Hard shaking 60-70 mph: is this the end?

[avgwarhawk]

My first thought is a spark plug wire having to much resistance under load and creating a misfire.  However you did a power braking with raised engine rpm and no issues noted.  My next thought is your u-joint. U-joints can act up at certain speeds.  They can act up under all speeds. It was a 2 week 15 miles per day period before I could precisely diagnos a failing u-joint.  Sometimes a diagnosis will not present itself without some drive time. The issue is driving upgrade in low. This condition is throwing a lot of torque in the driveline. I would concentrate my attention here.

Edited February 3, 2019 by avgwarhawk (see edit history)

[avgwarhawk]

3 hours ago, Ben Bruce aka First Born said:

  Ben, I will be serious now.  Have you checked the tires? A loose belt on a radial can do strange things.  A break in a bias can be just as strange.

 

  Ben

Good place to look. 

[Bloo]

Lots of great advice in this thread so far. Some thoughts...

 

Slipped belts on tires: You can often see this from across a parking lot if you know exactly what it looks like. Back in the days when radials first became popular, you would hear about bizarre "pulls" and such caused by slipped belts. Maybe. I haven't run across a slipped belt in decades that couldn't be seen with the car on a hoist.

 

Jack it up a wheel at a time (assuming you don't have access to a hoist) and turn each wheel while watching. While looking from the outside in, across the tread, look for runout. The tire should be pretty round. Probably not as perfect as you would expect, but round. Now, look straight at the tread and turn the tire so you can watch the circumferential lines of tread. A slipped belt will make a really distinctive s-bend in a line of tread that should remain straight while rotating. A slipped belt also makes a "high spot" on the tire (runout), and this spot is usually roughly egg-shaped. It could be about 1" x 2" to 3" x 5" or even bigger. If the tire has been driven on even a few days, this egg shaped spot will be worn heavily. As you turn the tire you will see the tread grooves get much shallower as they cross the high spot. Once you see these things, you will probably see a little bit of steel belt somewhere, too. Maybe one tiny strand of stainless wire will be sticking out of the tread in some random spot. It will never be noticed until you get your eyeballs right up to the tire.

 

I would re-balance the tires. I don't really think that is the problem, but I would eliminate the possibility. The tires I used to prefer on big US-built cars back in the day were the second generation "radial t/a". They had really deep tread, and sometimes required re-balancing during their long life. The tires on your car have a similar appearance.

 

Ignition problems: A high resistance plug wire can be found with a multimeter. Bad insulation is also a possibility, but I have found that failure almost always begins with high resistance. Testing with a multimeter finds bad plug wires 99% of the time. The only trouble is it is sort of hard to get a reliable connection with your probe at the spark plug end where the connector has been hot.

 

Back when I was in auto repair, I made a special test lead. I removed the insulator-and-center-wire assembly from a spark plug and soldered a test lead to it. I put a banana plug at the other end and use it for one of the multimeter leads. I am not sure I would bother making one of these leads for one car, but it does sure make plug wires easy to test. Resitance should be higher on the longer wires. Maybe a few Kohms, probably not over 10kohms. You are looking for outliers.

 

If this car has some metal covers over the wires or plugs, I would take them off until you get this solved. Running plug wires perfectly parallel, or super-close to grounded metal is just begging for trouble.

 

If you find a high resistance wire, replace the rotor too. The plastic at the center is highly stressed, and probably pushed way beyond the limit when a wire fails. Distributor caps, on the other hand, can go on practically forever. Inspect the distributor cap under a really bright light. You are looking for carbon tracks. Check your points for mountains.

 

Valves:  I have ran into burned intake valves exactly 3 times in my life. All 3 times I chased my tail for days. The symptoms are bizarre and unpredictable, and vary from car to car depending on how the engine is manifolded (I think). I only mention it because there is no abnormal valvetrain noise when it happens. Usually the car will be rough on transition, in other words it may shake violently when you crack the throttle suddenly. It may even snap back. The idle will probably be crappy, but only crappy enough that you know something is wrong. There is a HIGH probablility you will go through your entire life and never see this. A leakdown test would have found it right away....

 

Some in this thread have suggested a sticking intake valve. That is a much better possibility. Yes it should cause valvetrain noise. It wouldn't have to though. If the valve was hanging almost shut, the hydraulic lifter could take up the slack.

 

A flat exhaust lobe on the camshaft will cause a rhythmic snapping or backfiring in the carb. The higher you rev, the worse it gets. In some cases the intake noise starts to sound like exhaust noise. If the valve is still opening a little, the car will still drive, but the more air/fuel you put in, the worse it gets. This is a problem that would manifest worse in high gear, because the engine needs to pull more. Easy to see it cranking with the ignition disabled and the valve covers off.

 

I like Bernie's vacuum gauge on the windshield suggestion.  I have also used that trick.

 

U-Joints and Torque Tube: Yeah, this is probably it. Old Chevys with torque tubes commonly lose the bushings that support the u-joint and driveline at the transmission end. The driveline starts whipping around in there and making a bunch of vibration and noise. I'll bet its either the u-joint or whatever bushing or bearing Buick used to support the driveline. Torque tubes are a pain in the ass, but there are dozens of threads here about how to service Buick torque tubes by disconnecting the axle and rolling it back a bit. It should be cake compared to some of the things you have already done.

 

You have already been through all the bullshit that causes people who dip their toes in this hobby to quit. It really didn't slow you down much. Yeah, you'll have to fix it yourself to get it done right, but I think you already knew that. Don't even think about giving up.

 

 

 

 

Edited February 3, 2019 by Bloo (see edit history)

[Dennis Hagen]

I purchased a 56 Century a year ago and not knowing any mechanical history. It had a severe vibration about 50 mph, had trans fluid in rear end. Found torque ball and bushing were bad, appears car was driven with low trans fluid which took out the bushing and ball. Found a nos ball on eBay and bought a seal kit from Bob's, all seems to be good now.