Sharp knock[s] in the bottom when dropping the gas?

[vt]

1st gear, accelerate, sharply drop off the gas - and invariably get a heavy sharp knock somewhere in the bottom, middle to rear. Sometimes it is not one knock, but two or three. Sometimes the knock is delivered if I even gently ease the gas when approaching the curb to park. Doesn't happen on second gear and up (can't be sure, though, this is not my car, I just drove it around the block to diagnose the problem).

So, what does it sound like? I initially thought that was backfire (heard it when the car was driving away), but now that I drove it I would rather think that it's either the drive shaft lash, or, even worse, suspension or diff mount.

Any more clues?

I will have the car on the ramp as soon as I can, it would help if I knew what I should be looking for.

[Coldswede]

I'd look first at the differential mount. Try backing the car in reverse, a bad diff mount will be much more pronounced in reverse.

[vt]

[boardsportsrule]

yes, that's the part he is refering to, although it can be had for much cheaper than listed on real-oem.

it's quite simple..5bolts to take off, and put back on, and makes a world of difference. As stated above, drive in reverse and see if it happens more, if it does, that's pointing towards diff mount, if not..let us know!

[vt]

Dumb question - is the visual inspection of this part when the car is jacked up going to reveal anything?

(my hunch that it won't)

In case it doesn't, am I likely to see anything when I take this part off?

[Tony533i]

Something else to look at. My car was making the same noise in the same location. Turned out to be the exhaust (Bosal factory style aftermarket replacement) was hitting the body when I took off and then let off the gas. A sharp crack, crack, crack at about the parking brake area. I realigned the exhaust, put the transmission to exhaust bracket back on and the noise was gone. This may not be what yours is but is worth looking at.

Tony

[vt]

[2ndeagle]

I had the same problem described. Try the CSB (center support bearing). Bet it's toast. If you can address the problem soon enough, you can save yourself the expense of a driveshaft. Take a look at the transmission bushings while you are under there. If you have to change the CSB, may as well change the trans bushings while you are there. And vice-versa.

[Mike W.]

[vt]

[wkohler]

If in fact your diff mount is bad, it will show itself when putting the car in reverse (if a stick, you'll have to accelerate in reverse, of course). A visual inspection will show that the rubber has pulled away from the metal frame. You want the M535i/528e mount at $70, rather than the $2xx 535 mount.

I was thinking center support, but it only seems to happen on deceleration, so there goes that.

[vt]

[tn535i]

M535 diff mount 33 17 1 129 786 ~$62 when I bought one last but I don't remember where.

Braking won't produce this clunk, only engine brake decceleration or acceleration in reverse, same force on the differential actually. That thing is not so robust in the reverse accel direction and the clunk is probably from when it slaps back into place.

[vt]

[boardsportsrule]

Try checking it in reverse, that should clue you in to CSB or diff mount to check first...eventually you're going to have to climb under the car and get dirty..

[vt]

[California Dreamin']

What car do you have?

If it's not the center support bearing (and I agree with above about accell vs. decel) then the next thing I'd look at is the harmonic balancer. If your car has one - not sure about eta's.

[vt]

[2ndeagle]

[vt]

[2ndeagle]

That sounds more like diff, if not attributable to accel or decel. You're going to have to crawl under there. You have the potential problem areas defined, the rest is visual and probing.

[tn535i]

[vt]

[wkohler]

MAKE SURE YOU ORDER THIS ONE!

http://www.autohausaz.com/search/product.....e/Differential%20Mount

[vt]