bad vibration

[Greybeard]

I read a story once about a guy that had a vibration under his truck that he could not isolate so told his girlfriend to drive while he hung under the truck listening and looking.  .....Don't do that!  It ends up very bad.  At least it did for him when the driveshaft took a hold of his jean jacket ...

[philo_beddoe]

No bars!  What if i laid down on a creeper under there, like keanu reeves in "speed" under the bus??

[philo_beddoe]

There is always this option....

[philo_beddoe]

Interesting site on auto vibrations.

http://www.agcoauto.com/content/news/p2_articleid/302

[zieg85]

Did you ever have your mechanic run the engine without the torque converter connected to the flexplate?

[philo_beddoe]

No, but i will, the t converter got sent out to be balanced.  Havnt had truck in like 9 days, starting to go in and out of consciousn e s s , , huh what, what just happened....

[philo_beddoe]

Well, its a lot better, but the vibration is still present. I am replacing the motor mounts and the h. balancer/damper. Will report back soon. After this there is nothing else.

[Greybeard]

Questions; how long have you had the truck? When did the vib start? Any changes immediately prior to it starting? New brakes, new tires, new anything?

You say it happens when in park but only when rapidly accelerating it? That sounds to me like a miss. Like the fly weights in the distributor are binding or not moving smoothly. If the vib goes away at a high idle (maybe 3grand steady) and only happens when adding fuel that would be the thing I would look at.

If the vib is not present at idle it is likely not the t-converter, that would vibrate at any engine speed. However, since the harmonic balancer is rubber isolated from the crank it would not make much impact at idle but would get progressively worse as the engine speed increased (think of a seriously out of balance tire moving at 5mph as opposed to 55 mph). An out of phase harmonic balancer will destroy an engine pretty quickly. I think the simple test to see is in an earlier post (timing light).

Motor mounts will not cause or contribute to a vibration in the engine, they will only conduct what is already there. Find and fix the cause of the vib before spending money on mounts unless they are obviously trashed.

Check the fan itself, I don't remember that being mentioned yet, it should have five blades IIRC and they should be staggered not uniform around the circumference. If it has a replacement fan you are on your own. But a bent fan will cause a vibration. I bent one the very first time I went river running because I neglected to loosen the fan belts. But I had a direct drive fan at that time too (no thermal clutch). It hit my radiator but did not puncture it although it made some hellish noises. Lesson learned. Deep water and direct drive fans with tight belts do not mix.

[philo_beddoe]

A new dist cap was put on when we did valve seals. I will start with the harmonic balancer first. You make a good point about that. Thanks.

[Greybeard]

When you did valve seals? 

Are you certain you got all the valve train tightened down correctly? A valve not opening or closing completely will cause a vibration too I believe. Not closing would cause a popping or sucking sound but not opening an intake would simply create a weak firing cylinder. Make certain all the retainers stayed in place.

[Captain Swampy]

No bars!  What if i laid down on a creeper under there, like keanu reeves in "speed" under the bus??

Pictures (preferably video) or it didn't happen! LOL

[Captain Swampy]

I'm gonna try to reread this whole thing and put my thinking cap on. I wish I could come over and look at it.

[Greybeard]

I agree Swampy, it needs ears and eyes on it. There is a lot going on between the t-converter and fan but overall not a lot of it can cause a quiet vibration. Most of it will be a metal on metal sound or have some other noise associated with it. I was just reading another thread and was thinking maybe the vibration is really resonated from under the truck due to exhaust noise.

[philo_beddoe]

Haha,,,anyway...truck is at a shop that is real solid. Never been stumped. I mean c'mon, were talkin about a 77 chevy here,,,not a ferrari. Hopefully its just that stupid balancer/damper.

Question, are those flex plates balanced like a rim,,,with weights?

[VileZambonie]

Late to the party here. . .

After your preliminary checks, the first thing you should have done was performed a cylinder power balance test. Would have saved you a lot of aggravation and helped you pinpoint the problem quickly.