Please help me diagnose vibration 1966 289 4V :SOLVED!

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I posted this over on the VMF site, but I know the members on this site better and have more history here so I figured I'd post here as well.

I’m looking for some assistance determining the cause of a vibration I am experiencing with my (new to me) 1966 289 Coupe.

66 Front View.jpg

I have an extensive folder of receipts and notes that came with the car, but no first-hand knowledge of work performed over the years.

Here are the vehicle particulars:
  • 289 4V rebuilt a decade ago. Very few miles put on the car since rebuild.
  • Engine runs well.
  • C4 automatic. Original I believe.
  • 2.80 open rear end.
  • New tires and balance.
Vibration:
  • The vibration is evident driving the car at 55-60 mph. I would describe the vibration as a 2-5 Hz sensation. I can feel it in the wheel and some of the interior trim “buzzes”.
  • Shifting to neutral while at speed causes the vibration go away.
  • With vehicle in park or neutral I can reproduce the same vibration. I do not have a tach, but I’d guess at +/- 2500 rpm or so. Probably the same rpms as vibration when underway.
Where I am:
  • I have replaced the harmonic damper. It looked bad, but no improvement.
  • I suspect either a bad torque converter or an incorrect or incorrectly installed flex plate.
Is there any way to trouble shoot the torque converter/flex plate without pulling the transmission? Can I unbolt the torque converter from the flex plate and start the engine without damaging anything?

Other ideas?

Thank you for your wisdom!
 
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At least you know that the vibration is most probably in the engine/flexplate/torque converter. If you can reproduce the vibration with the car in neutral by just revving the engine, and by putting it in neutral while driving the vibration goes away, 99% of the time the vibration is in the engine/flexplate/torque converter. First thing I would suggest is to remove the accessory belts, start the engine and see if the vibration goes away. If it goes away then you know the issue is in the accessories or the engine fan/water pump. Since you have ruled out the balancer, then the only thing that is left is the torque converter/flexplate or an internal engine vibration from and incorrectly balanced assembly (which is very rare). Check your motor and transmission mounts too. Could something in your exhaust/engine be touching the body/chassis of the car, something that is transferring vibrations from the engine to the chassis. I once had a car with a vibration that would only happen under acceleration, when the engine twists/lifts on the drivers side under acceleration or heavy load one part of the exhaust would just touch the rear bumper and you would get this vibration, would not do it at idle, you had to accelerate the car for everything to move a bit and the exhaust to touch.
 

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As noted, pull the belts first. I've repaired a couple cars with failed fan clutches that caused a horrendous vibration.

Having replaced the balancer, I'd then take a look at the counterweight on the flexplate. It's possible that a 50oz imbalance flexplate was used on the 28oz 289 rotating assembly. Hopefully, it's not the other way around and it's a 50oz 5.0L assembly in a 289 block.
 
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Thanks for the thoughts guys.

I have pulled the accessory belts and the vibration is still there. I guess we can rule out the fan, water pump and alternator.

I have run the car while up on a lift. I cannot find any exhaust issues. The motor mounts and transmission mount look like they are new. I think I recall seeing receipts for such in the file.

Is it possible to check for 28oz vs 50oz flexplate through the same access I use to get to the flexplate bolts?
 
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Just as an alternative idea, sometimes vibrations at a particular rpm are exhaust related. If you can get someone to operate the throttle, look/feel under the hood for induced vibrations, and especially under the car. Since it’s rpm related and not mph related, you can do this. Check every connection, clamp, etc to see if it’s buzzing; I like to use a think metal tube that I hold in my hand to feel it. It might be a dead end but it’s a possibility.
 

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Is it possible to check for 28oz vs 50oz flexplate through the same access I use to get to the flexplate bolts?

Yes, the counterweight is welded to the engine side of the flexplate. The 50oz units on Rock Auto have a counterweight with a window 180° opposite. The 28oz flexplates only have the weight.

50oz flexplate

1663000868402.png
 
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Here's a photo of my flexplate counterweight.

20220912_093436.jpg

I don't have the opening on the opposite side.

That said, my counterweight does look a tad larger than the one in the photo Dennis posted.

After a little research here is a photo of a 50 oz plate from Summit (no hole opposite).

Summit Flexplate.jpg
The weight on the Summit flexplate above looks more like the size of mine.

Wondering if I do infact have a 50 oz plate installed?

Edit to show another photo from Blue Print Engines of 50 oz SBF flexplate below:
BPP529610_1978x.jpg
 
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Was the damper you removed a 3 bolt (for the pulley)? If it was, it was a 28 oz. imbalance. You can't tell one from the other just by the size of the weight. Hole size and placement are other variables. Can you see a PN sticker/stamping anywhere? Chuck
 
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Just as an alternative idea, sometimes vibrations at a particular rpm are exhaust related. If you can get someone to operate the throttle, look/feel under the hood for induced vibrations, and especially under the car. Since it’s rpm related and not mph related, you can do this. Check every connection, clamp, etc to see if it’s buzzing; I like to use a think metal tube that I hold in my hand to feel it. It might be a dead end but it’s a possibility.

Thanks Steve- I'll bribe my wife to sit in the car while its up on the lift and snoop around more closely. Not really a buzzing vibration though, more like a deeper harmonic resonance kind of vibration. At this point I am more than happy to chase potential dead ends...

Was the damper you removed a 3 bolt (for the pulley)? If it was, it was a 28 oz. imbalance. You can't tell one from the other just by the size of the weight. Hole size and placement are other variables. Can you see a PN sticker/stamping anywhere? Chuck

Chuck- The damper I removed had three bolts for the pulley. From the condition it was in I'm pretty sure it was the original 28 oz damper (see photo below). I replaced it with a new 28 oz damper. The vibration is the same with the new damper.

289 Harmonic Balancer.jpg

So much for the low hanging fruit. I was really hoping the problem was the harmonic damper...
 
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Unfortunately, at this point I think you must find out if the flexplute is 28 or 50 oz. imbalance, somehow. If you confirm it is 28 oz. then all that remains is a bad convertor, bad balance, or worn/damaged pilot hole in the end of the crank. Hope for a bad convertor. I'm out of ideas. Perhaps someone thinks of something we've all missed so far. Chuck
 
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When you are removing the trans and replacing the flexplate make sure that both your dowel pins are present. If you are missing one or both of them, it can cause the transmission to be a bit out of center, and you will get a vibration from the converter and the transmission not being centered. There was a thread here not too long ago about a car with a vibration and it ended up being the hub on the torque converter was welded out of center, there was a video of it. If you are taking the torque converter out you may want to take it to someone that can check it out.
 
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When you are removing the trans and replacing the flexplate make sure that both your dowel pins are present. If you are missing one or both of them, it can cause the transmission to be a bit out of center, and you will get a vibration from the converter and the transmission not being centered. There was a thread here not too long ago about a car with a vibration and it ended up being the hub on the torque converter was welded out of center, there was a video of it. If you are taking the torque converter out you may want to take it to someone that can check it out.

Good advice on the dowel pins, thanks. I'll see if I can find the video you reference.

Presently I am just planning to pull the transmission aft just far enough, supported by my transmission jack, to replace the flexplate. I am hoping to leave the welded in H-pipe exhaust system in place and in tact.

The TC is clearly not original. You can see that it is painted non-OEM blue from the photo I posted in post #8. The TC has a stock stall speed. I know Precision of New Hampton paints all their rebuilds blue so I am hoping that is what I have. They are a known USA remanufacturer with a good reputation.

If the TC looks suspect at all I'll drop the C4 completely and replace it.

If anyone is interested I'm going with a heavy duty ATP flexplate:

ATP® Z-503 - Automatic Transmission Flexplate

It'll be here Monday and I'll get it in early next week.

Thanks again everyone for the help!
 

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Before pulling the flexplate try unbolting the TC, rotate it 180° relative the flexplate and bolt it back up. Run the engine and see if it makes a difference. Easy and cheap AND fast! Worth a shot.

(edited to correct autocorrect)
 
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Well it has been a while but I thought I'd post an update.

Good news! Problem solved! Vibration is gone.

Life got in the way and I only just yesterday had a chance to work on the car. I pulled the transmission and replaced the flexplate with the new one I got from ATP.

ATP® Z-503 - Automatic Transmission Flexplate

Apparently the wrong (or a defective) flexplate was installed when the C4 was rebuilt.

Thank you everyone for your thoughts and suggestions!

Oh, a side note... using a crowfoot flare nut wrench on the impossible to reach hydraulic cooling lines on the transmission is a lifesaver!
 
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