P38A Strange vibration...

Hi

Picked up a lovely 1998 4.6 HSE yesterday and on the whole all is well. It just passed the MOT with only an advisory for grotty number plates.

Air suspension is leaky, but I'm on top of that.

The reason for the post is that there's a most strange vibration happening and I'm struggling to understand what could be the cause.

When you get to motorway speed, a vibration takes over the whole vehicle. It feels exactly like a wheel balance issue. Except two strange things.

1) Once the vibration has set in, slowing down does not make it diminish
2) Putting the vehicle in neutral immediately cancels the vibration.

Any ideas?! It's baffling me.

Cheers,

Steve

Removing drive suggests rear diff or maybe prop hanger bearing.

Cheers!

Any diagnostic suggestions to narrow this down?

I'd check your UJs and while you're about it check the VCU, simply because it is easy and gives peace of mind.

With the vehicle in neutral, I jacked up the nearside front wheel and using a bar was able to slowly turn the wheel and observed the shaft turning. It wasn't jammed, nor did it move freely. I had to apply moderate force.

I conclude from that that the VCU is OK, but would appreciate some verification!

A further symptom that I didn't mention in the first post is that there's judder on lift-off as well at lower speeds.

Give it a full service ...

I have every intention of doing so... But a full service is a big undertaking and I want to start by targeting the one problem area!

Amazingly, it has a full uninterrupted service history. It had one owner and has been serviced every year since new right up until 2016. Every year without fail.

Call me cynical but, a car with a full service history can sometimes encourage garages to take short cuts. I always mark my oil filters with the date to make sure they get changed.

Col

The full service is just as likely to address the issue .. even just changing oils and filters, especially in an automatic gearbox when appropriate, can produce wonders ..

Just looked through the receipts. Transmission oil was last done in April 2012 and it's driven 4681 miles in the intervening period. Interestingly no mention of the gearbox oil filter. I've had a look and it doesn't seem like a difficult job.

Engine oil and filter was last done in March 2015 but the vehicle has covered just 325 miles in that time period... The oil looks absolutely spotless. It seems pointless doing this again after just 325 miles?

I'm not trying to shirk away from doing what's necessary but I don't have endless free time or money, so I need to choose my battles!

Going back to the suggestion of checking the UJs - visually, they look fine to me but I don't really know what I'm looking for. They are an inexpensive part and again don't look particularly difficult to change. These are original parts as far as I can tell - certainly not mentioned in any of the paperwork.

Fair comments, I dunno how long it takes oil to degrade, but ..

"2) Putting the vehicle in neutral immediately cancels the vibration." suggests gearbox related, but that might also mean engine or transfer box. Rather than replacing stuff that doesn't look bad or worn too much it might be worth finding and paying a local independant mechanic or dealer who will check it out for you and at least give you a list of possibilities.

stevelup said:

With the vehicle in neutral, I jacked up the nearside front wheel and using a bar was able to slowly turn the wheel and observed the shaft turning. It wasn't jammed, nor did it move freely. I had to apply moderate force.

I conclude from that that the VCU is OK, but would appreciate some verification!

Click to expand...

Sounds fine.

The lift off part. Change the oil in the diffs. That degrades very fast.

Have you checked whether the exhaust is dropping slightly on to the cross member. Might just need tightening up, mine would cause vibration in Drive, but no vibration in neutral

Thanks everyone - lots to be going on with at the weekend!

Is it definitely a drivetrain vibration? My old p38 developed a vibration at certain revs that went away when N was selected. I honestly replaced and changed every moving part I could get my hands on only to find it was the roof skin had come away from the body shell and was vibrating at certain engine speeds. Headling off and some new RTV paste and it was cured. I spent thousands on that p38 trying to find the vibration. Only to discover some £4.99 RTV paste was needed. I strong advise you check your roof skin.

Hope that might help