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Discussion Starter #1
I was talking to a mechanic-friend of mine who used to be a tech at Ford about the "clunk" that a lot of mn12 cars get when letting off/getting on the gas and when shifting to R from D etc, etc.

He thinks the "back lash" inside the diff. is one of the reasons, and says that this is very common in Ford rear ends.
He also says that the reason for the clunking not being bad when it's cold outside is that the diff.Fluid is thicker when cold(I thought EXACTLY the opposite!!), so it performs better...

IF this is true, would using a thicker than recommended diff. fluid get rid of the clunk, or make it clunk less??

is anyone running a thicker diff.fluid? if yes, why?
or are most people running the recommended weight?(is it a 75w-90?)

Thanks for any opinions.
 

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I think you'll find that the IRS mounts are much more of a problem than anything in the rear itself. If you don't think so, remove the front 2 IRS mounts and look at how they've compressed over time. I have never seen your car, but I guarantee the problem exists. This may not solve 100% of the problem, but its a major part.

I run 80W90 Motorcraft, and don't really think a different weight will affect the clunk much at all. .008"-.015" of backlash isn't nearly enough to cause the kind of clunk I think you're describing, IMO.
 

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I agree with silver95.

The IRS cars blow out the irs bushings like nobodies business.

Also check the U-joints on the driveshaft.

JH
 

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Discussion Starter #4
Siler95bird,
Justin-H,

Thanks to both of you for your replies.

you guys pretty much said what I've been thinking, and I'm glad my thoughts are being confirmed.
I know my IRS bushings are bad, and I've alreay made an appt. to get them changed next week. (my bird has 93k miles right now, and I'm excited about the difference they might make next week)

I'm also getting my u-joints checked very soon.(I have huge difficluty in taking time off work, otherwise I would've done the irs bushings a lot sooner; have known they were bad for a long time.)
I do think that the reason for my "clunk" and some vibes is irs bushings and/or u-joints.
If those 2 things don't cure it, I'm moving to the tranny mount as next step.

QUESTION: does any of you have the oem recommended torque amount to be used when installing the bushings? I'd really appreciate it.
How many bushings are there in total?
I was told my "rear IRS diff. bushings" are the bad ones by the shop.

(I'm not getting the poly, just the oem ones, I don't think I need the polly's for my kind of driving, and i've heard that they sometimes transfer more vibes, although more durable)
 
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