Engine Vibration at highway rpm/speed

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cli55er

cli55er

Tem Greene's Nephew
Joined
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Guys, I got a real head scratcher here.

1970 Dodge dart with 71 340 block and internals. The car vibrates badly at highway speeds in a pulsating vibration. vrrruummp........vrrummmp....vrruummmmpp. like that. it's done this ever since I was a kid and my dad owned the car.

here is what I have done to try to fix this problem.

1. found out the motor was a 71 external balance motor, but it had the wrong harmonic balancer and flexplate on it. so I change to the correct external balance harmonic balancer and B&M flexplate with the notch cut in it. this did not solve the problem 100%......so I moved forward with number two below.

2. I had the transmission rebuilt by a very very reputable shop and a new "zero" or neutral balance torque converter with 3000 stall made for it. At the same time I also had a new driveshaft made from yoke back....new u joints, new yoke...and computer balanced. Also at the same time I had the third member rebuilt and went from a 3.23 to a 3.73....new bearings, new gears, new pinion, new pinion bearing, etc....same yoke though.....it was a good yoke....7260 u joints.

none of that has solved my vibration problem. not a dang thing I've done above. the motor is timed right...no misses. it runs pretty good around town. there is no vibration at low speed and around town....but when I get on the highway...3500-5500 rpm....it vibrates like crazy.

wth am I missing here? the motor has not been touched when my dad bought the car in 1989, it always ran fine. it is stock 340 only 71 block with cast crank (no idea what happened to the original) it has a crane purple shaft cam and that is it. nothing fancy. stock 340 headers.

could I have a slightly bent rod or maybe a piston that is way heavier then the others? I didn't think piston weight mattered too much on an external balance motor. I really appreciate any insight you guys may have.

for pictures...go here Henry's 70 Dart Swinger
 
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one thing you never mentioned is pinion angle . you pretty much covered most of it .
 
stock 8-3/4 rear end. 489 third member case with 7260 pinion yoke. stock pinion snubber. stock trans mount and engine mounts. not sure how I would check pinion angle nor how I would adjust it with stock setup.

the rear end was replaced by my father and uncle. it came from a valiant or something. When my dad bought the car it had the smaller rear end...7 something. so they found this 8-3/4 at a junk yard, pulled it from that car, and stuck it in the dart. I believe the vibration was there before this switch out and the build sheet on the car IIRC says 8-3/4 rearend.

I did put new leaf spring bushings in a few years back...nothing out of the norm there though.

how do I check and then adjust pinion angle?
 
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what area of the car do you feel the vibration from? This may sounds crazy but, do you have a bent rim, a bad tire or bent axle?
 
I never knew there was an external balanced 340 in 71 I thought that did not happen until 73 .
 
71 340's were not cast crank, but forged crank and not externally balanced. Are you sure you have the right balancer and flex plate?
 
I just did a google search...looks like shims are used in conjunction with the shock mount. so maybe the angle is wrong and my dad didn't really consider it when he changed out the rear end.

the vibration comes from center of car in trans tunnel area. shakes the whole dash and hood a bit too. its a pulsating vibration that I can vary with speed and pedal pressure.

I have new rims going on it from wheel vintique.... as I thought about rims being bad too and they actually are bad as the seats for the lug nuts are wobbled out a bit from years of torqueing and/or over-torqueing. the new rims are black steelies in 5x4 pattern with new tires. I pick them up from being mounted tomorrow.

the axles seem fine to me, but I only removed them to put in new seals...I did not replace the bearings as I don't have the special tools for that and honestly they looked to be in great shape. the vibrations don't really seem to come from the back of the car. it's more in the trans hump starting at the firewall and going to about under the front seat area. that is why I thought for sure rebuilding the trans, new torqure converter and new driveshaft would have fixed it. but nope.
 
i switched out to different rims a long time ago, one wheel a little water in it and had similar symptoms. the wheels must have gotten low air pressure, stored outside, and got water in one. bam! vibration...oh just read...new wheels...

My vote is converter, or balancer
 
okay so its a 72 or 73 block (edit see below)...my memory is fuzzy since 2014 when I checked all that out and started my blog. I did check the casting numbers on the crank...its def cast crank. it has casting lines in it. and I check the numbers on the side of the block...its not a 70 block....it had blue paint on it as well. (edit see below...it is indeed a 70 block and remembered incorrectly, my apologies) there are pictures of it in my blog. I def have the right balancer and flexplate now. it did not before. I have a balancer with the weight bolted to it and the b&m flexplate with the notch in it.

torque converter is brand spanking new neutral balance.
 
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see post 37 in this link....I was mistaken....it is indeed a 70 block...my memory is not what it used to be. its a 70 block that was at some point painted blue....and has a cast crank in it.

Henry's 70 Dart Swinger
 
clementine said:
i switched out to different rims a long time ago, one wheel a little water in it and had similar symptoms. the wheels must have gotten low air pressure, stored outside, and got water in one. bam! vibration...oh just read...new wheels...

My vote is converter, or balancer
Click to expand...

wouldn't you feel the vibration in the steering wheel too though? I do not have any side to side movement in the steering wheel.
 
i'm sorry guys...my brain is fried from this day and i'm trying to remember things from 2014 when I found out I had a cast crank in a 70 block and thought for sure at the time that changing to the correct flexplate and balancer would have fixed it. after all I've done now...i'm just throwing my hands up and about to give up. pinion angle sounds like a good place to start as was suggested. I appreciate that suggestion. I guess I will find out about the rims/tires tomorrow after I get the new ones mounted and take it for a spin. this car is just a driver and we have owned it since 1989...I was 8. i'm just trying to make it as good as I can get it so I can enjoy it...but the vibration is enough to drive me bonkers.
 
I take it you replaced the transmission mount with a new one and the bolts are nice and tight to to the transmission and the cross member when you rebuilt the transmission. Do you feel the vibration when the car is sitting still and you rev the motor up or just when the car is moving and the engine is at high rpms?
 
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Is it crazy to put *** up on jack stands, remove wheels, run wheel nuts down to hold drums on, and see if it can be replicated? maybe remove driveline so you are isolating things?
dunno....might get you some answers....might spray trans fluid all over......put a plug in it?
 
Smart phone?
Tremec has an app that you can use to check pinion angle.
A couple wedges may be all you need. Or shims under transmission mount.
Generally the oscillations can be narrowed down by where they occur,steering wheel, floor or seat. An out of balance engine would vibrate at the same rpm in neutral,if it doesent it could be driveline.
I recently experienced a vibration from brake shoes dragging,drum would warm up and go egg shape so to speak. So it was like double wheel speed. Turned out to be a bad brake cable.

Engine mounts correct and in proper location?
 
I think I remember 7 degree difference from pinion to frame for stick and 4 degrees for auto......been awhile though. magnetic chassis tool thingy is what I use.
 
It does not do it in neutral or Park when slowly revving motor throughout the rpm band.

It only does it when driving and only at highway speeds. 60-75mph.

Oscillation is def coming from the floor and trans tunnel. Trans and engine both have brand new mounts. It is def mounted tight, but I need to retorque since remounting the crossmember and driving 100 miles or so now.

Brake drums and shoes are brand new. Back right does rub a bit on the backing plate, but is wearing down now and the vibration was there when the old original bell shaped drums were on anyways.

Taking out driveshaft and running the trans/motor in drive at that rpm and speed may help eliminate a few things... but I agree I may spew trans fluid from rear seal on trans since the yoke would be missing. I guess I could take my old yoke from the old driveshaft off just to plug the trans. I guess that test would eliminate driveshaft rearward if the vibration was still there. If the vibration goes away that tells me pinion angle or something driveshaft back is the culprit and driveshaft is brand new. So my bet at that point would be pinion angle since the vibration is in the tunnel directly down the middle of the car. Also the third member is brand new as well. I’m suspecting that when my dad put the 8-3/4 rear axle in from the junk yard... they may not have thought about checking the pinion angle.

Thanks for the app suggestion. I’m going to research this pinion angle so I can check it out. New rims and tires will be on tomorrow so I can eliminate the originals as the problem as well.

Thanks for the suggestions guys! I really want to track this thing down and fix it. I feel like if it was in the motor that it would run badly and would do it when revving in neutral. So I guess crank and rods/pistons must be okay.

The pinion angle looks okay to me... but 4 degrees would be hard for me to see with my eyes.

Also.... magically lost my Speedo when driving it few days ago.. haven’t looked that over yet... but my guess is snapped speedo cable. I need to spin it with a drill to see if it still works. Surely it isn’t the speedo gear on the trans... that was replaced to compensate for the change to 3.73 gears.
 
Perry cheap. . and useful

20180403_215103.jpg
 
You mentioned you have a new drive shaft, did you get it made the same size as the one it replaced or did you measure it and get one made to suit? A half inch shorter than it should be will have the same symptom you describe.
 
gold66cuda said:
You mentioned you have a new drive shaft, did you get it made the same size as the one it replaced or did you measure it and get one made to suit? A half inch shorter than it should be will have the same symptom you describe.
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Driveshaft was made off original one. It was the original Chrysler driveshaft. The new one is the same minus spicer adapter ujoints and it does not tapper at each end. When I put the driveshaft in and slide it towards transmission I have about 1/2” to move it over and line it up with the pinion yoke. Then I slide it back toward the pinion and secure it. Seems normal to me. I did not measure.
 
yeah man, sounds like you need to isolate the problem. vibration coming from butt is one thing, steering wheel is another, various pedals will tell another tale. put her *** up on jacks and try to replicate problem. then start removing possible culprits.
 
High speed vibrations at 60 MPH and higher are usually caused by a bad tire, rim or a bad brake drum or rear disc rotor. Lower speed vibrations at 35 to 45 mph are usually driveline related and could be an out-of-balance drive shaft or improper u-joint alignment. If the vibration can be felt in the shifter, it is likely a front of the driveshaft balance problem or a bad u-joint. If you feel it in the back seat or in the floor at the rear, it could be a rear of the driveshaft balance problem or u-joint. However, a bad rear tire, drum or rotor (if you have them on the back) can cause the same vibration in the rear. Before pulling the suspected driveshaft, swap the front wheels for the back and back to front or borrow a known balanced set of wheels to test drive the car. If you haven’t balanced your wheels for a while, it wouldn’t hurt to do so to ensure they are indeed balanced.
 
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I wouldnt run it with a yoke in trans. That 5 lb yoke would be a dangerous thing if it comes out at 3000 rpms. Even running it on axle stands is a little sketchy.
 
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