I can’t find the vibration source in my car

Question:

I’ve repaired/restored/driven many, many older cars over the years, but this issue has me stumped.

I have a 1960 Buick with a 401 “Nailhead” engine and automatic transmission. The engine and tranny were professionally rebuilt and then sat for a few years. I just put them back into the car. The tri-lobed flex plate was already on the engine when it came back from the shop. I attached the transmission to the engine and then put both in the car at the same time.

Now there’s a very consistent and pretty significant vibration in the engine around 1500 to 2000 rpm. It runs smooth below and above that rpm range. It seems to get a little worse when the engine is hot.

I’ve tried one to five ounces of weight at every point around the flywheel. No difference. I tried moving the flywheel in relation to the flex plate (which can be done with the transmission in the car) but there was no change. I tried removing the aftermarket front balancer/pulley and re-installing it at different points but there was no difference. I tried one to five ounces of weight at different points all around the balancer but there was no change. I tried removing the belts one by one but that made no difference either. All of the rocker arms are tight and appear to be OK. All of the cylinders appear to be firing OK.

When I put a vacuum gauge on the engine the needle “flutters” a little. Various timing, dwell, and idle mixture settings don’t seem to make a difference. The transmission seems to go into Forward and Reverse OK and moves the car. I drained the oil and saw no metal particles or anything strange in it. I’m at my wit’s end on this one.

Answer:

As you know, the Buick Nailhead family of engines was externally balanced and if the balancer, flywheel and flex plate are not all lined up properly for correct balance you normally will get a vibration. Your efforts to move the parts around and apply various weights to different areas of the balancer are ambitious but I’m not convinced that this work would eliminate a simple imbalance as the cause.

So, where to go from here? I’d suggest eliminating other possible causes that are external to the engine and if that doesn’t yield results I think you need to go back into the engine.

Other possible, outside-the-engine causes include a bad engine or transmission mount, imbalanced drive shaft, problem in the rear end, imbalanced wheel and/or tire, or a loose accessory component.

I’ve relayed the story in this column previously of my efforts to find a severe rpm-specific vibration in a 1968 Z28 that turned out to be a broken alternator mount bracket. Within a certain engine rpm range a frequency caused the incompletely secured alternator to vibrate so violently that it shook the entire car. So with that in mind, carefully inspect the alternator, air conditioning compressor, engine cooling fan, etc. to make sure nothing is loose or broken.

If it becomes clear that the cause of the vibration in your car is the engine itself then I think your only option is to take the engine out and apart and have a competent machine shop balance the entire reciprocating assembly.