rear end noises

[zcarguy]

Here is one to think about,

I have a 73 240 that is mostly stock except for the usual upgrades; Round top SU's, urethane bushings, etc. Every thing is either new, rebuilt, or very well maintained.

My problem starts when I am on the freeway. When I push the gas pedal down, it sounds like I have a solid steel diff mount (which I do not). It does not do it when I am crusing, coasting (at speed with the clutch pedal in), in first, second, third or fourth gear less than 50 or so mph. It only happens when I am at a decent speed (50 mph +), in fourth gear the gas is on (even just a little).

The car is an ex-automatic and the drive train has always been in very good condition. I have checked EVERYTHING that moves at all (over and over for the past several months in fact!) and everything is in great condition, very solid and tight (in fact the car drives perfectly in all other aspects, it does not shake, shimmy, rattle, clunk, pull or anything). All bushings and bearings are new (except for the ones in the diff itself). U joints are not new but a few mechanics have looked at it and all agree that they are in fine condition.

I rebuilt the car almost from the ground up and have done alot of work to it so I can not say that the problem arose at any one time.

The one thing that I can see that might be a problem is that the mustashe bar is VERY close to the two big flat steel links that go from the lower control arm to the chassis vertically. By close I mean less than 3/32", just enough to pass day light thorugh. Maybe when the drive train is loaded up it twists enough to contact these points and transmit vibration directly to the chassis.

Any ideas?

[2ManyZs]

It's possible that is why the noise is getting into the car as loud as it is.

Have you pulled the diff cover and inspected the ring gear? It could be two-fold, the ring and pinion are worn or you might have a bearing going bad in the rear, and the suspension mounts and the mustache bar being so close that it could be transferring the noise and amplifying it.

I'd check the diff just to be sure if everything else is in as good a shape as you describe, it is one of the last possible places to look. Good time to change the diff fluid if that hasn't been done in a while, and check the old stuff for metal shavings carefully before you re-fill it.

[Daniel]

I once fought a similar demon.Under such conditions as you spoke of.Turns out my problem was when I install my driveshaft I replaced one of the four bolts/nut/washer that attach the driveshaft to the rear end.The other 3 were factory.When I replaced all four with the exact size,length,washer and nut the problem went away.I must assume that the speed was just enough to magnify the super slight weight difference and cause the vibration.I woundn't have believed it. Daniel

[zcarguy]

Thanks for all responses and the great ideas of things to check...will do this weekend!

Something I did last weekend was is I pryed the mustashe bar away from those solid vertical links I described in my first post and stuffed a piece of fuel line between them. This dramatically changed the noise but did not eliminate it, which may mean I might be on to something.

my questions are 1)why are those pieces of metal so close? The frame shop said it was straight and nothing looks bent visually. 2)How close are those pieces of metal on other peoples cars? Maybe mine aren't really that close and it would not be a big deal to "tweak it" just enough.

Any thoughts would be great!

[2ManyZs]

Any way you can post a pic?

I'm wondering about one thing, does your car have the original R-180 or has it been converted to an R-200?

[zcarguy]

Hey......now there is an idea.....I can post a pic! I will get out there tonight and take a picture.

Any way, it is still the original R-180, 3.5 to 1 (the original diff because it was an auto before I converted it to a 4 speed). When the diff was out I did remove the rear cover and looked at it. It seemed to be OK on visual inspection. I was tight as far as I could tell and did not make any noises when I would turn it by hand. That was all I could do with it with limited tools. I installed the rear cover and filled with redline MT-90. It has been about a year since I assembled that part of the car (you know, the back half) so things may have gone bad since then, it seems unlikely because I only drive it maybe 2-3k miles per year, but any thing is possible.

I will get some pics up tonight

Thanks!

[zcarguy]

here is a pic of the rear end. as you can see there is some fuel line stuffed between the two pieces. I cut open the fuel line length-wise kind of flat. That fuel line is really only about 1/8" thick and it was a REALLY tight fit, I had to pry fairly hard to open that distance enough to fit in that fuel line in. Any ideas?

[2ManyZs]

Well, since you took the time to post the pics, I went out in the garage and looked under the old race car. Guess what, it looks the same. :stupid:

Now the only thing that comes to mind is something might be moving. Have you thoroughly checked the front diff mount? Have you checked the mounting bolts on the uprights?

I'm thinking front diff mount for one reason, the mustache bar is somewhat "springy" and if the front mount is loose or broken, the diff could be moving enough to deflect the mustache bar and cause the contact. The uprights are also somewhat springy as you may have noticed... One thing you might try is this, if you can get a long enough extension on a ratchet to get to the mounting bolts on the uprights, try loosening them a bit, and see if the uprights will move back any. If I remember right, there is a wee bit of play in those holes in the uprights, perhaps that will give it just enough room to keep them from touching.

It looks as if you or someone may have replaced the mustache bar bushings, so that might be one thing to ignore, but check to see if the mounting nuts are tight.

[Daniel]

This is a long shot but here goes.All I read about rearends and driveshafts be they datsun ,gm whatever say to mark the position to the rearend before removal.Seems they are balanced at the factory to optimum true.I always do this .If you did not you may try rotating your driveshaft connection to the rearend.You only have 3 positions left to try. Have fun!! Daniel

[kinfish]

I checked my '71 240Z and measured 1.75 inch clearance between the mustache bar and the mount plate. I have an R180 rear end which I installed several years ago to replace the worn out factory unit. Do you have the mustache bar installed backwards? There might be difference in drive shaft length between the OEM automatic and the 4-speed and this was done to make it fit?

[zcarguy]

Thanks kinfish for that info. I have owned several 240's and I could not remember the distance between those parts, it dosen't seem right that they are as close as they are!

I had thought about the mustache bar being in backwards so I had taken it out to flip it around but to say it would not have lined up fight would have been an understatement... I mean it was WAY off.

The postition of the diff in the car should pretty much be determined by the front diff mount. Although I didn't remove it from the diff it could still be on there backwards. I posted a pic of it...can any one out there tell if it facing the right direction? In any case, the length of the drive shaft kind of is what it is. From what I have read the drive shaft from the auto is the same as the one out of the four speed and different from the five speed. When I did the conversion I was told that is should be fine as long as I stuck with four speed. (can any one correct me on this?)

I at one time owned a 71 also and it was one from before Datsun moved the diff backward because of the dreaded harmonic at speed (which mine had) which could be why there is alot of space between the parts on your car (unless you fixed that?)

Any way, If the front mount ends up being corrent in its current position, I just might use old 'crystal blue' to re-contour the mustache a little to get some extra clearance unless someone has a better idea?

[Brett240]

i had a very similar problem a couple of months ago. a vibration occured above 45 miles an hour only when accelerating, at first i thought it was the fly wheel coming loose:( but it turns out that the tailshaft was coming out of balance. after a couple of quarter mile runs at the local strip it was so bad i had to tip toe home. one balance job and two new universal joints later (even though they looked OK) and the car is the smoothest its ever been. hope this helps