Annoying Vibration

[Wookie]

I have noticed an annoying vibration that happens when my car is free-wheeling, and the vibration varies with speed. Once you let the clutch out, and the wheels are being driven again, the vibration ceases. Any ideas?

[mrLEE30]

why would you need to free wheel with the clutch in anyway?

but to your problem could be allsorts of things. propshaft centre bearing,prop U/J siezing/excessive play, driveshafts bent, gear box mountings knackered, engine mountings knackered etc etc.

is there any sound with the vibration? if so where does it appear to come from. does it happen when freewheeling in neutral or only with clutch in?

mrlee

[Wookie]

It seems to be coming from the rear of the car, it's almost like a droning sound. It happens when freewheeling in neutral and with the clutch in.

[mrLEE30]

had more of a think about this and it could be your layshaft bearings (there are two), this would explain why you get vibration and noise under no load, but nothing under load. My advise is just to leave it alone, to change requires full gearbox stripdown = expensive, better to change the box... unless its very bad i wouldn't bother with it. Could also be the clutch pressure plate/clutch release bearings

but to keep thinking as you mention the drone it could be prop centre bearing on its way out (easy change) diff bearings on their way out (not such an easy change) but i cannot explain why this would cease under load as it normally the opposite case (i.e. vibrates under load) for these cases. Also worth noting that vibration and vibration noise can very rarely be dectected accurately with the ear, especially a droning noise, amy sound from the rear but could be from anywhere.

just had a thought... could be simple, make sure your exhaust is not touching the body underneath anywhere, could be touching when engine under under no load and not when the engine is under load and thus generally moves on its rubber mounts.

mate best bet is to leave it for now and stop freewheelin about and let us know if it get any worse over the comming months, you can PM me if it gets worse and i will have another think


mrlee

[gareth]

the first and most simple thing to check is your wheelbolts (don't laugh, just double check and don't tell anyone!!!).

after that, the most likely things are:

- the propshaft flexi (guibo) coupling on the tail end of the gearbox. this can crack and split with age. i've also had broken bolts before now.
- the propshaft centre bearing supporting rubber. this too plits and allows the prop to flail about a bit! you'll probably have to drop the front of the exhuast and front heatshield to get at it.

driveshafts are generally very strong and rarely cause problems, even with big power conversions

[Martinaston]

At a guess i'd say the propshaft centre bearing is not pre-loaded correctly or the rubbers on its way out and the guibo's probably gone soft as well.

[Wookie]

Thanks for the advice guys! My first guess was the diff and/or prop centre bearing. Could low oil in the diff/gearbox cause a vibration? I have had a battle and still haven't managed to drain the diff and change the oil.
Secondly, I don't know if it's a gearbox or diif sound, but it whines, varies with engine rpms - obviously when the car is being driven, and is quite loud outside the car. Any takers?

[mrLEE30]

a whine is generally diff, a knocking/cluncking sound is generally layshaft, a low pitch drone is normally prop bearing/prop. all vary with engine speed and/or road speed

make sure when you change the diff oil you remove the filler nut before you remove the drain nut, as once you drain it and cannot open the filler nut you are knackered. and lack of diff oil it self will not cause vibration, but it will cause wear which will cause vibration, changing the oil will not solve a worn out bearing!

Again i stress vibration from the prop bearing/preload/misaligned splines normally is worse when under load, not vice versa.

mrlee

[Wookie]

I have done a bit more listening, and have noticed the same vibration happens if you take your foot off the throttle while driving. When you hear the droning sound, you can feel vibration through the seats of the car. Any more ideas?

[mrLEE30]

in that case would suggest 1. check your exhaust is not rubbing on the floor of the car
2. inspect the rubber donut that connects the prop to the gearbox (if it is cracked then that needs replacing
3. change the prop centre bearing.... do a search on removing the prop as they are balanced as two pieces and if you split the prop it needs to go back together on EXACLTY the same splines. i.e mark the two shafts before you split the prop. also remember the prop centre bearing needs preloading, bentley manual explains how.
4. if not the above then could be prop u-joint, easier to get a second hand prop then change this


BUT if its not too bad then just leave it till it gets worse and easier to identify. It will not cause any damage if its only a slight vibration/droning noise.

mrlee

[gareth]

i'm willing to put a beer down on it being the prop centre bearing or giubo... thes turns the seats into vibrators!!! my wife certainly missed them when i sorted my prop!!!