• Welcome to ClassicBroncos! - You are currently viewing the forums as a GUEST. To take advantage of all the site features, please take a moment to register. It's fast, simple and absolutely free. So please join our community today!
    If you have problems registering or can't log into your account, please contact Admin.

pulsating engine vibration

m_m70

Contributor
Sr. Member
Joined
Jun 14, 2001
Messages
1,459
Loc.
Pacifica, CA
getting a weird engine vibration at around 1800 rpms. If I hold it at that rpm it pulses in and out. It is strong enough to rattle the mirrors. them pulsating comes and goes around every 2 seconds. it smoothes ou at a higher rpm and lower as well. replaced balancer, carb, wires, cap and rotor.running pertronix module. any thoughts? I was thinking about replacing the dizzy just to see if that makes a difference.
 

Lmfp

Bronco Guru
Joined
Jun 28, 2011
Messages
1,496
Loc.
Lake Charles, LA
Warped flexplate? Have no clue if that would do it.. It couldve lost a weight or sumtin. DeffinAtely not something i would mess with till every other imaginable thing js ruled out.
 

hucklburry rev2

Sr. Member
Joined
Jan 17, 2006
Messages
759
I had a fan clutch go bad and had rpm specific vibration. You could pull the fan and see if it goes away. You could pull belts and see if it goes away, don't run it long with no water pump though.
 

Broncobowsher

Total hack
Joined
Jun 4, 2002
Messages
34,834
I have a few idea...
First off take the belts off the front and confirm it isn't an accessory drive. Because that will be about the only easy fix, and I doubt that is it.

Automatic? I think you may have a torque convertor coming apart.
If you are getting that much shake there is something out of balance. But 2 second phasing, you have 2 things messed up and it takes 2 seconds for the balances to line up and go 180 out from each other. That sounds about like the speed difference you would have in a torque convertor between the turbine and impeller without a load.

There is an art to the science of vibrations. It can be hard to track down and can be the wierdist of things.
 
OP
OP
m_m70

m_m70

Contributor
Sr. Member
Joined
Jun 14, 2001
Messages
1,459
Loc.
Pacifica, CA
some more info.........
3 speed manual trans....no torque converter. Has the same vibration with clutch in and out. Spark plugs all look good. Motor has 15000 on rebuild. I will remove the belts and see if that helps. no fan clutch but will remove the fan to see if that does anything. Thanks for the responses. Hopefully its not something major:cool:
 

Broncobowsher

Total hack
Joined
Jun 4, 2002
Messages
34,834
Is this something new? did this just start, if so what was recently worked on when this started? Did it gradually start happening or was it suddenly there one day?
 

Lmfp

Bronco Guru
Joined
Jun 28, 2011
Messages
1,496
Loc.
Lake Charles, LA
Ya know what my old truck did that. Pull the belt off of the water pump. Mine truck did the same thing and the pulley ended up blowing of the water pump and really jacked stuff up.
 
OP
OP
m_m70

m_m70

Contributor
Sr. Member
Joined
Jun 14, 2001
Messages
1,459
Loc.
Pacifica, CA
I always have had the vibration. At first I thought it was in the drive train so replaced the drive shaft, u-joints, shimmed the leaf springs but that didn't help. Got some good advice from this forum (as always) to check the motor by bringing up to the same rpm while in neutral and found it happening then.
I only put about 2000 miles per year on this truck so it gets put on the back burner of projects but I am off work for a couple weeks and jonesin for some quality time with the truck!
 

73azbronco

Contributor
Bronco Guru
Joined
Nov 11, 2007
Messages
7,796
did it do it since the rebuild? Maybe put the wrong flywheel on it or didn't do a lik of balancing the rotating assembly.
 
OP
OP
m_m70

m_m70

Contributor
Sr. Member
Joined
Jun 14, 2001
Messages
1,459
Loc.
Pacifica, CA
motor was blown when I bought the truck. dumped a Kragen engine in it. new motor mounts, clutch etc..... Just out scrrewin around with it and it seems that the crank pully isn't turnin that true%). I would think that if the flywheel and balancer weren't right it would vibrate all the time?? I think I will pull the pullies and see if that helps out. I know not to run it too long.
The truck runs really well it just bugs me at this rpm. Somthin aint right.
Thanks

Mike
 

hucklburry rev2

Sr. Member
Joined
Jan 17, 2006
Messages
759
did you use new flywheel bolts and torq properly? I had an issue with that once, but it was a constant vibration.
 

Timmy390

Bronco Guru
Joined
Jan 1, 2011
Messages
5,609
Loc.
Conway, AR
Had a pulsing vibration on my 69 Mach 1 and I changed everything or so I thought. Turned out to be the trans monut was shot. The rubber was 40 plus years old and mostly all gone. $10 part and it was fixed.

Tim
 

RG75Bronco

Jr. Member
Joined
May 23, 2010
Messages
139
Loc.
Jacksonville
Back ground: Purchased a 75 Bronco that had rotten tires and the 302 and C-4 was worn out. Never drove the truck over 35 mph when I decided to take it down for restoration. I have a similar issue, but mine does not start until 51-52 mph. Up to that point it is smooth as glass. It shakes so violently that you have to slow down. New drive shaft, engine, harmonic balancer, flex plate, torque converter, motor mounts, pulleys, C , radius arm bushings, and trans/t-case mounts. The ball joints are good. Replaced a rear axle that was wobbling thinking that would be the fix and it did not. Nope! C-4 rebuilt that did not fix the problem. Checked the drive shaft in another bronco and it was smooth. Pulled the drive shaft ran the engine up to 75 mph and no vibration. So under a load from the rear end it has a vibration. The t-case works fine and the rear end has the typical wear. Another bronco friend who has owned 3 over the past 30 years is at a loss as I am in determining the root of the problem. It is getting frustrating. So do not feel alone m_m70.
 

Lmfp

Bronco Guru
Joined
Jun 28, 2011
Messages
1,496
Loc.
Lake Charles, LA
Back ground: Purchased a 75 Bronco that had rotten tires and the 302 and C-4 was worn out. Never drove the truck over 35 mph when I decided to take it down for restoration. I have a similar issue, but mine does not start until 51-52 mph. Up to that point it is smooth as glass. It shakes so violently that you have to slow down. New drive shaft, engine, harmonic balancer, flex plate, torque converter, motor mounts, pulleys, C , radius arm bushings, and trans/t-case mounts. The ball joints are good. Replaced a rear axle that was wobbling thinking that would be the fix and it did not. Nope! C-4 rebuilt that did not fix the problem. Checked the drive shaft in another bronco and it was smooth. Pulled the drive shaft ran the engine up to 75 mph and no vibration. So under a load from the rear end it has a vibration. The t-case works fine and the rear end has the typical wear. Another bronco friend who has owned 3 over the past 30 years is at a loss as I am in determining the root of the problem. It is getting frustrating. So do not feel alone m_m70.
Thats a different issue than the original post. Yours sounds like deffinate driveline issue since it only does it while moving. Is your rear axle housing bent?
 

Lmfp

Bronco Guru
Joined
Jun 28, 2011
Messages
1,496
Loc.
Lake Charles, LA
Srry if that sounded smart a$$ it wasnt ment that way. But yeah these are deffinately annoying issues to track down.
 
Top