Diagnosis and Repair of Your Imperial Engine Vibrations

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Question from John (331):

The 331 Hemi in my '52 4-door sedan has a vibration that is most noticeable > at engine speeds above 1500 rpm, and especially if the car is not in gear. The PO, a friend, once replaced the water pump and fan. He says that the vibration began with the installation of the new fan. I should remove the > fan to see if this is the case --- but --- it seems hard to believe that these crude sheet metal stampings could ever be that much out of balance.


From Dan:

The fan is the first thing I would check. Just because it's new doesn't mean it's balanced. Just take the belt off and try it for a minute. At least that can be eliminated if it still vibrates. The other thing could be the harmonic balancer. If it's worn it may have slipped.

It can be rebuilt. A good source is The Damper Doctor. I know he's in Ca. but I don't have his address anymore.

From Demetrios:

John, its worth investigating the harmonic balancer, but the torsion vibrations due to a bad balancer may not be felt (because they are ... torsion). It is certainly worth checking though, because it can damage your crank if I am wrong (not unlikely).

Another thing, are you certain that the engine vibration is not caused by some cylinder(s) missing? The question would be, why would it miss more at the higher rpm or lighter loads. Hard to tell. A bad valve would be more noticeable at lower speeds, but it may be worth doing a compression test. Does this happen more when the oil is cold? I have had some experience with some modern engines where the hydraulic lifters may become a nit too enthusiastic at low temperature and not allow the valve to close. Of course, that happens at higher speeds because the oil pressure is higher. Fat chance, but you never know. If your car has too high regulator oil pressure or too low valve springs, it might happen. Then, as you increase the load, the leakage through the bad valve(s) is not enough to get the cylinder to miss. By removing the plug wires one by one till you observe no change, you may be able to test my theory of vibration caused by certain cylinder(s) going "off line".

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