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Old 07-06-2011, 06:59 AM
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Strange Highway Vibration

A brief history with the car. Its an 84 Caprice 2 dr with a worked 350 (From Jasper Performance) a TH350 from FB Performance and a converter from Edge Racing (Best thing I ever did). Rear is an 8.5" 10 bolt with 3.90 gears.

So I've had this issue for some time now, just never really sat down and thought about it, but since I had the radio off and was just cruisin it started annoying me so figured - why not make a post.

Anyway, the car/drivetrain is smooth as butter up to about 3500 rpm. If I start to cruise past that, I get this "rrr rrr rrr" vibration that kinda changes with engine speed/load. Mostly starts around 3800rpm, but if i start cuising at 4000, the "rrr rrr" gets faster paced and slightly more aggressive feeling. If I pop it in neutral, it instantly goes away and I can coast at 80mph without issue. Ive also tried duplicating the issue by slowly accelerating and cruising in 2nd gear, varying loads to see what happens, and there is 0 vibration. It definitely seems speed related, starting gently at 70, and getting more aggressive towards 75/higher speeds.
As a side note, and this may be unrelated, my rear trans mount bolt seems to have loosened and fallen out, maybe due to this vibration? It has been replaced of course. Just extra info.

Any idea what this could be?

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Old 07-06-2011, 07:51 AM
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Driveline angles or driveshaft balance. Have you checked the u-joints?
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Old 07-06-2011, 08:12 AM
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Being it goes away in N it has to be somewhere between the converter and the balancer, not in the drive line. It sounds like a balance or fitment problem with the flex-plate/converter. At least, that is what I would suspect first.
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Old 07-06-2011, 08:33 AM
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Quote:
Originally Posted by lmsport
Driveline angles or driveshaft balance. Have you checked the u-joints?
This is going to be my next step. Have the u-joints checked/replaced, as well as balance/straightness.

Quote:
Originally Posted by sqzbox
Being it goes away in N it has to be somewhere between the converter and the balancer, not in the drive line. It sounds like a balance or fitment problem with the flex-plate/converter. At least, that is what I would suspect first.
It does go away in N, but also isnt apparant in 2nd gear at same engine speeds. This same vibration was also there prior to my converter swap which was 2 months ago. The flexplate was also replaced during this time (Figured why not). Vibration has been exactly the same before and after.
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Old 07-06-2011, 10:27 AM
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If its motor or converter "RPM" related you will be able to rev the motor up sitting still and feel the shake. If its Speed related I would check Driveline and Joints first.
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Old 07-06-2011, 10:56 AM
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Quote:
Originally Posted by prostreet6t9
If its motor or converter "RPM" related you will be able to rev the motor up sitting still and feel the shake. If its Speed related I would check Driveline and Joints first.
Im hoping to bring it to my buddys shop tomorrow to have them pull the shaft and check everything tomorrow. I hope its just that and nothing else major!
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Old 07-07-2011, 09:11 AM
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Just a quick thought...it seems this vibration goes away on a full tank of gas... Could my driveshaft be too short? Is it possible to have ran for years like that without breaking something? Could have just been a coincidence... With the car on the ground, how much slip yoke should be exposed? Ill measure mine tomorrow before I take it out with the wheels loaded.

Last edited by 84Caprice57; 07-07-2011 at 09:16 AM.
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Old 07-09-2011, 09:58 PM
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If you are cruising at 3000 plus rpm, a lightweight aluminum driveshaft will help with the vibrations , from the increased propeller shaft speed. A car is not a boat and is better off cruising 2000-2800rpm.

Check it yourself, seat the yoke pull back 1.5 and measure center to center.

If any , the shaft being to long will cause problems. Weird driveability type complaints arise.
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Old 07-10-2011, 07:01 AM
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Change the U joints. The symptoms you describe are the U Joints. Great description by the way.
Be sure not to damage the driveshaft changing them or you will have more trouble. If they are original they will have plastic holding them in. I heat them with a torch until the plastic oozes from the little hole used to inject it at the factory.When it oozes it is soft enough to release the joint, then I apply pressure and the joint cap comes right out.You can press them but it puts a lot of stress on things and you can mess some stuff up.
You should look to see if the rear is a double Cardin style as they are a challenge for a novice, not knocking you , just making you aware of things,if you are up to the task then go for it.
The Double Cardin is used on real long shafts to eliminate vibrations,as long shafts are prone to , when driveline angles change from suspension travel while driving.
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Old 07-28-2011, 10:23 AM
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So FINALLY I got some time to bring the car to the shop, the shaft is out, and at the driveshaft shop getting balanced as I type this. Couple of things to add here:
1) Driveshaft shop is telling me my shaft is too short - which is def the case. You can see clearly where the shaft is riding. Hes going to give me a slightly longer yoke to get a better fit as long as the tube is straight.
2) Tailshaft play? I did the wiggle test to see if anything is loose. The rear end side was solid, no play or wiggling, however, the tailshaft side had some up/down side to side movement. With the driveshaft out, You are able with your hand to move the tailshaft around a bit. Is this supposed to happen and if so, by how much? It seems like quite a bit of movement so Im wondering if this is the cause of the vibration. I found a post elsewhere about some guy with his 69 Nova with the TH350, had the same vibration after 60mph, combined with his rear trans mount loosening - which are exactly my symptoms.

Any thoughts?
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Old 07-28-2011, 10:31 AM
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Yeah, I've noticed that play before. 700r4, and my 350 both have some play with the yoke out of the tailshaft so it must be normal to some extent. We'll see what other poster's have to say about that. The tailshaft bushing might be worn though if there's much play with the yoke in.
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Old 07-28-2011, 10:38 AM
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Quote:
Originally Posted by sqzbox
Yeah, I've noticed that play before. 700r4, and my 350 both have some play with the yoke out of the tailshaft so it must be normal to some extent. We'll see what other poster's have to say about that. The tailshaft bushing might be worn though if there's much play with the yoke in.
Play is def there with the yoke in, it seems the whole tailshaft can move. However I did here some play is normal. My friend is having his trans shop guy come by in a bit to take a quick look. I also put in a call to FB Performance (Speed shop who built the Trans) to see what they say as well.
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Old 07-28-2011, 10:42 AM
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The wiggling is normal.

You should change the bushing in the extension housing.
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Old 07-28-2011, 11:18 AM
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Quote:
Originally Posted by 123pugsy
The wiggling is normal.

You should change the bushing in the extension housing.
Very easy job with the right tool. I did the one in my 700r4 a few years back. A friend mechanic loaned me his puller/installer. The bushing isn't very thick so it's only going remove just a little side play unless it's really bumed up bad.
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Old 07-28-2011, 01:01 PM
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Quote:
Originally Posted by sqzbox
Very easy job with the right tool. I did the one in my 700r4 a few years back. A friend mechanic loaned me his puller/installer. The bushing isn't very thick so it's only going remove just a little side play unless it's really bumed up bad.

I had a guy bring in his pickup with a drive shaft which was too short.

The bushing was totally chewed up.
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