"Thump" while driving

[BobG]

Just got a '72 240Z - first time owner... car sat for a bout a year in previous owner's driveway. Ran new fuel lines and ran new wiring to the alternator and VROOM she started up immediately.

 

While driving around this weekend (first time out) there is a noticeable "thump" in the rear end when I let go of the gas.

 

Is this normal?

 

There is no loss of engine power, no loss of steering control, no effect at all. Just a "thump".

 

A friend used to have a 260 and said it did the same thing.

 

Anyything to be concerned about?

[Domzs]

Depends on the noise , I would check all your bushings and u-joints . And make sure everything is tight .

[WRC17]

I would agree.  My car does the same thing and I know for a fact it's the bushings because they are all just about gone.

[sito]

Possibly bushings, driveline slop, diff/mustache bar. Like everyone before me said, it depends on the type of noise and is it every single time or only occasionaly? Since its a new old car I would go through the entire car especially the suspension. 

[Enginerd81]

I just got my '76 a few weeks ago and am trying to track down a similar thing. Previous owner replaced, diff mount, mustache bar bushings and trans mounts. I tried a new driveshaft and the halfshafts all feel tight. There is some slop in the drive train - can rock driveshaft back and forth with no motion from the wheels. Wheel bearings are on their way out but not to OMG status yet. All rear suspension parts are tight otherwise. Diff strap is intact.... Short of diff backlash I'm at wits end. I have a slight thunk when I get off the gas or quickly engage the clutch.

 

Here's some good resources:

http://forums.hybridz.org/topic/105207-the-dreaded-diff-clunk/

[Enginerd81]

I especially like the last line from this article,http://datsunzgarage.com/probs/

 

CLUNKING REAR END
This is the bane of all Zcars, it's almost impossible to completely fix. It can have many causes but it's rarely caused by the internals of the differential, especially if it's an R-200. The problem is that if you tighten things in one spot it seems to immediately shows up in another: these cars will find any slack in a heartbeat. If there is one thing that Datsun should have designed differently it's the rear end on these cars.

If you hear it thumping the floor behind your seat it's a broken/stretched differential mount...the rear driveshaft flange can actually jump up far enough to strike the handbrake linkage. This sounds like a ratcheting sound on hard acceleration. If you hear it farther back its likely one of the causes below:

• - u-joints on the halfshaft ends are loose and worn-out (common, replace the joints)
• - tighten the bolts/nuts on both ends of the halfshafts
• - driveshaft bolts loose
• - worn out mustache bar bushings (replace with urethane)
• - shocks bad or loose in strut
• - replace the control arm spindle pins
• - ball bearings/grooves in halfshaft body worn (replace halfshaft)
• - tighten the two large nuts that hold the differential to the mustache bar
  
  Even after you try all these you may still have a clunk. The only solution then is learn to shift more smoothly.

[Phantom]

I originally had this in my 280Z with the original drivetrain and it was the front differential mount.  Replaced it and the problem went away.  Now I have an LS1 and T56 and the problem is permanently cured as the rear of the T56 is bolted directly to the nose of the R200 using a pair of C channels to create a torque tube that replaces the rear tranny mount and the front differential mount.  Been driving it 34,000 miles now with no issues.

[Norman V]

I originally had this in my 280Z with the original drivetrain and it was the front differential mount.  Replaced it and the problem went away.  Now I have an LS1 and T56 and the problem is permanently cured as the rear of the T56 is bolted directly to the nose of the R200 using a pair of C channels to create a torque tube that replaces the rear tranny mount and the front differential mount.  Been driving it 34,000 miles now with no issues.

 Hey Phantom. Any pictures of that setup?