vibration ONLY when I let off the throttle

[irish44j]

This is really just a sanity-check. The rallykar ran fine yesterday, but on the (3-hour, all-highway) drive home I was noticing a slight "rumble" or vibration from the mid/back of the car that only happened when I let off the throttle or had very light throttle.

On the throttle hard it is smooth (well, as smooth as an M42 car with solid engine mounts can be). Seems to happen in all the top gears (hard to tell in low gears) when I let off it kind of rumbles. By the time it got home, it almost felt like I had a loose wheel (lugs and wheel bearings are good, so it's not that).

Other (possibly related) symptom is that recently when I accelerate hard(ish) and then lift quickly, the rear end of the car seems to "step out".

1. when coasting in gear or out of gear, no vibration/rumble
2. when on the gas at half or full throttle, no vibration/rumble
3. rumble/vibration only apparent when I let off the throttle or coast in gear, or are on light throttle.
4. transmission shifts great, and smooth
5. diff fluid was replaced a month or two ago (full synthetic for LSD, same stuff I've always used)
6. I have not greased any DS fittings in a good while....

I have my suspicions about what it is, but want some unbiased opinions before I take the car apart. This is a rallycross car, so it is REALLY dirty under there, lol. The less time I spend underneath searching for problems, the better. :)

[jalopi]

you sure the wheel bearings are good? kinda sounds like a wheel bearing issue. I guess it could be the axles or the diff too.. hard to diagnose over the internet though.

[Todd Black 88]

Let me know what you find, as I have had the same issue for years. Everything is new in my car except the diff, but I have swapped in three diffs and still have it.

[irish44j]

you sure the wheel bearings are good? kinda sounds like a wheel bearing issue. I guess it could be the axles or the diff too.. hard to diagnose over the internet though.

yep. and a wheel bearing issue would manifest when coasting, on throttle, etc. This issue isn't speed-dependent at all, it's only dependend on the throttle load on the rear end.

And trust me...this is a rallycross car. I know well what a trashed wheel bearing sounds like, lol.

[irish44j]

Let me know what you find, as I have had the same issue for years. Everything is new in my car except the diff, but I have swapped in three diffs and still have it.

my initial suspicion is that a u-joint in the DS is bad, or seizing. I never got around to greasing the DS before this season, and we're running in dirt and gravel and mud so all kinds of junk gets up in there. ALso could be tat the slip-joint collar is loose.

Just a PITA to get to all that since my exhaust isn't exactly "stock" lol.

[Todd Black 88]

I have a brand new rebuilt driveshaft, new csb, new axles, wheel Bearings, t/a bushings, subframe bushings, you name it.
Sound is still there for me. It's right at lightest of throttle, like coasting, and it's there.....grumbling.

[irish44j]

well, just got under the car and took the heat shields off, etc. CSB looks good (it only has about 10k miles on it), guibo still looks brand-new. All bolts are tight. when I have a helper spin the wheels no clicks or noises from the DS and no sign of any binding of UJs. Both wheels spin smooth and zero bearing play. Diff is still being an LSD :)

ONE things I did find though is that the "cap" on the outer CV on one side (the metal cap piece on the actual CV joint where the outer boot attaches) has come off at an angle, and as you spin the wheel it slips on and off.

So.....that may be the culprit. Not a mechanical issue though and I don't know why it would only affect things when at lightest throttle, but I guess it could cause enough imbalance to cause a vibration. Also since this is a rallycross car I suppose it could have let some mud or water or crap into the actual joint. So I guess the plan will be to put a new axle on that side (probably do the bearing while I'm at it since I have a spare sitting around anyhow), and see what happens from there.

Else it's just a grumpy, 29-year-old diff I guess. Already looking around for a replacement. I've been thinking about changing ratios anyhow so might be a good time to see what is out there.

[jalopi]

Just re-read the first post, holy crap you've got balls of steel driving 6 hours to and from a rally event. And here I thought driving 3 hours was bad for me going to Summit :drink:

But yeah man, I don't know what it could be.. could be so many things. But considering it's rally, I guess something could've come loose and/or fallen off.

[irish44j]

Just re-read the first post, holy crap you've got balls of steel driving 6 hours to and from a rally event. And here I thought driving 3 hours was bad for me going to Summit :drink:

But yeah man, I don't know what it could be.. could be so many things. But considering it's rally, I guess something could've come loose and/or fallen off.

lol....almost all of us drive there (including the four BMWs at the last event). I tow there for 2-day events but gas cost is too steep for just the one-day. If something breaks, that's way I have Triple-A :)

It's less than a 3 hour drive there though, not 6 (unless you meant combined there and back). When we went to Ohio (6-7 hours) I trailered the car there.

[FredK]

Any progress on finding the culprit?

I just replaced my driveshaft with a Beyer Driveline reman, new CSB, and nearly new flex disc.

I still have a vibration mostly when I accelerate HARD then let off to partial or no-throttle. I am beginning to suspect there's a little bit of binding in the diff clutches and steels, so I'm going to try to squirt some friction modifier into my diff.

I have poly engine and transmission mounts.

[TobyB]

a u-joint in the DS is bad, or seizing.

Or the driveshaft is slightly out of balance, or the center support's not preloaded enough.

I've also had slightly bent diff flanges do that- a few thou of runout seems to do it. More, and it's driveline vibes all the time.

t

[FredK]

I did a diff fluid change with 4oz FM and nothing changed. Diff does seem a bit quieter though.