Engine vibration at high rpm

[FlakeyJake]

Expert Jeepers,

I just got a rebuilt 360 in the 88 GW from Western Engine Supply in Denver ( thanks for the recommendation ). I guess I'm satisfied, it seems to be OK. However, shortly ( a couple of weeks ) after I got it back, it developed a ( what seems ) serious vibration as RPM increases. It starts when on the road at about 60mph and at about 70mph it resonates enough to give you a headache. I don't have a tach so I don't know the exact RPM but the speeds illustrate the rpm range. It is not powertrain related as it occurs in park or neutral as well.

At the time of engine rebuild, they replaced several other "items" that were supposedly in need of replacing. The harmonic balancer was on this list. I'm not sure what the harmmonic balancer does, but it sounds to me like it may be responsible for managing engine vibrations... Could it be that this has failed?

Does anyone have any idea what this might be?

I'm planning on doing a TFI-type upgrade when I get around to it to eliminate the possibility of it being a broken wire causing misfire or something, but it just seems that the vibration is worse than that.


Any experience & expertise welcomed....

[jeepbob]

I would suspect either the wrong flexplate(most likely) or a broken flexplate. The flexplate is engine size specific on AMC motors as is the balancer if either is not correct you will have a virbation.

[FlakeyJake]

OK. Pardon my ignorance. But, what and where is a "flexplate"?

[Snakeyes_Tx]

Flexpate is commonly mistakenly called "Flywheel" Automatics do not have flywheels... they have flexplates! IT's what is bolted to the back of the crank and the starter uses to turn the motor.

I suspect you have a bent driveshaft, or a bad U-joint, or a failing wheel bearing. Start small and cheap!

[FlakeyJake]

I don't believe it can't be a drivetrain issue as the vibration is there when in park or neutral.

I assume, then, that the flexplate would have been replaced with the longblock they put in.

And, then, if this is the issue, would require another engine pull to resolve. Am I correct in that assumption?

[Michael]

Man that would worry me. In my experiece you balance the bottom end with the balacer,crank,rods/pistons,and the flex plate. If one of them is missing chances are you would have the vibration you are talking about at higher rpms. Also check you engine mounts. Does the motor shake or move a lot when you rev it up???

[Sgt. Dave]

Take a close look at the harmonic balancer. Is it really new, or did they "forget" and put an old one back on? Just to isolate the engine from any accessory drive possibilities, pull all the drive belts off, and do a quick run up and see if the vibration is there.

BTW, when you are doing those neutral or park run-ups, stay clear of the fan. They come apart sometimes!

[reddog]

Maybe the harmonic balancer is loose?? It is the pully looking thing on the front of the motor attached to the crankshaft. It has a pully attached to it that runs the accesories.

Kerry

[Snakeyes_Tx]

<BLOCKQUOTE>quote:</font><HR> I assume, then, that the flexplate would have been replaced with the longblock they put in. <HR></BLOCKQUOTE>

Nope... Flexplates aren't included in rebuilds.. those are one of those things that they assume you should know to change. When I rebuilt my motor, I replaced the flexplate, all the rocker bridge bolts, and replaced all the "lifetime warranty" accessories.

[FlakeyJake]

Just an update on this weird vibration. I finally got the "TFI" upgrade done with the MSD Super Conductor wires and the MSD Blaster-2 coil and the MSD cap/rotor. I opened up the spark gap to .045 and finally last night installed the MSD 6A ignition. All this in an attempt to eliminate the possibility of a miss causing the vibration.

Now it seems to start easier, idle more smoothly, and I do not feel that annoying vibration so much now when in park and the motor juiced up :D ( still don't have a tach ), but it is still there ( albeit not as bad ) in drive at 70mph.

Now I'm thinking that something is out of align or loose or something. Several of the posts above discussed the flexplate as a possible culprit, do you (collective) think this could be the problem? It's not a horrible earth shaking vibration now, now its more of a droning which comes and goes at at maybe 1hz at 70mph.
:confused:

[J20 project]

Flexplate or driveshaft, Take your pick and work on them. If you take your shaft(s) off and take them to a competent shop they can evaluate them and balance and or replace bad parts as needed. If you need the flexplate then it's easiest to jus tpull the tranny and replace it. About $50.
My .02. J20

[redneckd out]

is it possible to have a warped flexplate or loose flexplate????? or what about the torque converter??? are both of the differentials angled towards the t-case?????do you have a lift???
just some questions
blake

[2j20's]

am gald this came up because am having the same vibration 60-70 but i have a t18 in my 76 20 i have had front and back driveshafts done frontend align with all new front end parts new tires and am about to go crazy.i got the jeep just a year ago and it had a new motor in it already and it started this about 3 mouths ago all that is left is the hamonic.it looks to be in very good shape.david

[Mick]

Just went thru this with my 85 cherokee,,,, vibrated bad at highway speed,,, we checked everything we could find,,, finally the flexplate broke $300 to fix. New flex plate, no vibrations. I was told the other was in the process of cracking and probably had cracks that would have shown up if it had been pulled and inspected.

[Dude]

The AMC 360 is externally balanced. Meaning that the stroke of the crank is too long to correctly balance the assy w/ just the material on the crank webs. As such, the dampner and flexplate are both used to balance the engine - thus, being called externally balanced. Quality replacement dampners/flexplates should be matched close enough to original parts to avoid a problem. However, other dampners/flexs will fit. The guy that just balanced my engine gave me a detailed explanation of the process. He warned that should the dampner/flex be replaced, you should statically match their balance with the old piece. Does the problem go away after 75mph? May wanna just drive faster.

[2j20's]

NO the 75 mph dose not help!!! i did that.so now i have replaced the motor and trans mounts and its still there. so can you buy a new harmonic damper and the flywheel and get them balance to what the old ones are ? i know they are balance with the crack in a rebuild will this work,also while am at it what about the pressure plate ? has anyone use the centerforce or sould i stay a factory type.David