Art Brock

I still have an annoying vibration when driving my car.
Car is a 1966 coupe with a 1993 302 & 1966 C4.
I feelthe vibrationmainlyin the seat & a little in the steering wheel.
It's around 2000 RPM. I can shift to 2nd, the RPMs jump & vibration goes away. Shift back to 3rd, about 2000-2400 RPM & vibratin reappears.
If I shift into Neutral it also goes away (so I think that eliminates tires).
It's what I would call a rhythmic vibration, where the car sorta shudders about every second. Almost what I would think a bad tire would feel like.
It also gets worse after I run the card hard.
Last night on my way home from work I did a couple of 0 to 60 runs & a couple of 1/4 mile runs. (The country road was straight, NO houses & NO traffic).
The vibration got a LOT worse for the rest of the trip home. It did that once before after I got on it hard.
What would change after a few hard runs?
What would change after a few hard runs?
I have had all tires balanced.
have replaced motor & trans mounts.
I checked the HD & it looks great.
The torque convetor bolts are tight.
U-joints don't have any play (I have not replaced them).
I think it's in the drivetrain some where but where? I hate to start spending money replacing everything until I get the right thing so I figured you folks could give me some ideas where to start.
Could it be U-joints, driveshaft, rear end,tranny, wheel bearings?
Can it be the motor? I doubt that but I'm at a loss now.
Is there a way to check the rear end without pullling it out? Can I put her on jackstands and check for play by turning the wheels back & forth?

Gun Jam

You could have a bent rear axel.

jack the car up so both rear wheels are off the ground. Make some type of thing that will hold a pointy object like a pencil at 90 degrees to a flat spot on the wheel (further out from the center the better) Place the point of the pencil so its very close to the flat surface (.003 of an inch is good) turn the wheel by hand and watch to see if the point attempts to change that gap either by running into the wheel or pulling away from it.

If you don't have nice wheels with a machined surface on them (a flat spot) then remove the rear wheel and use the flat section on the axle.

If you notice any movement at all then your axle/wheel is probably the issue. if you can easily see the gap change then your axle or wheel is the problem.

Free test at least.

Gun

Mr. Classic EFI

It may be your harmonic ballancer, I had a simular harmonic vibe, and changing the H Ballancer fixed it.
If you can acurately set #1cyl on TDC, then do a static timing check, you might get an indication the outer ring of the ballancer has slipped. The old one pulled off mine(92 5.0)dident visually appear to have slipped, though it apparently had.
Please keep us informed as you trouble shoot this problem and fix it.

jcomp

After a few hard runs, your transmission temps are higher.

I'm a bit confused by your description, can you clarify? It doesn't vibrate with the transmission in neutral, no matter the engine or vehicle speed? It only vibrates in 3rd at 2K RPMs? Does it vibrate in first or second at the same RPMs?

In general, a vibration in the driveshaft, differential, or suspension will only change with vehicle speed. A vibration from the engine will probably be apparent sitting in the driveway with the car not moving but with the engine revved up.

If your vibration is dependent on which gear the transmission is in, then maybe the C4 is the culprit (but I don't know how).

Finding a vibration like this can be frustrating. I had one in a car that turned out to be a loose front control arm bolt.

stang05gt

Do you have 50 oz flexplate as well as damper in your 5.0 engine.I think late 302 are balanced differently from earlyer 302.

Art Brock

UPDATE
I drove "Marilyn" to work this afternoon and did some troubleshooting.
To answer a few questions first, I do have a 50oz HD & flexplate. I found TDC & it lined up with the 0 mark on the H, so I think it is OK.
OK here is what I found out today.
In 3rd gear the vibration starts about 1900 RPM and continues to about 2600. Vibrates worst at about 2300 RPM.
Go to about 2700-2800 RPM and she runs pretty smooth.
While it was vibrating I shifted to 2nd and vibration disappeared.
Stopped and left car in 2nd, ran up to over 60 MPH and no vibration. Held her steady at 2300 RPM in 2nd and no vibration.
Shifted to 3rd and as soon as I hit the 2000 RPM area it starts vibrating. Shift to 2nd again and it goes away. Shift back to 3rd and it starts vibrating.
In summary it seems no vibrations in 2nd at any RPM or speed and vibrations in 3rd in the 2000-2600 range.
So that sounds like the tranny must be having "issues".
Guess that would maybe explain the running worse after a few hard runs also.
Anybody ever heard of a vibration in high gear only.
Anything you can think of I can try besides having tranny rebuilt. (I may go the AOD route if that is necessary).
I have replaced the fluid & filter lately & it was doing this before I replaced them.
So do you agree this is a tranny issue or could it still be something else.
Thanks for the replys and I hope someone has a good idea on how to fix this.

67mustang302

Could be some worn bearings in the tranny on the shafts, such that it vibrates at a certain shaft speed and load, and you only feel it at a certain rpm in a certain gear. But it sounds like the tranny may be the culprit. Could go find a junkyard AOD maybe? If you want you can pull your trans and see if you have any play in the input or output shafts. Also, what condition is your torque converter in?

Gun Jam

yep sounds like that trans is the problem child or at least related to it.

good hunting

Gun

blueovalfan

I would agree its the tranny. I had the same problem with my old 3 speed. Sounds like its time for a rebuild. While its out I would recommend you have your driveline balanced and new u-joint too.