70 GTO Vibration Problem
 

I have a 70 GTO convertible that has had a vibration in it years! In that time frame I have replaced everything that moves. I have changed rear ends, engines, trans, drive shafts, brake rotors and drums, wheels and tires, and suspension bushings and related parts. The vibration is still there, it's like a pulsating harmonic type vibration that resonates through the car that kicks in at about 2600-2800 rpm and gets worse with more revs and mph. Now that I put a more powerful engine in it this summer it's worse. It was just annoying with the other engine, now it just pisses me off with the new engine. I had everything balanced with the new engine. It vibrates so bad you can't see anything in the rear view mirror! Could the pinion angle be causing this????? What should the angle be? How do you actually measure the angle? I was going to install adjustable rear upper control arms, but I'm not sure where to start. I've noticed this vibration in other A body convertibles but not as bad as my car. Any input is welcome, thanks.

you did not montion the drive shaft.

Yes I've tried those as well, currently has a new one, with joints.

Does it go away when you put it in N and coast?

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Without a doubt you have given this a lot of thought and $$. Hopefully someone here will help. Bet it doesn't vibrate in neutral at the given RPM, that's too easy. If the mirror / steering wheel are shaking but not so much in your butt it's an indication of front wheel- tire- rotor balance. Can you find an old school tire balance machine that uses spinner and strobe light. The wheel-tire stays on, and is spun on the car ? You can individually check each wheel.
Put it on a dyno ? Where you can tour around the car as it revs the desired RPM.

Here's a thought. Are the factory "cocktail shakers" in place. These were installed for a reason. Convertibles are notorious for vibration and the "vibration dampeners" (cocktail shakers) were the solution. Your symptoms are exactly why the factory installed them. Higher rpms and speeds cause the harmonics, the shakers balance/cancel them out. Your car should have two in the front and two in the trunk (one per corner).

Swap wheels front to back, side to side, etc...confirm each wheel is balanced accurately, also confirm your new driveshaft is "balanced".


Tim john---

I haven't tried that, but I will.Thanks

I haven't seen one of those style balancers in decades, I used to work at a Goodyear dealer and we had one in the 80's. I've had the tires balanced, and I've put different wheels on the car, same problem. I've replaced the rotors and wheel bearings. I don't feel it in the steering wheel, it's in the chassis. The dyno is a good idea, there's a place not to far from me that does that, I think I will check into that, Thanks

As far as I know the A bodies didn't come with the cocktail shakers, I believe they were only on the 1st gen F bodies. But I will double check, my car currently doesn't have them. Thanks

If you put it in N and it doesn't vibrate, it's before the trans. If it does, it's after the trans or chassis related.

Another thing to check is to rev it to the 2600-2800 RPM while parked, see if it vibrates. That will also tell you if it's engine or chassis related.

Usually, if it's RPM related, it's engine related, if it's speed related, it's a chassis issue.

I've seen people have pulleys and fans out of balance that they kept swapping for config to config, car to car, and it followed them.

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Here is the information that I use for both driveline angles and vibration diagnosis:


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If you follow this thought process you can fix any vibration. It's all about separating those areas that don't require any attention from those that you need to focus on.

Having said that - I'm beginning to believe there are some inherent beaming modes in A body convertibles. I worked on a '70 Buick convertible that could not be improved until the owner added a roll cage.

K

New engine has new balancer, water pump, ps pump, and alt. I was just playing with the timing up to 3000 rpm and it runs smooth while setting the timing. But i did transfer over the pulleys, they were bead blasted and repainted, and look to run true on the engine, but I give them a closer look while running. Thanks

I had a vibration issue with the LeMans once, and it ended up being the yoke on the rear, the 'nubs' that center the u-joint caps were worn, and the u-joint would shift making it 'wobble'. It took a very close visual inspection to catch that.

Keith - What's the source of that diagnosis chart, etc? I'd like to have a copy on hand...

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Looks like some things got packed away during my last office move; I believe it is from a Chevy S/T truck service manual.

I also have a (very interesting) Pontiac Vibration manual from the mid 60's but am hesitant to post that in case there are copyright issues.

K

Since the vibration got worse after the new engine was installed, how good is the engine and trans mounts? Sounds like one may be grounding out or the exhaust is grounding out.:)

The mounts are new, I put in the polyurethane motor mounts from Bulter and the trans mount was replaced a few years ago and is still in good shape. This vibration is definitely rotational due to the harmonics that I feel. I was an auto mechanic for 10 years and have fixed my share of vibrations, but this one has me stumped.

Maybe the reason its worse now with the new engine is because of the stiffer motor mounts, but that still doesn't explain the cause.

Engine firing frequency!

;)

When you say "rotational due to the harmonics you feel" does that mean it comes and goes?

If you are feeling it increase and decrease: that's a "boom/beat", which indicates you have two root causes. The increase and decrease is a result of the two out of phase amplitudes adding and subtracting. You would need to isolate and eliminate one of the root causes.

Does that sound like a good description of what you are experiencing?

K

Get a syrometer (sp) and find out what HZ you get at what RPM or vehicle speed. From there you should be able to calculate what it is.:)

Engine frequency? I have had 3 different engines in this car over the years and the vibration is the same.

It starts out as a pulsating vibration and as speed and rpm increase so does the vibration, I guess at higher speeds and rpm the frequency of the pulsations increase making it feel like one steady vibration, this car at 90-100 vibrates a lot, just feel it through out the car. Because I have changed out all the drive train components over the years is why I was questioning the drive shaft angle or some thing related, It's a new drive shaft but who says its balanced correctly, I'll rotate 180 and see if any thing changes. this vibration has been with this car since I restored it back in the 80's, I was really hoping it would go away with the new drive train, and it has not. Maybe it's time to tear in to the rear end as well?? I bought it rebuilt from a guy that does rear ends, 12 bolt 3:55 gears. Thanks Keith

I had to Google that! Sirometer is a vibration tach generally used on small engines. Not sure exactly how that would help? What would the frequency tell me?

First, I would determine if it was the engine or drivetrain, using the techniques already posted. Personally, my '65 had a mirror-blurring vibration for years. Turned out to be a worn front driveshaft yoke. A new yoke and bushing in the muncie fixed it. I have also run into cars where the locating tang was hammered/worn on the rear yoke, making it nearly impossible to center the driveshaft. My gut tells me you have a driveline issue. Also, check the weights on the rear brake drums, etc. A run-out gauge will help you check driveshaft run-out and wheel run-out, which is also not a bad idea. I feel your pain....vibration is a fun killer.

iam sure you check this, e-brakes hangin up?

That's not what she said! :D

That's the joke: all cars get really quiet when you shut the engine off. A muffler engineer taught me that one.

;)

But, in all seriousness, engine firing frequency is a 4th order disturbance of engine rpm (there are four firing pulses per revolution for a V8 engine). If it gets close to another distubance, like 2nd order driveline, it can result in the "boom/beat" phenomenon I described.

All of these components are rotating at a specific speed during the disturbance (in rev/min, which can be converted to cycles/sec = frequency, in Hz).

The hope is that if you can isolate the frequency of the disturbance you can back calculate which components are spinning at that same rate (either the base frequency or some harmonic of it), thereby pinpointing your suspect component. It virtually eliminates all this guesswork.

It simulates on a smaller scale how we quickly diagnose and repair vibration problems at the GM Proving Ground, except we use power spectral density plots and/or order tracking on one of our Noise & Vibration dynomometers, analyzing several strategically placed accelerometers at the same time.

Disclaimer: You do, however, have to be careful where you place the vibration tach. If you set it on the steering column, for example, and the column is going off on it's own resonant frequency rather than the forced frequency of the actual disturbance, it can cause some confusion.

K

pggto -

Did you do this yet?

You've really got to "split the dictionary" to know where to start.

This first step will eliminate half the car, so you know you don't have to worry about that part of it and can begin homing in on where you need to focus.

K

Is the tranny crossmember bent in any way? Another thing to check is how close your engine sits to the engine crossmember. (Does the fan come real close to hitting the bottom of the shroud?) Sometimes (especially on cars that have been drove hard or have a lot of miles) the engine crossmember tends to "spread out" ever so slightly, letting the engine sag and throwing off the pinion angles. Just something else to check.:2cents: Still comes back to the basic pinion angle deal.

Heres a question for you vibration guys since we are on the topic. Could it be a slightly bent pulley? I have a bad vibration that comes in after 2500 RPM and gets much worse with vehicle speed but its never present before 2500rpm in any gear and it only shows up strong in 3rd 4th and 5th.....I think I have a crank pulley that has a small bend in the outer portion of the ring from when I put in in a tire to haul it. Its not messing with the fan belt but the bend is obvious. Im going to tap it back in place tomorrow and report back. I guess im thinking out loud since we are on the topic.

Nah, since it's gear related, my guess it's behind the trans. Do you have a spare slip yoke? If so, pull the shaft, slide the spare in, and see if it wiggles. First guess there is the tail housing bushing/bearing is bad.

With the car on the ground, how much remaining 'slip' area on the yoke is showing? Same question when it's on jack stands and the suspension is drooping?

When the shaft is out at this point, take a close up of the rear yoke and post it. Also, grab the rear yoke and wiggle it. A pinion bearing is also a possibility.

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Really ? I had to shim my engine mounts up slightly to get the fan to quit rubbing the bottom of the shroud. I tried everything trying to determine why after all these years it started rubbing. Shimming fixed the problem, but I never knew the cause.

Are you sure you got the right engine mounts? Did this start after changing the engine mounts? Frame spread is somewhat common, but it takes a lot of like 'general lee' jumps and stuff to spread it that much. You can take measurements from the chassis service manual that will tell you if in fact it's frame spread.

Trans mounts too, seen where the wrong trans mount causes issues.

And now of course you need to verify your pinion angles.

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As seen in another "vibration" thread. Cliff Ruggles had reccomended that unknown vibrations can be found in the transmission bushings such as pilot bushing and tail housing. Have you checked these.

Tim john---

There is another thread here from about a year ago addressing this issue. A PY member actually manufactured spacers to put under the motor mount brackets. I purchased some myself but havent installed them yet. I corrected my problem (Milodon oil pan rubbing the crossmember) with modified motor mounts before I saw these spacers. I intend to install them the next time I need mounts. I only had to jack mine up about 1/4 of an inch at the mount. Made a bigger difference overall than you may realize. I beleive in my case it was due to high mileage. (395,000). Plus some hard driving.:2cents: Your vibration is likely a combination of different things, but it is something to address.

I'm on engine # 8 in the driver, and get about 100k+ out of each, and if you look at my crossmember, and the current headers on the car, you would agree that I don't baby that thing. So don't think miles has much to do with it.

I'm not convinced it's frame spread some are experiencing, because it takes a considerable amount to show up like that. It would more show in an alignment than in shroud or oil pan clearnace.

On the shroud, my first thoughts would be the core support bushings are worn, and the bolts were tightened. That would raise the core support up to 1.5 inches or more, and cause shroud clearance issues.

Pan to crossmember clearance, my first thoughts would be due to the engine mounts. I've seen such variations in mounts that cause all kinds of problems, wrong part (box/part swap), poor manufacturing, and incorrect application (cross reference wrong). Think about it, if it spread that much, don't you think it would show up in an alignment? Those front upper control arm mounts would have to move like more than an inch, and that would need like a stack of shims to get right.

Second thought would be trans mount is too high, wrong app again. And that would show up as a vibration, since it would whack out the pinion angles.

Original Poster - Guess we're all still waiting for you to complete the diagnosis as suggested, before we can continue. Have you isolated the area the vibration is coming from yet?

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I have not had an opportunity to work on the car in a few days, I'm hoping to get back to it shortly. Other obligations keep getting in the way. I also need to get something to measure the pinion angle, I currently don't have a way to check it. Thanks

All of that withstanding, the mounts for early A-bodys do seem to all be of inconsistent quality. I do also have a '74 Oldsmobile with 500,000 miles and the engine still sits where it should in that one, but the Oldsmobile mounts seem to be closer to the original location than Pontiac mounts nowadays seem to be. (Just from what I have seen) I guess it sounds like I was implying that the frame could be spread out severely. My mistake. I should have mentioned that the combination of a "little here" and a "little there" of things like this, the inconsistent motor mount height, the varying tranny mount height, worn axle flanges, bent driveshafts, can all combine together to become a problem caused by multiple things as this, and not one "single" problem.:)

Absolutely, all the little things do add up, and those seem to be tougher to isolate too. I have a bad habit of over-engineering stuff, and in a lot of cases am searching for complicated problems when it's something easy staring me in the face. So any chance I have to provide insight in some areas of thinking out loud, especially, I try to soften that logic/process. A bunch of times I've found solutions to my own issues in that process!

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I suppose that is what I am doing to try and throw the original poster "a bone" is thinking out loud. :)In my case (only after I had installed my Gear Vendors unit) did I have a vibration that was driveline related at about 60-75 mph, that wound up being a lot of little things that added up. fixing each little thing made it "better", but in the end it was the fact that they had cut my driveshaft 1/2 too short (causing the yoke to wobble ever so slightly in the back of the GV unit). Had a new driveshaft made up at the correct length by Dennys, and that solved it. Over the years, I had developed a similar vibration in my Oldsmobile, and it wound up being the driveshaft had been "opened up" in the rear very slightly, by some improper u-joint replacement.:rolleyes: You could slide the shaft back and forth in the caps ever so slightly. It would not allow perfect centering of the shaft when running. It was only about 1/32 of an inch, but that was enough to cause a vibration.

I see that you replaced the trans already, but was it used? I had a terrible vibration in my car and after suggestions from ppl on this forum, including Cliff, I replaced the pilot shaft bushing in the output shaft and the tail housing bushing and it solved my problem.

I had the original trans rebuilt buy a very reputable shop, I hope they replaced the bushing, but I will will check for play when I get back under the car hopefully this weekend. Thanks