything is the halfshafts. If the splines are starting to twist etc they could cause the shaft to run out of true

[PSL184]

I should head my own advice !!! (change causes effect)

Before the winter I changed the diff (4.55 to a 4.22). I had done a few short runs around town and all seemed well..... Then I realised I hadn't been upto 50mph on the "new" diff so carefully checking it revealed the flange to be running out of true.... So, old faithfull 4.55 back in and now runing smoothly again

[bmcecosse]

Great success!

[MarkyB]

Good grief, it must take some force to bend the differential input shaft.
Jacking the car up on it presumably.

[katy]

Is the shaft bent, or just the flange out of true?

[PSL184]

It was just the flange out of true. Jacking could well have caused the problem or it could have been dropped at some point and landed on the flange....??