My car started developing some squeaks and hollow almost imperceptible clunks when driving on washboard surfaces. It's definitely coming from the rear. Although the possibility exists that this clunking may be coming from a loose part such as the rear bumper cover, I'd still like to go over the suspension.

Last time I worked on the rear suspension, I installed brand new CATS shocks, installed 1 inch lowering springs, shock mount isolators, half-shaft spacers to correct the camber, anti-roll bar bushes and anti-roll bar links. That was about 13,000 miles ago. At that time, I did not replace the two large rear A-frame bushes (MJA2595AC), the differential strut mounting bushes (MJA3715AB2), the rear bottom differential bush (CAC4713), nor the rear differential pendulum top bush (CAC77601).

Now that I'm at 95,000 miles, are any of those bushes that I have not previously replaced likely to be the cause of the clunk? At what mileage do those rear suspension bushes mentioned above typically begin to fail? I have not yet crawled underneath the car to assess their status. I remember reading threads that said that replacing the rear A-frame bushes results in a massive handling improvement. My original plan was to drop the entire IRS and install a Limited Slip Differential and replace all those bushes as well. Currently, the LSD is out of my budget. For the time being, could I get away with just replacing the A-frame bushes or should I do the entire thing and re-visit the differential at another time?

Has anyone replaced all of those rear suspension bushes with the powerflex polyurethane upgrades? Do the polyurethane rear bushes noticeably affect NVH? I had initially gone to a full polyurethane front end but had to return to OE bushes for all four lower control arm bushes to improve NVH to a tolerable level. Sometimes I consider doing the same to the top bushes but then I remember how that front bolt can be a ***** to remove and I decide against it. The roads here are truly craptastic.

Fun fact, I installed the first pair of upgraded shock mounts ever made by Wayne, the prototype pair that was tested in the Puerto Rico's lunar surface style roads, 15,000 miles ago, back in 2021. This was before I decided to lower the car. Those mounts are still in as new condition. Excellent job Wayne!!

 

Nobody?? I'm asking about these bushes that the red arrows are pointing to. Particularly interested in the really big ones circled in yellow.

1. Will ride quality and NVH noticeable worsen if I replace these bushes with polyurethane?
2. At what mileage/age do they typically fail or deteriorate?
3. Could any of these be the source of a clunk in washboard roads?
4. Can I get away with only doing the ones circled in yellow for a year or two before I go back in and do the rest?

 

1: My garage has always told me to stay away from poly bushes but our roads are pretty terrible so if you're roads are in good condition it will likely be fine. The thing is those metalastik bushes have lasted 20+ years which is a long time for any car bush. Polybushes might not last as long but they may also last longer.

2: No idea, I'm at 90k miles and still OK on most original bushes.

3: Any of those could be the clunk. Drop links as well.

4: Yes but it might be best to just replace them all in one go.

Others with more experience might give you more details though but I do think poly would be a good way to go but it might not be great for you're roads so if you repalce them all with poly and the ride is terrible you might have to be repaired to redo it all with OEM bushes.

 

^^ This....

 

1. Yep, you're right. I couldn't live with them too long in the front because of how bad our roads are. Ended up going back to metallastic for the lower wishbone bushes. I was wondering if the impact in the rear would be as bad as it was in the front. Thank you for pointing out what I was failing to see. 20 years is a long lifespan for the metallastic bushes. Any potential improvements on longevity and handling that the poly bushes may provide will not offset the high likelihood of worsened NVH. You're persuaded me that metallastic is the way to go.
2. I figured. I haven't read many posts about failing rear end bushes except for the big ones at the subframe.
3. I'll check my drop links. I did replace mine in 2021, but the roads here are terrible...
4. Ideally yes, I would love to do it all in one go BUT, because my 14HU diff is already seeping at the output seals to the halfshafts, I need to do a rebuild. But if I have to start opening up the diff then I HAVE to also use that opportunity to upgrade to an LSD, whether that's the Quaife ATB or to a 15HU. That is where the project exceeds my budget.

The Quaife on its own is £940 + shipping from the UK. The parts to DIY recondition the 14HU are also expensive. An already reconditioned 14HU with the Quaife already fitted is £2,115 or so + shipping from the UK.

Then the option of the 15HU with posi also has many issues. The biggest issue is finding one. Locally I haven't found any. Online, I have found a few for between £1400 and £1900 + shipping from the UK for reconditioned units. If going that route, dealing with the offset input and different style driveshaft inputs is the easy problem to solve. The big problem would be the gear ratio. Most posi 15HU have 3.58, or 3.77 or 4.08.1 gear ratios. The unicorn XJR6 3.27.:1 is the closest to the gear ratio of our 3.06:1, but that is very hard to source. And even that ratio will still result in ABS errors, Incorrect Part warnings, inaccurate speedometer, inaccurate odometers, and worse fuel economy. As far as I know, the ECU/TCM issues have not been solved for the 4.2 ZF 6HP26 cars. Likely this would require a standalone ECU or some way to spoof the TCM. Chirpy was able
to solve this for the 4.0 cars with the Mercedes 722.6 transmission.

So given those challenges, I think the most prudent route for me to tackle the rear suspension all in one go would be to rebuild my diff and DIY install the Quaife and hope to god that I do not mess up the pre-load and backlash.

Maybe I could get some transmission shop or driveline specialist to rebuild the diff and install the Quaife for me while I take care of the rear suspension.

 

Why do you HAVE to have an LSD? You will be dumping a ton of money into a vehicle that continues to decline in value with each passing year....

I understand the suspension repairs. They are critical. But a tremendously expensive LSD is not....

 

XK8 XKR 14HU 3.06 RECONDITIONED DIFFERENTIAL UNIT | Simply Performance

They offer an LSD option in their rebuild.

 

I haven't done the back on my car yet, but when I did the front I used original style rubber ones, and I thought that in order to not overtax these rubber bushings that they should be tightened up with the suspension travel at the mid point of the range! Was I dreaming? or does this make sense? The rubber parts are vulcanized to the metal parts, not slipping in side the metal!

 

It's a car that cost me nothing to acquire because it was my late father's car and has sentimental value to me. I don't see myself selling it. The declining value is irrelevant to me.

I might as well make improvements to improve my enjoyment of the car. Why dismantle the rear end and fix the diff leak and not take the time to upgrade? If I'm going to be in there, then I might as well go for it. It's not a question of if, it's a question of when. And if I can do this in one shot rather than having to dismantle the rear end twice, I'd actually be saving money in the long run. An LSD will be in the car eventually.

It's fine if you don't agree, it's not your car.

Yeah, I know, I quoted their price. £2,115. They also sell the Quaife unit on its own, for £940. So, I think I'm better of rebuilding it myself and fitting the Quaife.