Technical Chasing a vibration-Solved

So long story short, finally have my 51 Pontiac Fleetleader (basically a Chevy fleetline with Pontiac trim) back on the road. It has a rebuilt 350/350 combination sitting on Chassis Engineering biscuit front mounts along with standard gm tranny mount. Engine and tranny have about 5000 miles on them during the 90’s primarily…has been mostly parked since 2006. It always had a vibration related to rpm in park and neutral, not a death shake but just an annoying buzz in the steering wheel and body. It has always run nice, no apparent misses.
I have been trying to chase down the vibration the past few weeks and don’t seem to be making much progress so figured I would outline what I have done so far and see if someone can spot something I am missing.
First thing…eliminate fan, water pump etc by removing fan belt. Vibration still there.
Second…basic tuneup plugs,wires, carb overhaul and setup, cap, rotor, triple check firing order, vacuum test (steady and in proper range). Vibration still there. Starts and runs nice with no nasty noises.
Third…checked for any body or chassis interference with engine, trans, exhaust. Nothing found and vibration still there.
Fourth…replaced harmonic balancer with new one. Confirmed engine is a 350 (I don’t have the details with me right now but it checked out as your basic 350) and confirmed proper flex plate with no obvious cracks or runout. Vibration is still there.
Fifth…disconnected torque converter and pushed it back (maybe a 1/8” gap when all the way back with no issues rotating the converter by hand). Vibration still there.
Sixth…tried various levels of “tight” for the Chassis Engineering biscuit front mounts. No difference in vibration noticeable.
At this point I buttoned it all back together and it runs and drives fine… except for the buzz in the wheel and body that is dependent on engine rpm, parked or driving.
Anyone have any suggestions? Thanks in advance and sorry for the long

 

What RPM?
Only in park & neutral? driving?
have you done anything with the steering wheel ?

 

Are the "biscuits " rubber or urathane?
And how old is the transmission mount?

 

No tach but basically from a bit above idle up. Intensity seems highest at what I would guess is about 2000 rpm.
Steering is untouched. Previous 235 and cast iron power glide (pre 1990) did not vibrate.
I believe the biscuits are rubber…whatever CE sold around 1990.

 

I6 has different harmonics than a V8.
Try clamping something to the steering column.

Of course there’s the other weird ass stuff like finding 3 different kinds of pistons in a rebuild. Yeah that’s happened and no fixin’s for it.

 

So you know that the engine is a 350, but what about the harmonic damper? Could you pull the damper off and run it long enough to see if the vibration changes? While it's off the engine take some pics of the front and back side of the damper and post them here.

 

2 piece rear main flex plate for 350 has no weights.
1 piece rear main flex plate had weights but bolts to nothing else.
400 flex plate has weights and will bolt up to 350 2pc rear main.

The 350 damper has a few configurations and the 400 is it’s own thing

 

The damper is new and the proper one without the cutout. Installed properly with a tool and seated nicely. I did start it without and still felt a vibration. The steering wheel buzz seems magnified from what the body buzz is…guessing likely due to its unsupported length. It might have something mismatched inside…came as a long block from a reputable company but as we all know, shit happens.
Appreciate the help!

 

The best description I can give of the vibration is a buzzy motorcycle, which is why I am leaning to the mounts. The car itself was pretty quiet with the 6 and power glide so it seems to be more engine or mounts to me. The engine seems smooth but I know harmonics can magnify out like a ripple in a pond.
The flex plate is the correct one..no weights and seems to track straight so I think it is ok, especially after disconnecting torque converter made no difference.
Has anyone experienced this type of vibration with the biscuit mounts before?

 

I'm gonna go way out in left field here and suggest, possibly, exhaust resonance. What kind of hanger's are you using and how is it routed? is it close to anything that could transfer vibration? If this is a stupid suggestion please slap me for bringing it up!

 

Exhaust resonance is a possibility. System was patched together and hangers are quite tight. I will experiment with that when I get back in the garage. Thanks

 

Also, one comment was how old transmission mount is…was new when this combo was installed and had vibration then. Looks good still but I will look closer at it. I am camping for a couple days but trying to develop a plan for attacking it next week. I appreciate the responses as it validates some of what I have tried and gives me a bit of direction.
Individual cylinder temps were checked on the exhaust manifolds and all were close to same temp so I don’t think it is misfiring. The more I think about it the vibration feels like my old Japanese vtwin bike did at certain rpms.
Oil was changed and no nasties found at all so I don’t think it is self destructing…fingers crossed…although I just made arrangements to get the old 235 and power glide back if I need it.

 

above all else, don't leave us hangin!!! good luck

 

Nope…I will report back for sure. I have used this forum for years as a resource and hugely respect the talent and knowledge of the members. I hope it isn’t some stupid little thing I am missing but on the other hand I hope it is some stupid little thing!

 

I had a bad torque converter do that in neutral and park

 

Yeah..that was one of my possibilities so I disconnected it from the flex plate and isolated it. Vibration was still there so I am fairly confident it is either internal to the engine or some interference with body/exhaust.

 

Based on what you have posted, I'd be looking at the exhaust, or engine mounts. If the engine mounts are the biscuit type, have you tried loosening the bolts to see if it affects the vibration?

 

I guess it’s possible but how is the exhaust going to shake the steering wheel?

Take the wheel off and see if you can better spot the source without the distraction.
can you easily rubber isolate the column, radiator hose under clamp?

 

Yes, I have tried tightening and loosening the bolts. No difference. Think I will back them right off and try lifting the engine a quarter inch or so off the biscuit and see if that changes the vibration. I agree on the mounts and exhaust as likely suspects so my first approach next week will focus on that unless someone thinks of something I have missed looking at. Thanks!

 

31vicky..I will check that but I do feel some buzz in the fender as well…interested more than if I put my hand on valve cover.

 

Has the engine been rebuilt? If so, I would say it did not get balanced properly. I know that's the last thing you wanted to hear but you changed the balancer and disconnected the torque converter and confirmed you have the right flex plate. That is all that's left.

 

Rockable…yup, bracing myself for that by arranging to get my 235 back.
Engine was rebuilt and I have never been inside it so unfortunately the possibility of improper balancing is definitely there. I should have got aggressive finding the vibration when it had a warranty 30 years ago but life got in the way.

 

Just out of curiousity, would it be possible that an intake manifold leak could lean out a cylinder or 2 and induce a misfire? Grasping at straws.

 

Exhaust can certainly amplify a minimal vibration. Loosen it at the manifolds and see if it helps.

I replaced the a/c clutch on a Ford 289. The original had a very heavy damper made into the pulley, the replacement did not. I isolated it by removing the belt.

What I am getting at... If it is spinning, it can create a vibration. Look at everything !

 

Wonder if the vibration will go away if I put Corvette Valve Covers on it!!
Just kidding..I think.

 

It's certainly possible. What was the vacuum reading when you checked it? If it was a strong steady reading, you might be grasping a little.

 

It just might! But only if it's painted orange!

 

I believe it was 17 or 18” at idle, no fluctuation at all…very steady. We are at about 3200 ft elevation.
Damn, it is painted blue.

 

Drop the downpipes and start it.

 

some flex plates i have seen have a small weight welded to them. could flex plate be issue???
it is pontiac blue,right.