V6 Drivetrain Vibration

Post by **xantia_v6** » 25 Dec 2009, 20:12

I am back down in Burgundy for a couple of weeks, managing to avoid the -15 temperatures of last weekend.

The high speed vibration that was present before the transmission rebuild is still there and if anything, worse.

I don't really notice it in the UK, because it only starts at 120 kph (75MPH), and only when the engine is under load, but by 130 kph on a slight uphill, with 350kg of "luggage" (a marble fire surround, an oven and a good collection of DIY tools) the car shakes quite badly.

I don't think it is just wheel balance, as that always starts a few MPH lower and this vibration disappears as soon as the accelerator is released, and this also seems to rule out out-of-round tyres (but the fronts are near due replacement).

The drive shaft intermediate bearing was inspected and seems to be OK, but I wonder about the inner CV joints?

Another contributing factor may be the state of the top RH engine mounts, both the vertical and horizontal rubbers are broken to some extent, so I wonder if the engine can get into an unstable state where it is oscillating on the mounts? To test this theory, I rotated the horizontal mount through 90 degrees so that it prevents any excess travel of the engine, despite being a bit broken. I have not taken the car out to test it yet.

Any other suggestions?

Deanxm · Post by **Deanxm** » 25 Dec 2009, 21:13

Hi V6

I would have thought that you and Jim would have noticed a worn driveshaft joint when you had them off the car?
The engine mounts are my bet though, the xm has started doing this and on rough roads you can feel a short of juddering through the car as the engine is shaken about, i pressed a new bottom mount in a few months back which helped reduce the jump when coming on and off the throttle but with the top o/s mount split and soft its still letting the engine pivot back and forth a bit and im sure this is the cause as when it was packed with silicone all the wobble and snatching vanished, i will press the new rubber into a spare mount and fit it over the holiday which should have things back to normal.

Hope whatever it is, its cheap and easy

D

lexi · Post by **lexi** » 25 Dec 2009, 21:39

The mounting seems first port of call. I suspect they get mangled with disconnections and heat from that engine? You could change the wheels around to back to front and see if there is any improvement. Wheels and tyres are funny things but usually speed related rather than the torque/ speed thing you have.

Wonder if a temporary type of fastening/clamp could be made to hold things tighter on that mount while you test We used to chain the engine down on fourby to stop too much movement off road......bit rough for the Cit V6 mind you

Post by **xantia_v6** » 31 Dec 2009, 20:03

I have been out on the Autoroute a couple of times with the tweaked mount, and the vibration at speed is considerably reduced. There is however a noticeable engine vibration for about half a second when accelerating briskly from rest.

I am considering the option of forking out for a new engine mount (about £80 I believe).

Post by **CitroJim** » 31 Dec 2009, 20:14

xantia_v6 wrote: I am considering the option of forking out for a new engine mount (about £80 I believe).

Which one Mike?

The rear (cast part) was just over £50 when I bought a new one.

Do you mean the one actually on the engine itself with the big donut? I seem to recall the one on the broken V6 was gone a bit too...

Post by **xantia_v6** » 31 Dec 2009, 20:25

CitroJim wrote:
xantia_v6 wrote: I am considering the option of forking out for a new engine mount (about £80 I believe).
Which one Mike?

The rear (cast part) was just over £50 when I bought a new one.

Do you mean the one actually on the engine itself with the big donut? I seem to recall the one on the broken V6 was gone a bit too...

The big donut on the engine bracket, to which the fork attaches.

When I get home I will try swapping it with the one from my other car to check that it cures the problem.

HDI · Post by **HDI** » 31 Dec 2009, 22:48

My money would go on engine mounts too. The clue is in your comment on the load conditions where the vibration occurs. The torque reaction of the engine is absorbed by the engine mounts , if they have become soft or worn they will allow the engine to oscillate when the load reaches a certain point.