Vibrations - off throttle

Paul_nz

Paul_nz

Member
When driving down hill and I have backed off the throttle, engine / transmission vibrations are amplified, if you apply just enough throttle to drive the engine, then the vibration tends to lessen. If I put the gearbox into neutral, there are no vibrations. Any thoughts / suggestions as to what I need to look at?

Paul
 
I assume when you put the trans in neutral you take your foot off the throttle and the engine is idling even though the car is traveling at speed? If so, that indicates the vibration is in the engine. If it continued to vibrate at speed when the engine is idling, the vibration would be in the driveline. There is a sort of hummm like vibration on decelleration that is normal to the engine. The trick is knowing when the normal vibration becomes abnormal. Check with friends.
 
Yes the car is traveling at speed when in neutral and the vibration disappears, if in gear and I back off the throttle, the vibration is definitely more noticeable, but reduces (not completely) when the throttle is back on again.
 
U-joint, definately. We had the same problem in a car going to Detroit in 03. When the car was pulling or backing off it would vibrate till you couldn't see out the mirror. Push in the clutch and all vibration went away and the motor reved smooth. We pulled the rear and fixed it in a motel parking lot in about an hour.
 
Does the vibration occur if you rev the engine real high then take your foot off the gas pedal while the car is STATIONARY ? If so the vibration is in the engine and not in the drive line.

Bob
 
I agree with Jim. If you have another car or two to ride in look for a comparison. Your description doesn't sound like a real "shaker". They typically have a fine vibration when descending a hill on compression. The audible portion of the "experience" takes some getting used to and adds to the perception.
 
Thank you all for your replies, I will go for a ride in another car and compare mine, I am due for a full engine overhaul soon and will look at the u joint then.
 
I agree with Jim and Marco about taking an experienced Model A guy for a ride.

Engine vibration is normally most severe when backing down for the simple reason that there is minimal fuel/air charge present on the compression stroke to cushion the piston's reversal; it all happens as a sudden jerk on the rod bearings. If ever your mirror is going to vibrate, it's then.

The exhaust gases provide some cushioning on the exhaust stroke, even though the exhaust valve is open. Piston reversals following downward strokes aren't such shockers because of the big circle the lower rod bearings travel in.

Otherwise--if you're out of the normal zone--sounds like a U-joint problem.

Steve
 
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