A Surge of Vibration

[tarquin]

Now that I have the GT up and running for the first time in 2 years, I am getting a surge of vibration that cuts in at 80mph and 4700rmp in 4th only. The front suspention is all over the place and is going in for a geometery setup soon, but it feels more like its coming from the transmission. Once I get passed 80mph/4700rpm it goes. When it comes in, it vibrates and shakes the gear stick and transmission tunnel. I also have a chirping noise coming from the back of the engine that rises with the revs and is not present when its cold or below 1000rpm and dont think they are connected.

Anyone??????

[spdkilz02]

would the tranny mount have anything to do with it???

[tekenaar]

Originally Posted by spdkilz02 would the tranny mount have anything to do with it???

. . . and/or torque tube 'donut' . . .

[Dmcbrass]

The reason the vibration is speed dependent is due to resonance. The vibration is originating from a source somewhere between the output shaft of the transmission and the rear wheels. As vehicle speed increases, the frequency increases until it reaches a frequency that something in the car is sympathetic to. High vibration can result from the source (the thing the vibration is originating from) being excessive or from high transmissibility ( components in the car being overly responsive to that source). The source could be things like a bad universal joint, tire out of balance, bad bearing, etc. High transmissibility can be the result of weak shocks, loose bolts, structural cracks, weak transmission and/or center support mounts, etc. If it was my car, I would start by inspecting the tires for obvious defects and then rotate the tires and see if the origin of the problem moved. I would then look for bad universal joints, loose bolts, etc.

[tarquin]

Cool, thanks guys, it gives me a starting point. As I said, the front suspension has to have its geometery set up, so I'll start there and work back.

[baz]

have the wheels balanced too just in case

[jvandyke]

Yesterday, all of a sudden, I have a serious vibration at specific rpm, (about 1100) and it's not suspension related as it happens in the driveway. I'll look at fan and other spinning things, the explanation above, the "resonance" and such is spot on, once it kicks in the whole car shudders "to the beat" steering column, steering wheel, a light touch is all that is needed to placate the wheel but the noise is annoying and obviously something isn't cool.

[tekenaar]

Originally Posted by jvandyke Yesterday, all of a sudden, I have a serious vibration at specific rpm, (about 1100) and it's not suspension related as it happens in the driveway. I'll look at fan and other spinning things, the explanation above, the "resonance" and such is spot on, once it kicks in the whole car shudders "to the beat" . . . steering column, steering wheel, a light touch is all that is needed to placate the wheel but the noise is annoying and obviously something isn't cool.

I take it that the car is not moving when it happens? 1100 RPM (if accurate) is just "off-idle", i.e. beginning of transition phase of carb and mechanical advance in distributor.

Ensure that distributor mechanical advance advances and snaps back smoothly and enrich idle mixture a bit.

[jvandyke]

rotor snaps back fine, I did recently slightly lean the carb, 1/8 turn as it seemed a tad rich, I would not have thought that could do it but I just put it back and no effect.
I did mess with vacuum advance lines last night but this started BEFORE that. Now hooked up "properly".
Very weird. She runs fine just makes me nervous. Could it be something serious like a bearing somewhere getting ready to fly apart?

[dcm013]

Originally Posted by jvandyke Yesterday, all of a sudden, I have a serious vibration at specific rpm, (about 1100) and it's not suspension related as it happens in the driveway. I'll look at fan and other spinning things, the explanation above, the "resonance" and such is spot on, once it kicks in the whole car shudders "to the beat" steering column, steering wheel, a light touch is all that is needed to placate the wheel but the noise is annoying and obviously something isn't cool.

My 71 GT lights up at 1100 RPM in neutral - really drones/vibrates everything - until it warms up, then it pretty much settles down. I attribute it to the 36 yr old engine mounts. How's yours once it warms up?
Don

[jvandyke]

Makes no difference warm or cold. It really is bizzare, came on suddenly, I haven't changed anything recently, well I did do an oil change (first one) a few days ago and I adjusted the mixture ever so slightly but have since set it back.
I took my wife out for dinner, about 75 miles round trip. Everything went fine but for this annoying "resonance", it seems like it is always there just varies greatly in intensity. It didn't ruin the trip, it ran nice rode nice, it bothered me far more than her, since I knew it shouldn't be there.
When we got home, I let the car idle down through its "buzz zone" when it gets to the sweet spot it sounds like a really deep bass speaker feeding back, it's almost freaky, bit more/less RPM it eases off.
Maybe I did lose a motor mount. I'll check for stuff touching where it shouldn't. How else does one know if a mount is bad?

[jvandyke]

Checked mounts, "look okay" to me. Here's pics of the passenger side. Pacesetter header is up tight against it but is that a problem? hmmmmm
Fan looks like fine, it's a 7 blader, some tips show a little "wear" one worse then the rest but it's minor and old and I can't implicate it based on that.
Here's a couple pics for fun.

[namba209]

Originally Posted by jvandyke Maybe I did loose a motor mount. I'll check for stuff touching where it shouldn't. How else does one know if a mount is bad?

There's a couple of ways. One, is to put a jack under the pan with a piece of wood on it to protect the pan, and slowly jack up the engine checking to see if the rubber separates. The other is to try and rotate the whole engine by hand with a pry bar and check for the same thing.

[tekenaar]

Originally Posted by jvandyke Checked mounts, "look okay" to me. Here's pics of the passenger side. Pacesetter header is up tight against it but is that a problem? hmmmmm Fan looks like fine, it's a 7 blader, some tips show a little "wear" one worse then the rest but it's minor and old and I can't implicate it based on that. Here's a couple pics for fun.

Regardless of header wrapping, where's that motor mount heatshield?!!!

[jvandyke]

Originally Posted by tekenaar Regardless of header wrapping, where's that motor mount heatshield?!!!

Is there something missing? Is there supposed to be something more than the metal cover shown in the second picture?
I don't know what's up. When I stand over the engine and induce the vibration by moving throttle linkages nothing appears amiss, I can barely tell it's doing it. Inside the car it's extreme. Weird. There's some vibration of course, the gas line showed it most. I don't know. Runs good yet.
I'll make a sound file of it.

[opelwasp]

Motor mount heat shields usually obstruct the installation of headers, so most PO's remove and toss them. Very bad for motor mount though.

[hrcollinsjr]

Originally Posted by tekenaar Regardless of header wrapping, where's that motor mount heatshield?!!!

If you think much of your header I would pull that wrap off of it and get some type of coating that will protect it not shorten the life like the wrap.

FWIW,
Harold

[soybean]

Originally Posted by hrcollinsjr If you think much of your header I would pull that wrap off of it and get some type of coating that will protect it not shorten the life like the wrap. FWIW, Harold

I agree with Harold on the wrap. It will hold moisture and rot it from the outside in. Back on topic, even though picture 2 shows the bolt, etc. I have a motor mount that has sheered the upper part shown, from the body of the block. Try what Ron suggested. Hth, Jarrell

[jvandyke]

I got two new mounts and installed them last night. It did make a difference but not sure it "cured" the idle thing vibration thing. I noticed a big difference on the highway, over all much smoother and several annoying "rattles" (dashboard, door hardware, who knows where else) all disappeared.
Still seems to vibrate near that 1100 mark, and then again at around 3000, so I pass through that vibration coming and going through that rpm range. Can't say for sure it's cured because shortly into this morning's commute my exhaust broke apart. (that's a different thread) The few minutes before that emergency though I was pretty impressed with the vibration improvement.
That was a PITA to put them on, mostly because of the way the lower bolt is entombed by the frame. No surprises or big issues just a pain to line up the mount's bolt hole without being able to see it and put the bolt through at the same time. I used the jack under the oil pan method with a hefty block of wood, I had the passenger side of the car jacked up so I could crawl under. If (when) I have to do it again I will raise the whole front to give myself working room. Used a flexible extension on my socket wrench too. (which doesn't let you torque very much but the only torque spec I found in the FSM was vague at 40lbs which isn't much).

PS I started stripping off the heat wrap too