hi guys, i posted a question about my irs making a clunk noise when changing gears,like there is play in it. i wanted to know if anyone can direct me to info on this type of problem.thanks
Are you sure the 'clunk' is coming from the rear end? Usually clunks while shifting have more to do with the u-joints of the drive shaft. Ona side note. I spent most of the day today alligning my drive line and couldn't be happier with the results. I used this write up as a guide line. Driveline Alignment. I also changed my diff. fluid with Mobil 1 75-140w oil and Motorcraft friction modifier and used Motorcraft PTFE lube on the slip yoke. My Cobra is smooth as glass up to 100 mph (so far) and no clunks or chunks coming from the driveline at all. I highly recommend you do this especially if your car is lowered. I don't have a lift so I spent several hours on my back smacked my head multiple times on various parts of the exhaust system. The results are well worth the time and aggravation!
I also have the clunk... the friends I have that have driven with me noticed it too but have no idea what it is. Id love to try to tackle the driveline alignment to see if that helps, but I dont have the tech skills yet.
A LOT of times it's the stock rubber differential bushings, they'll split and crack causing more movement in the center section which in turn creates wheelhop and strange sounds!
Lots of guys that strictly run their Cobras at the dragstrip will put solid steel bushings (Delrin)in to tighten up the IRS as much as possible, not the best though for a daily driver but Steeda and other companies make decent poly bushings that are superior to stock. You should get it on a lift to inspect the stock stuff before you go ripping it out though, it might not be the problem..
hey guys, just checked back. the "clunk" is at the rear of car when i shift gears or let on the gas or let off the gas. the car has 110,000 miles and i was told the prev. driver rode the car hard. (i gave 4000 for a 99 conv. cobra. i'm not bitching) but, i would like to follow up on any suggestions for diagnosing this noise. there is about a 3/4 inch back and forth movement of the drive shaft if i lock the emergancy brake. any suggestions? anybody covered a shimming of the rear end to tighten it up. thanks.
Movement in the drive shaft with the e-brake set is normal nothing to worry about there.
I would start by pulling the rear wheals and making sure all your suspension componets are torqued down to their respective specs( ie tie rod ends, control arm bolts, etc.).
Then get under the car and make sure all the componets that bolt your pumpkin to the subframe are tight. While you are doing this inspect for broken, craked bushings. Replace broken bushings preferably with performance aftermarket bushings.
Pull your driveshaft. Check for u joint wear. Replace u-joints as needed. You can take the driveshaft somewhere to have this done or just buy a new driveshaft. (preferable with an aluminum piece, if you find one for our cars let me know.)
When you reinstall the driveshaft USE PTFE lube from motorcraft on the slip yoke! You may want to pull your driveshaft just to do this. A 3oz tube from Ford is 7 bucks and will cut down on clunks from the driveline area considerably.
Unfortunatley you won't be able to access the differential without completely taking the rear end apart. You can however(using gear oil pump) drain and refill your rear end fluid. At your mileage its probably overdue. Don't skimp use Mobil 1 75W-140 and Motorcraft friction modifier.
I am no mechanic but I have done all this on several Mustangs I have owned. Read up on everything get a Tech manual and be prepared to get dirty and smell like rear lube. It will be worth it. Also check your driving technique learn to work the throttle and clutch evenly and smoothly and brand new car will clunk and chunk if you drive it like a clod! Good luck!
Many of the IRS cars have a rear driveline clunk (from the factory) just from the IRS mounting bolts not being torqued quite tight enough. There was even a TSB about it in '01. Retorquing those bolts can often mitigate the problem.
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-- BK
'01 Cobra Black/Charcoal
'84 GT T-top Black/Charcoal SEFI
I had the "clunk", too. Replaced the IRS bushings, with correct front bolts and that did not completely cure it. Recently, i had to re-seal the rear cover. Those rubber bushings on the IRS differential are SOFT and allow a lot of movement of the center pumpkin. I replaced them with poly and the clunk is gone. You can use delrin or aluminum but I wanted less road noise.
The fronts are no problem - easy. The rear bushing is a slightly different story. First, Ford puts a thread lock on that is really tough to get the bolts out - you have to use a breaker bar the entire way. Then, you need the tool to remove the old rubber bushing - it's included with the MM kit. Make sure you use a wire wheel on the old bolts then use the blue locktite when you put them back in.
I had to drop the entire center pumpkin as I had to re-seal the rear cover and that is a bit of a pain. Have to remove the half axels, driveshaft, upper control arms, etc. The center section is about 70 pounds so it's no lightweight when you have to put it back up.
where can i buy these bushings? my car does it sumtimes, randomly? u think i can just bring it to a ford dealer and tell them what i read on here and wut it does and they can just replace the bushings for me? cuz im not mechanical enough or have the right equipment to do it my self.
99 cobra irs clunk/vibration. 
) and no clunks or chunks coming from the driveline at all. I highly recommend you do this especially if your car is lowered. I don't have a lift so I spent several hours on my back smacked my head multiple times on various parts of the exhaust system. The results are well worth the time and aggravation!
