Low Speed Vibration?
Thread Starter
Drifting
Joined: Oct 2006
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From: Beaverton Oregon
Low RPM Vibration?
My '86 runs just fine. It has the very responsive Doug Nash 4+3, which I really like (hey, it's a 7-speed car). I've noticed that the car seems to shake on the freeway at 60+ mph when in OD 4th. If I 'downshift' into just 4th, shaking goes away. What would cause this shaking/vibration at 1500-1700 rpm?
Last edited by All2kool; Dec 6, 2006 at 11:21 AM.
Melting Slicks
Joined: Aug 2003
Posts: 3,393
Likes: 392
From: Lakeville MI
Any idea of the frequency of vibration?
I'm thinking maybe your ecm is detecting some knocks under the low rpm and moderate load conditions while in 4th-od at these speeds.
Perhaps knock retard is removing and adding back spark advance at an objectionable frequency that you can feel.
Other thoughts: Clutch or flywheel imbalance, but I would expect this to be worse at higher rpm. Imbalance within the od itself?
Does it also improve in 3rd-od at the same or proportionately lower speeds like say 46 mph?
What octane fuel? Iron or aluminum heads?
I'm thinking maybe your ecm is detecting some knocks under the low rpm and moderate load conditions while in 4th-od at these speeds.
Perhaps knock retard is removing and adding back spark advance at an objectionable frequency that you can feel.
Other thoughts: Clutch or flywheel imbalance, but I would expect this to be worse at higher rpm. Imbalance within the od itself?
Does it also improve in 3rd-od at the same or proportionately lower speeds like say 46 mph?
What octane fuel? Iron or aluminum heads?
Thread Starter
Drifting
Joined: Oct 2006
Posts: 1,694
Likes: 0
From: Beaverton Oregon
Originally Posted by tequilaboy
Any idea of the frequency of vibration?
I'm thinking maybe your ecm is detecting some knocks under the low rpm and moderate load conditions while in 4th-od at these speeds.
Perhaps knock retard is removing and adding back spark advance at an objectionable frequency that you can feel.
Other thoughts: Clutch or flywheel imbalance, but I would expect this to be worse at higher rpm. Imbalance within the od itself?
Does it also improve in 3rd-od at the same or proportionately lower speeds like say 46 mph?
What octane fuel? Iron or aluminum heads?
I'm thinking maybe your ecm is detecting some knocks under the low rpm and moderate load conditions while in 4th-od at these speeds.
Perhaps knock retard is removing and adding back spark advance at an objectionable frequency that you can feel.
Other thoughts: Clutch or flywheel imbalance, but I would expect this to be worse at higher rpm. Imbalance within the od itself?
Does it also improve in 3rd-od at the same or proportionately lower speeds like say 46 mph?
What octane fuel? Iron or aluminum heads?
Melting Slicks
Joined: Aug 2003
Posts: 3,393
Likes: 392
From: Lakeville MI
The 86 iron head cars run around 32-36 degrees advance for the rpm and load conditions you have described.
I wouldn't expect much knock on 92 octane.
Edited and updated info:
The 86 aluminum head cars run around 41-44 degrees advance for the same rpm and load and could be more likely to knock.
I wouldn't expect much knock on 92 octane.
Edited and updated info:
The 86 aluminum head cars run around 41-44 degrees advance for the same rpm and load and could be more likely to knock.
Last edited by tequilaboy; Dec 6, 2006 at 01:52 PM.
Thread Starter
Drifting
Joined: Oct 2006
Posts: 1,694
Likes: 0
From: Beaverton Oregon
So I replaced the Spark Plug Wires today with some Borg Warner 8mm's. The 'vibration' in 4th OD has disappeared and the car runs like new. As I was replacing the wires I did notice the Distributor was quite loose and could easily be moved turned in either direction. Amazing what new wires and a tight Distributor will do for a car. Car is much more responsive, idles great, sounds better, etc. BTW - the Borg Warner wires were on sale at Schuck's for $19.95.
