Here's the problem:
1986 IROC Z
350
700R4
BW 3.73 posi
Typical mods.
Here are the symptoms:
Vibration at different/varying rpm.
Occurs under driving conditions or while parked or in neutral.
Can be modulated with the throttle. ie, more throttle under partial load increases the frequency and intensity of the vibration while easing off will reduce the intensity and frequency.
Some cruising situations are vibration free. Overdrive on the freeway (2200)or accelerating through 4000 rpm at part throttle for example.
Appears like harmonics and will be most intense at specific rpms. 1800 then again at 3600. (values approximate)
Is evident through the whole car. Through the steering wheel and seat as well as audibly. (Change rattles in the ashtray)
Remedies so far:
Torque converter. (back to stock from TCI)
Trans swap.
TCI converter again.
Engine balancing.
In this config the car ran 13.4 @ 102. Still shaking although it's hard to tell at WOT.
Another torque converter. (no name HD/HS generic brand)
With this latest converter the vibration is not evident in park or neutral however it is just as noticeable under driving conditions.
No vibration with torque converter decoupled.

Any thoughts?

 

Have any weights fallen off of your driveshaft? Might need to get it balanced.

 

I would say the driveshaft needs balancing, or it's the harmonic balancer on the engine.

*edit, I just re-read your post and saw the "de-coupled torque converter" part. Pretty well narrows it down to the driveshaft.

 

"Occurs under driving conditions or while parked or in neutral. "

Converter-engine. You've swapped converters and it's still there. Simple things come to mind: motor/tranny mounts or some part of the exhaust/headers vibrating against a frame member with engine torque (under load).

 

I thought driveshaft too (and there's a new steel 3" driveshaft coming) however the vibration in park and neutral (less now than before) bugs me. I know the 3rd gens had poor quality driveshafts with urethane foam for dampening.
I checked the car front to back for contact points. Although the exhaust is suspect, (Hedman shorties with a single 3" cat back) it dosen't touch anything. Maybe it vibrates like a guitar string and creates the resonance.
Engine mounts are new rubber and the trans mount is polyurethane, also new.
I thought that maybe the driven part of the transmission pump was bent (input shaft driven in all gears including park) however my trans swap (used) still shakes.
Could it be the same condition in two transmissions? I doubt it.
Next upgrade will be LT headers with new exhaust. Spohn subframe connectors and then a driveshaft and.....

 

Didn't catch the "parked" part. Scratch driveshaft. It's impossible for it to vibrate the car if it's not spinning.

 

How's it run? Intermittent miss could cause it to shake a bit.

 

Wouldn't it be nice if it was that? I had 2 dead cylinder misses. Swapped plug wires. Didn't feel it at all until the converter locked.

 

No misfire.
205 psi cranking pressure across the board.
Good looking plugs.
20 mpg hiway.
Fresh ignition parts.

 

An intermittent miss is exactly that - intermittent. Some ignition parts can be faulty out of the box - the joys of electronic parts.

Can you recreate the problem anytime, or does it just show up randomly? If you can get it to happen late at night when it's pitch black, pop the hood and look. Jiggle a plug wire or two and see if you see any blue sparks lighting up the area.

Other then that i'm scratching my head.

After changing the torque convertor it doesn't happen in neutral you say? Could be a double problem. When going highway speed, if it's shaking, pop it into neutral and coast at idle speed. Still doing it? That would tell me it's now tranny and after...

 

I'm 100% sure it's not ignition related.
Other than a faint blue corona around the plug boots, there are no arcs from the wires or cap.
After the latest converter swap, the vibration in park/neutral is undetectable to the point where if it still didn't shake at speed, I'd say there was no vibration.
I haven't tried the neutral while driving test since the latest change. I used to be able to induce the vibration by bringing the revs up while coasting. I'll try that one again. It's cold here but the roads are dry.
Seems directly related to engine rpm however it's not the engine.
Yes, it could be more than one issue.

 

Ok, if you're convinced it's not ignition, then i'll let that go.

But do those other tests. Then you can at least rule parts out. Seems like it's more driveline now.

 

I've been troubleshooting this for nearly two seasons. I'm pretty sure it's a combination of things.
Like I said, exhaust, driveshaft.....there's not much else.

 

Torque converter decoupled. I am going to take that as it's not bolted up to the engine so your transmission is not being turned either. Correct me if I'm wrong.

I would have to go with exhaust harmonics on this one. What are your exhaust specs? I cannot remember what I had in my setup a while back, but at certain RPMs the resonance inside the car would just about give a headache. Real close to that of open headers. In my mind that could cause some vibration. The fact that if you take away some load on the engine changes characteristics and would affect your exhaust.

I am more interested in the severity of the vibrations in question. They will rattle change... if you let go of the wheel while the vibrations are occuring would the car dart off the road? If it is in the driveline, does it make driving difficult?
I've had low tire, a bad shock, incorrect pinion angle, plug wire that fell off, and they all produced vibrations but of different levels. A driveshaft would just about knock your teeth out though.

 

Did more testing.
3000 rpm seems to be a focal point.
Slipping into neutral at cruising speed and the vibration subsides. There's lots of other noise in the interior however it's obvious that the intensity of the 3000 rpm vibration is decreased. Bringing the revs up to 3000 while still coasting in neutral will bring the vibration on just like it does in drive (or any gear). Interestingly, if I'm at or near the vibration peak, I can modulate the frequency and intensity while working the throttle.
The resonance from exhaust has a different feeling than this. I do have suspicions about the exhaust though. Hedman shorties with a 3" cat back flowmaster. I'm wondering if the whole thing vibrates like a guitar string. I have a distinct vibration that can be felt through the steering wheel and seat. Images in the mirror become blurred.
I'll hopefully have an answer to the exhaust question in the next few weeks. I've got an appointment at the fab shop whenever their current project is off the hoist.

 

[quote=skinny z;3677186]
205 psi cranking pressure across the board.

Just wondering what your CR is?

The only odd vibration issue I have ever had, was loose (end play) pinion bearings in rear end. Always the same speed like 25 and 50 mph. Allowed the end of the driveshaft to move up/down. This was on a 190,000 mile vehicle.

Did this engine ever run smooth? What was just changed, if and when it started?

How about internal balanced engine, external balanced flex plate or damper.

This will be interesting when cured.

 

[QUOTE=pandin;3681658]Compression ratio is 9.97:1. That high static CR and my short (and 4* advanced) cam gives excellent cranking pressure.
The Borg Warner rearend in this car is noisy however its sound is easy to pick out. I've coasted downhill with the engine off and its obvious but not loud. Not the problem though.
The engine was a little rough partly because of the crappy header design and partly because the rotating assembly was never balanced after the pistons were replaced and the rods resized and new bolts. Since then I've had the engine balanced and now use a Fluidampr harmonic balancer and an SFI approved flex plate. With the engine decoupled from the torque converter, it runs as smooth as glass.
I made some changes during one off season and the vibration became very noticable. Rebuilt existing transmission, TCI stall converter, new engine mounts and a couple of minor things. I was blaming my transmission guy for a poor rebuild however a trans swap made no difference.
I made other upgrades the following season but the condition hasen't changed. My latest converter swap seems to have quieted the shaking while in park or neutral however it's still the same while cruising.

 

It's been a long time however it looks as thought the vibration issue has been resolved.
Unintentionally no less.
For the record, EVERYTHING in the drivetrain has been removed, replaced or rebuilt at least once. Check out this thread for the list.
I've been driving with this problem for a few seasons and had resigned to myself to accepting that this is how the car is going to be.
Recently I had a 700R4 3-4 clutch go away. This trans with the TCI Street Fighter convertor helped the car to it's first sub 13 second pass.
I found a low mileage 700R4 from a forum member, did a quick shift kit, basic mod procedure and installed it with same convertor. Now it only took 4 quarter mile passes to cook this transmission ( that's another story ) however, the vibration is GONE. I have only 1st and 2nd gears to test with but I can say at all previous test rpm's, there is no longer the ' harmonic vibration' that was painfully obvious before.
I can come to two different conclusions. Two previous 700's ( I'm on my third ) and 3 different convertors all combined to produce the vibration. This I find unlikely.
Or the relationship between the flexplate and the convertor had a stacked up tolerance with respect to balance ( and I have been using a new SFI flexplate and new TCI convertors as well as an engine balance). After the latest installation, the convertor and flexplate were bolted together not using the previous index marks. Now it's smooth as silk.
After investigating further and going through the GM manual, I've come across a chapter titled " Flywheel Assembly/Torque Convertor Clutch Vibration Test Procedure ".
Who knew?
The test describes checking for vibration in park or neutral, decoupling the torque convertor, rotating it 120 degrees and hooking it up again. Check for the vibration and repeat the procedure if the vibration is the same or worse. Looks like when I reinstalled this latest trans and convertor I inadvertantly came across the best connection.
It was a long time coming. And expensive too although I must say I've collected and installed a lot a top quality parts along the way.

 

Awesome. That explains why manuals tell you to mark the orientation of the flywheel and torque converter upon removal so they can be re-installed in the original orientation. Reckon the assumption is that it wasn't vibrating in the factory orientation so that setting should be maintained.

I've followed that procedure before but didn't really understand why. I'll add that vibration test/rotate 120 degrees to my arsenal for future work when the original parts are not re-installed. Thanks.

 

The strange thing is that this problem had been presented to several professionals ( engine builders, transmission shops, race chassis design/builders ) over the years and not one person suggested this particular test. It's even more interesting in that I performed a decoupled test several times, never indexing the components for reassembly and ended up with a similar vibration each time.
Live and learn.