MY2000 XKR, 94000 miles. Driving back from London, a knocking sound started from the rear of the car. Over about 10 miles it got louder. It changes frequency a little when the car is driven faster, but the frequency nor loudness of the knock is not proportionate.

The car is "driveable" but the sound makes it sound a very bad idea.

The sound appears under acceleration or maintenance of road speed. Under engine braking ("foot off the accelerator"), the sound is inaudible. Driving in reverse gear, it is inaudible/not present.

A local mechanic put the car on his ramp to inspect. The noise could be heard with drive engaged and the wheels rotating. Using a screwdriver as a listening device, he pinpointed the noise to the differential. He said there had been oil leaking from the diff, though I have not seen evidence of this on my garage floor. He has proposed to replace the diff, causing my eyes to water.

I'd be very grateful for any advice or stories of similar issues. eg

What other diagnostics could I/should I have done before committing? Are there any interim "sticking plasters".
Should I drive it to and from the mechanics or have it transported.

 

Seems an extreme(ly talented) diagnosis and recommendation from a quick listen with a screwdriver. Not to say it hasn't happened but diff failures aren't exactly common on the XK8/XKR.

There's a lot of bushes and bearings in the rear suspension, driveline and its mounting frame. I'd be eliminating those as a possible source before giving the mechanic open season on my bank account.

Graham

 

Many thanks - confirming my instincts....

 

Suspension and bushings would not be noisy while she is on the lifter. And trying to locate the source of the noise that way -whith running gear engaged on the lifter- is not that stupid.

On the other hand a diff failure occurs commonly with a constant bearing howling, not with an intermittend knocking. And replacing the diff should always be the last option.

I would have checked first how many oil actually still remains in the diff and would have filled it up if necessary. unfortunately the oil fill plug is not easy accessable.

no oil on the garage floor is definitly no evidence of a non leaking diff.

Fritz

 

Many thanks for taking the time to comment - very helpful. The only other potentially relevant recent change that I noticed was that reversing into my garage, I needed to apply the accelerator a little - normally I just use the brake and do not need to touch the accelerator.

So I should to direct a mechanic to:
• Check and eliminate as a source of the noise, bushes and bearings in the rear suspension, driveline and its mounting frame.
• If diff suspected, check its oil level, flush with oil, check for metal waste from content, refill and report on impact on noise/drive-ablity
• If diff confirmed, replace with [second-hand/reconditioned?]

[I guess I'm perfecting the art of stating the bleeding obvious....]

 

The jurid coupling or a half shaft universal would be the first places I would look. A knock inside the diff is weird. Make sure it's not a loose exhaust hanger or heat shield.

 

A diff will clank, clunk, hum, whine, but not knock usually more than once before it goes to pieces. You wouldn't need a screwdriver to hear it either. Diffs give little trouble at this mileage.
I highly suspect your top shock bushings are gone in both the front and the back. Open the bonnet, remove the plastic shock top cover where the wires go in. If you see light colored rubber dust, and the shock nut is way off center in the hole, there's your problem. Also look at the lower shock bushing in all 4 corners. If there seems to be grease on them, they are blown. You got shock service due. It will ride like a new car when you are done.

 

As far is i understood the mechanic was able to replicate the knocking noise while the car was on the lifter. the shock bushing may be worn but they dont produce any noise with the car on the lifter.

I would force him to prove his own diagnosis. Flushing the diff oil would have been my proposal, unfortunately the diff has no drain plug because of the "lifetime" oil fill.
Suction with a hose could be a solution. For sure you schould be aware of the the correct amount of oil in the diff, whatever the noise may cause.

Fritz

 

Everyone, many thanks for the advice, it is so helpful.

Having had nearly 100k trouble free miles (lucky me), I'm not used to resolving with issues like this and a bit of a "babe in the woods" when dealing with mechanics - they know so much and me so little. I might go back to local garage in Beckenham London that does simple servicing and MOTs for me and my wife- at least I have a level of trust with them, but I'd prefer more XKR expertise...

 

It's really just a car, mechanically. The electronics require special equipment and knowledge, but the drive train is not that different from other touring cars and saloons.

 

Ah well, in the end I took it to another garage without briefing them on the other place's findings. It was indeed the differential. When they disassembled it, they found a number of teeth had been knocked off the gears inside and suggested I avoid putting it into reverse unless the car was stationary. I do recall that 16 years or so ago, when I was accelerating away from a toll booth in France accidenetally putting into reverse at 20+ miles an hour (I was mucking around with the Jgate and gears....). It slowed down very quickly..... Too long ago to be that (nothing bad happened at the time), so I'll make sure that I have the car stationary before moving between forward and reverse now.

In terms of cost - garage number 1, with second hand differential quoted, £2,000 for cash (the guy had a breaker's yard and looked lovingly at old XK. Other quotes were c.1.6k. The selected garage kept it to three figures - just - at £990, with a second hand diff (not reconditioned).

 

'when I was accelerating away from a toll booth in France accidenetally putting into reverse at 20+ miles an hour'

Pretty sure the reverse gear solenoid wont allow gear selection unless you are going at a crawl. If it did, something is wrong

 

You got a good price. Mine was refurbished, total cost over $2000. Nice Job.


John

 

Let's hope it's another 17 years before I have to do that again.

 

The reverse gear solenoid shouldn't allow it but crap happens.

I did exactly the same thing with an Opel many moons ago, it spun up the wheels when it went into reverse.

 

The garage showed me the innards of the diff.
Around four teeth of a large bevel gear had sheared off and the remnants were rattling around the diff, damaging the teeth on other gear wheels. I was astonished by the size of the chunk that had come off - I thought that I might see some metal shavings......
Weird or what?