Jump to content

High speed rattle on deceleration or engaging clutch


gone2long

Recommended Posts

OK gang, here is another challenge.

 

Background: I am living in the boonies of SE Asia, and I am somewhat mechanically challenged, so the normal fixes don't always apply.  I have a 1979.6 Datsun 720 4x4, with a stock, 5 speed manual transmission and I had  a new clutch and throw out bearing installed about three years ago.  Have only maybe 5,000 miles on her since then. 

 

The truck was recently tuned and it is running well. Around the same time, I added 90 weight oil to the forward differential, which seemed kind of dry when I opened the plug.  Mia culpa : (   In any case, after adding the oil I noticed that, when running at high speed in 5th gear and easing off the gas pedal or hitting the clutch pedal on a downshift, I would hear a rattling sound, kind of like a BBBRRRRRRR - as if something was vibrating intensely right under the shifting boot. As soon as i take my foot off the clutch or hit the gas, the sound goes away.

 

I am wondering what could be causing this sound, and if it might be an indication of something serious.  The throw out bearing and clutch seem to be OK, as I have no problem shifting and engaging the clutch at low speed, and I do not hear the sound at low speed.  What might be causing it?

 

Thanks to everyone for all your help in the past.

 

GTL  

  • Like 1
Link to comment
  • Replies 27
  • Created
  • Last Reply

Top Posters In This Topic

Top Posters In This Topic

Thanks Mike.  Short answer is " I don't know".  Will check the manual.

 

BTW,  also had a new muffler recently installed.  Wonder if that could be the source of the rattling.

 

Appreciate your help.

 

G2L

Edited by gone2long
mistake
  • Like 1
Link to comment

More background:  As noted to Mike above, I also had a new muffler installed AFTER the differential lube.  I drove about 100 miles after the lube and did not notice the rattle. 

 

The rattle occurred during another 150 mile drive taken after the muffler was installed.  I suspected the muffler as a problem initially so I looked at it and the exhaust pipe, but it did not appear that there were any holes, broken or loose segments in the muffler or exhaust pipe welds.

 

The muffler, however, was "home made," by a local shop, so the muffler itself seems like an "X factor".  However, it also seems that there isn't much in it that could be causing a vibrating sound, no?

 

  

  • Like 1
Link to comment

B series trans are famous for bad countershaft bearings. I went through 3 failures on my 82 4x4. If you pull it apart before it fully grenades you may be able to just replace the bearing. The sure fire way to know if this is your problem is if the vibration is present in all but 4th gear. 

  • Like 1
Link to comment

Get the truck in the air high enough to get under it and then grab the exhaust pipe in several places and shake it around, see if you can get it to make the rattling sound you hear.

5th and reverse gears are both behind the center piece, dos it make the rattling sound if you go backwards?

  • Like 1
Link to comment

Grab the cold exhaust pipe and give it a good shake. Anything hit???

 

Check the engine or transmission rubber mounts for snugness. If loose or broken the engine torque will move it over enough to rattle the exhaust.

 

Ah wayno's got it.

  • Like 1
Link to comment
37 minutes ago, bottomwatcher said:

I was going to mention that. My 4th b series failure was 5th gear walked of the shaft. I can't remember symptoms but my 5 speed became a 4 speed. I drove it from California to Florida like that. Smooth trans just nothing in the 5 th gear position.

Huh,

That is interesting.  Again, you're  talking to a mechanical novice here, so if I get you right, what you and Stoffregen Motors are suggesting is that a loose bolt inside the tranny could be causing the problem.  The solution then, is to disconnect the drive train, pull the tranny and inspect/repair it.  Is that what you are suggesting?

 

If so, let's hope your wrong :  ) I will definitely get dirty and start rattling my muffler first : )

 

If I am not understanding you, please correct me.  Also, are there any simple tests I could do before pulling everything apart?

 

Thanks to all; you guys are great.

 

G2L

  • Like 1
Link to comment

Well you guys tickled my brain. I still have the innards of the 4th b where the nut backed off. Just pulled it out and the staked nut had backed itself out 10mm. I went to a c series after that and never have had an issue. This old b guts I kept with front and back housing removed. If wife threw a party and there were a bunch Male significant others with nothing in common I could always gather the guys in the shop. Throw the trans in a vice and everyone wanted to take turns shifting it and turning the shafts to watch everything move. Rednecks and scientists all returned to the party with greasy hands. 

  • Like 2
Link to comment

The loose nut was only on the earlier B transmissions is my understanding. Later ones have a groove in the main shaft and the nut has a thinner shoulder that can be hammered down into the groove (staked) to secure it. No way it will back off then. I recently had my '79 ZX 5 speed out and pried the shoulder up and it easily tightened about 1/8 of a turn, probably from wear. If you have your 5 speeds out probably wouldn't be a bad idea to check and do the same.

 

The earliest B (5 spd.) was on the '77 280z and 620 truck so probably these are prone to the nut backing off and like I said my '79 is well secured by staking. I had a '78 620 that lost 5th and then 2nd. I replaced it without taking apart to see what the cause was but in retrospect this was mostly because transmissions were a big mystery to me then. 

  • Like 1
Link to comment
4 hours ago, Stoffregen Motorsports said:

There is not easy way to tell if the nut is loose, but you should rule out the other possibilities before pulling the trans.

 

If you do find that 5th gear is loose on the shaft, let me know and I'll give you some pointers and advice on pitfalls to watch out for.

Thanks.  Much appreciated. 

  • Like 2
Link to comment
23 minutes ago, datzenmike said:

The loose nut was only on the earlier B transmissions is my understanding. Later ones have a groove in the main shaft and the nut has a thinner shoulder that can be hammered down into the groove (staked) to secure it. No way it will back off then. I recently had my '79 ZX 5 speed out and pried the shoulder up and it easily tightened about 1/8 of a turn, probably from wear. If you have your 5 speeds out probably wouldn't be a bad idea to check and do the same.

 

The earliest B (5 spd.) was on the '77 280z and 620 truck so probably these are prone to the nut backing off and like I said my '79 is well secured by staking. I had a '78 620 that lost 5th and then 2nd. I replaced it without taking apart to see what the cause was but in retrospect this was mostly because transmissions were a big mystery to me then. 

Thanks Mike,

If my history is correct, my 720 was made in the main Japan factory of the time ( don't have the name at my fingertips)  at the end of 1979.  In 1980, they began introducing the "Nissan 720."  Based on your experience it seems as though the staking fix should apply to my vehicle as well.

Let's hope so.

Best, G2L

 

 

  • Like 1
Link to comment

Check the transmission and engine mounts and give the exhaust a good hard pull in several directions. The exhaust manifold has a heat shield for the ATC that easily rusts out and could be vibrating. There may be heat shields on some of the exhaust pipe or the muffler.

  • Like 2
Link to comment
15 hours ago, datzenmike said:

Check the transmission and engine mounts and give the exhaust a good hard pull in several directions. The exhaust manifold has a heat shield for the ATC that easily rusts out and could be vibrating. There may be heat shields on some of the exhaust pipe or the muffler.

Yeah,

 

The problem with this vehicle is that stuff like the exhaust manifold heat shield (and lots of other stuff) was ripped out by the importer when this truck was imported to the PHL from Japan as "surplus".  Also, I had to have the entire exhaust system Oki rigged when stuff like the rubber mounts could not be found in this country.  From all the discussion on this thread, it seems very likely that the Oki rigged exhaust and/or a poorly built muffler could be the cause.

 

But again, if I can grab the cold exhaust pipe and get a rattle out of it, why do I not get that in 4 of 5 gears, and at lower speeds?  Why is it happening when I let off the gas or engage the clutch at high speeds?  Maybe the vibration I am hearing has something to do with back pressure at high speed?  The new muffler currently installed has nothing in common with the original, stock muffler.  Could it be that the "new" muffler is simply the wrong size or shape?  This one is round v. the original oval, shaped muffler.

 

The good news is that I am back in the town where I had the new muffler built and installed, so that will be my first stop.  I will let the builder shake and rattle things in my presence, but it would help to get your opinion on the above so I can talk intelligently to him as to what might be the problem - not that it might do any particular good  :  ).  But maybe, just maybe,  if the build is actually the problem,   could get him to make something that works : )

 

Tell me what you think,

 

G2L

Edited by gone2long
spelling
  • Like 1
Link to comment

PS - Interesting that you mentioned the motor mounts. Had not thought of that at all.

 

I know that one of the engine mounts needs to be replaced, but again, why noise only at high speed, and when I come OFF the gas?  If engine torque is responsible for the sound, the sound should increase when I hit the gas, not when I come off the throttle, right?  Not trying to contradict you; just trying to understand.

 

Thanks  again,

 

G2L

  • Like 1
Link to comment

Not a definite answer but many noises are all about harmonics.  It may be that the frequency of vibration that allows the noise to occur is only really struck under the conditions of 5th gear or decel.

 

Mostly these guys are listing all the stuff that is easier than the trans.  You can deal with most their suggestions without tearing the truck apart.

 

I've never owned an L series powered 720, but I've owned 5 or 6 720s.  After pulling a trans both directions, i firmly believe its easier to pull the engine with trans than it is to pull the trans from underneath.  Just in case you have to pull yours.

  • Like 2
Link to comment
11 hours ago, gone2long said:

 

But again, if I can grab the cold exhaust pipe and get a rattle out of it, why do I not get that in 4 of 5 gears, and at lower speeds?  Why is it happening when I let off the gas or engage the clutch at high speeds?

 

Take a look at your engine when you rev it up. You'll notice that it torques to one side when revving and quickly flips over to the other side when you let off. It's more pronounced when in gear and under a load. There's a lot of movement that can rattle something on deceleration only. If the engine or transmission mount is loose or broken it's even worse.

  • Like 2
Link to comment

Thanks Mike and Lockleaf for the tips.  Have not gone back to the muffler shop yet, but when I do, I'll have another look at the motor mounts as well.  Could very well be the root of the problem, 'cause, as mentioned earlier, I know that one of them is loose.

 

Will report back on what I find.

 

Thanks again,

G2L

 

 

  • Like 1
Link to comment

Latest Update:

 

Went to the muffler shop.  Guy rattled the exhaust pipe, muffler, and resonator.  With the truck running, he found leaks in the resonator weld, and starting tapping it.  His analysis was that it was shot, and all the interior had essentially decayed and that we needed to replace it.

 

We cut it out and tapped some more.  It sounded very hollow and was kind of light, so he took that as a verification that it was shot.  Nothing rattled around when I shook it, however.  Put on a new, home-made one, which was a little heavier and did not make a hollow sound.  Sounds fine when idling or running at low speed.  Have yet to test it in 5th running at high speed and coming off the throttle or engaging clutch, but I explained those symptoms to him, and he seemed convinced that the resonator being shot was causing them. 

 

Comments anyone?  Think he knows what he's talking about?

 

Will report back after I test her at high speed on the highway.  BTW, there are no rubber connectors holding the exhaust system in place.  Could not find the parts, so the exhaust is Oki-rigged onto the frame using welded rebar.  Not a great solution of course, but the only one available at the time.  The vibration from the road and years of use/rust probably caused the resonator welds to leak, so it was probably a good idea to replace it anyway.  I'm pretty sure it is the original, dating back to 1979.5.   

 

The latest work cost a total of $23  (Philippine peso equivalent) including the new resonator and the labor.  Can't complain at that price.

 

That's it for now.  Thanks for all the great advice - G2L

 

Edited by gone2long
missing word
  • Like 1
Link to comment

Um... rebar holding the exhaust up, and your muffler shop guy didn't point that out? Yes, that sounds like something that would cause a rattle or noise.

 

You can't find exhaust hangers either? You don't need original parts. Any crappy hanger will be better than rebar.

 

Try these - https://www.summitracing.com/search/part-type/exhaust-hangers?sortby=Default&sortorder=Ascending&keyword=exhaust hangers&kr=exhaust hangers

  • Like 1
Link to comment

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.