2004 Mustang 3.8L V6 vibrations above 65 MPH and differential leaks

[joeybuddy96]

tl;dr: I'm at a loss as to where some oscillations are coming from at 65 MPH.
I think there's leaks at the pinion seals and at the rear cover gasket. I'm putting together a shopping list of what all I need to replace. I think there are two pinion seals, one on the driveshaft end and one on the pinion end. There's a lot of oil buildup on the brake rotor mud guards, so I'm thinking that's gear oil rather than brake oil, meaning there might be leaks on the axle seals also. *If* there's something wrong with the axle bearings or differential bearings causing wobbling, now would be the time to replace those. I just don't know which bearings or seals in the differential exactly are causing it. Rebuilding a differential is a very precise job, with like .002" tolerances in adjusting the pinion and ring gear alignment, so I only want to replace what needs replacing. I've seen rebuild kits that include carrier, pinion, and setup bearings and races; a pinion seal; and a housing cover gasket. It might also mean getting new crush sleeves or spacers, clutches, and carrier & pinion shims. I don't know if it's possible to put things back how they were or if everything needs to be readjusted from scratch. I think the differential ring gear size is 7.5", the gear ratio is 3.27, it has 28-spline axles, and it's an open type, if I'm reading this website correctly. Idk if changing to a 31-spline axle differential matters, but I guess I'd switch to that if I end up needing to replace a lot of the parts in it or end up buying a completely new one; the problem would be that I think you have to have a 8.8" compatible housing that could hold a new 8.8" compatible carrier. That means taking an 8.8" housing off at a salvage yard from a V8; I'd be able to install axle dampers if I got one of those.

I'm getting vibrations once it hits 65 MPH, at about 2100 RPM; the vibrations increase in intensity over like 10 seconds, and decrease in intensity over like 10 seconds. I've revved it to 2100 RPM in neutral, but there are no vibrations there. I've changed out the ball joints, control arms, and inner/outer tie rods. Given that the vibrations aren't constant at 65 MPH, and have a rise/fall cycle, I don't think it's the motor mounts or any rear control arm bushings; it's a pattern that belongs to some kind of rotating part. I don't think it's the drive shaft, because if it were, I'd be feeling it at neutral at 2100 RPM. I don't think it's the rims, because I've swapped where the wheels are before; I could try swapping them around again, since after I changed the control arms, the vibrations decreased. It could be that one of the wheels needs to be re-balanced. I don't think it's any wheel bearings, since I'd be hearing that loud roaring sound if it was; I've had a bearing go out before, it's hard to miss, lol. That leaves the rear axles and the differential. I'd like to think it's the rear axles or axle bearings, since the differential has like two dozen different parts to it. I don't know how the axles could possibly get bent out of shape, since they're inside the axle housing. If it is the differential, I have no idea which part it could be; I've never had to disassemble one before. I could just forget all that stuff and get a pre-built differential, but that'd be about $400 more than rebuilding it, but if that's not where the vibration is coming from, that'd be pointless. It's a labor & time vs money trade-off.

 

[gkomo]

I mean it’s probably something in that diff but just wanted to say the driveshaft doesn’t spin in neutral so it could still be the driveshaft.

 

[joeybuddy96]

I mean it’s probably something in that diff but just wanted to say the driveshaft doesn’t spin in neutral so it could still be the driveshaft.

Yesterday I drove it in low and 2nd gears on mostly smooth roads at 2100 RPM to test it out. I couldn't feel any of the same kind of vibrations that I get at 65 MPH.

 

[ctandc]

Do you feel it in the wheel or in your seat?

 

[LILCBRA]

I don't think it's the drive shaft, because if it were, I'd be feeling it at neutral at 2100 RPM.

Yesterday I drove it in low and 2nd gears on mostly smooth roads at 2100 RPM to test it out. I couldn't feel any of the same kind of vibrations that I get at 65 MPH.

I would lean toward your driveshaft vs the differential and axles. The reason you don't feel anything in neutral is because the driveshaft isn't under a load. The reason you don't feel it at lower speeds is that it hasn't reached a high enough RPM to induce harmonics that would be more prevalent at higher speeds. Replacing U-joints is a lot cheaper and easier to boot, I'd suggest starting there.

 

[joeybuddy96]

Do you feel it in the wheel or in your seat?

Both. You can hear the interior trim and stuff rattling, too.

 

[ctandc]

Both. You can hear the interior trim and stuff rattling, too.

Driveshaft is the most likely culprit - as someone else suggested. Easy thing to try - remove it, and turn it 180 degrees and reinstall it. Of course before you reinstall - make sure you don't have a failed u-joint right off the bat.

If the vibration changes at all - you've found your culprit more than likely.

 

[joeybuddy96]

Driveshaft is the most likely culprit - as someone else suggested. Easy thing to try - remove it, and turn it 180 degrees and reinstall it. Of course before you reinstall - make sure you don't have a failed u-joint right off the bat.

If the vibration changes at all - you've found your culprit more than likely.

That sounds like a good idea. I already bought two new u-joints before I saw your comment. I planned on changing those out anyway. If doing all the replacements I have planned doesn't work, I can always try flipping the driveshaft around or taking it to get it rebalanced.

I also got a cover gasket, pinion seal, output shaft seal, and bearings & seals for the axles (because one of the axle seals appears to be leaking). I only want to get all this stuff done once, and get it all done correctly. It's my daily driver, so I can't just get this done halfway and wait a week for more tools or parts to arrive if I get stuck at some point during the replacement process.

I did have another quick question. I'm going to be replacing the pinion seal (because I think it's leaking, see photos), which means taking the pinion nut off and putting a new nut on (the new nut arrived with some dry threadlocker pre-applied to it). I want to make sure I'm not ruining the bolts that attach the flange to the yoke; I don't want to bend those bolts or mar their threads. Do I *need* to buy one of those pinion flange holder wrench thingies, or is there some other way of getting the nut on and off without special tools? They're basically just a thick metal plate with some holes drilled through them. I don't know what brand to get, if so. There seem to be a lot of $20 ones that aren't compatible with the 7.5 differential flange, but I'm sure there are some that sell for around $30 that'd work (again, assuming the holder required for the job).

I'm going to be doing the replacement with the car up on jack stands, so there won't be much clearance or room to get a lot of leverage. I plan on not using an impact wrench to get the pinion nut on and off, because I'm not going to be using a beam torque wrench to measure drag--I'm just going to mark the nut and the pinion, count how many turns it takes to remove the nut, and line it back up *exactly* like it was when I removed it and no more than that (I've seen some articles that say to tighten the nut slightly further than that mark to make sure the crush sleeve has the same force on it as before, but I've seen more people saying to not to overtighten it). I really don't want to mess up the backlash and have to figure all that out or get a beam torque wrench I'll never use afterwards. I wish I could heat up the nut with a propane torch to make it easier to tighten down, but if I do that, I think it'll melt the threadlocker and rtv on the nut, so idk what to do about that. I can't even put oil on the threads because it might interfere with the threadlocker on the nut keeping its grip on the threads.

I plan on putting a small amount of RTV on the yoke splines (not the pinion splines), but I don't want to put any on the seal itself, the inside of the cylinder, etc.. The seal itself absorbs oil and swells up to block gear oil from leaking out, so I don't want to disrupt the seal's surface contacts in any way. I've seen videos of people packing pink or red grease (probably high-tack wheel bearing grease) inside of the seal where the spring is, but I don't know what kind of grease to use for that; all I've got right now is regular old dark gray grease; I forget what kind it is (probably some kind of #2 synthetic); the grease and the gear oil need to be compatible if I'm going to try doing that. Do I have this right, do I need to put oil, grease, or RTV anywhere else?

 

[michaelleewebb]

are you sure your tires are balanced ? i'm sure you have but i don't see anywhere where you stated that you had them checked.

 

[joeybuddy96]

are you sure your tires are balanced ? i'm sure you have but i don't see anywhere where you stated that you had them checked.

I got them balanced when I bought them, but they haven't been re-balanced since then.
Right now there are weights on:
FL inner & outer
FR inner only, no outer
RL inner only, no outer
RR no inner, outer only

Regarding the pinion yoke holder, if it wasn't clear already, I'm not crushing the crush sleeve again, so I don't think it'll take as much force to torque the new one down to spec, but removing the old one will still require a holder, I think. Ideally, I'd be wrong about that and I wouldn't need to buy one. Jegs has one for $15 (part number 555-81658) and Ratech makes one (product number RAT-18001) that is available for $20 from a bunch of different retailers. A couple of reviewers have mentioned that the Ratech one rounds out the 1/2" drive hole and could only use it with a 3/4" breaker bar or 1/2" to 3/4" adapter. A redditor said I could, "Buy a 4 ft length of 1.5 inch angle iron from Home Depot and drill your own holes, grind the center so the socket fits, done." A 4' length piece costs about $25, so it wouldn't be much different, I'd have to put in a couple of hours of work and electricity trying to get that done, though, assuming the drill bits I have would work and assuming I wouldn't need to buy a drill press (both of which might be big "if"s). I could repurpose the steel for other stuff later on, which I wouldn't be able to do with a prebuilt yoke holder.

 

[joeybuddy96]

I snapped one of the bolts putting the differential cover back on. It now has a slow leak, even with the other bolts secured. The gas tank cover makes the space too tight to fit a drill in to get the bolt extracted. If it's the body of the car behind that cover, then removing that isn't going to help anyway. So now I'm trying to figure out how to gey some kind of right angle adapter or tool in there that is compact enough that it'll actually reach the broken bolt. Let me know what your suggestions are. I could try to use a right angle drill attachment (possible brands are Titan and LaBear), with a drill chuck (Neiko brand). Right angle die grinders might be a possibility, but I've reas that their RPMs are too high for drilling. I can set my air compressor to a lower output, so I don't know if that's really a problem. I don't know if regular drill bits even fit onto die grinders, so someone tell me if they're compatible or if I'd need to buy another adapter for one. I do own an electric angle grinder (not a die grinder) and there is an angle grinder arbor to drill chuck attachment on Amazon, but it has almost no reviews and angle grinders have higher RPMs than drills, and trying to control that tool along an axis perpendicular to its intended angle in tight quarters seems like a good way to injure myself. The easiest solution would be to get a right angle air drill from Harbor Freight, but those are $70. There's a Matco right angle drill I can get used for $50, so I'll get that one if I try that solution.

 

[DadLife0724]

Jack up the body and let the Axel come down a bit. Might give you the clearance you need.

 

[joeybuddy96]

Jack up the body and let the Axel come down a bit. Might give you the clearance you need.

I completely forgot about that, lol. I'd had the jack stands under the axle housing the whole time I was working on stuff. I'll have to try that next.

 

[joeybuddy96]

I wasn't able to reach the broken bolt by jacking it up by the torque boxes. I ended up having to buy a right-angle air drill. I wasn't able to get the bolt out by drilling it out. It's just too hard to get it centered with the differential cover still on. I think if I really wanted to extract the bolt, I would have to completely take off all of the bolts, drain the gear oil, and use either a wide diameter drill bit or a conical deburring bit to make a centered depression in the bolt so that I could get a hole drilled for the extractor bit. It hasn't been leaking, at least, so the other bolts seem to be enough to hold the cover on tight enough to prevent leaks.

I got the extension housing seal, extension housing bushing, and the front differential seals replaced. I should note that the yoke nut was only at 3 in/lbs of drag. When I replaced it with a new nut, I tightened it down to about 11 in/lbs. The vibration is still present.

I guess my next step would be to get the wheel rims professionally checked for being out-of-round and re-balanced. As far as I can tell by eyeballing every rim while spinning the wheels while they're off the ground, they're not out-of-round (I know you can use tools from around your own garage to do it, too). I plan on buying a used tire and rim from the salvage yard. The front right rim probably ought to be replaced bc it got marred in Decwmber. The tire on it is new, so it's going on a rim that the rear left tire is on. That tire has little to no tread toward the inside edge. That in itself could possibly be causing the vibration.

Other than that, I think the next step is to get the driveshaft rebalanced. I don't think that's likely, bc once it's balanced, it's not like anything has physically changed about the driveshaft. It's not aluminum, it's not like it could be bent.