Not So Good Vibrations

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MOPARJ

MOPARJ

What can I upgrade now?
Joined
Nov 3, 2006
Messages
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Location
Thousand Oaks, Ca
Currently experiencing a vibration at speed, but more closely correlated with engine speed. The car is running a 318/904 with built up 2700 stall TC and 3.91 gears. Between 2900-3300 rpm, or about 57-63 mph, the vibration is most notable. If I pop it in neutral or park and Rev it to the same point, the vibration/harmonics are still there. Any feedback on places to look? Motor runs excellent with great power, pressure, and oil consumption otherwise. 46k since rebuild. Trans also shifting fine.
 
Did you check your motor mounts? I had an old truck do something similiar. Tore it apart trying to figure out what it could be until I finally to it to a friend. An hour later and I was back in action.
 
Check your trans mount as well. If broken or using anything else but a rubber Mount, this can be the culprit.
 
What have you done?

"Eliminate."

Run the engine before it warms up with all the belts disconnected

If this is an engine / converter problem, it should vibrate "just as well" in your driveway as moving, no need to drive the car

Is this "something new" with an existing build, or is this a new engine / trans combo?

As above, don't discount motor mount problems, or headers / exhaust touching something
 
First, does it doo it sitting parked at that RPM?
I'd want to find that out first thing.

If so make sure the flexplate is ok.
 
In response to above questions, yes it will do it in park or neutral between 2700-3300 rpm. No this is not a new combination: same motor since rebuild 46k ago and same trans and converter for the last 7k miles. Has started in the last 1500 miles, which has been over the last 7 months.

The motor mounts we're not replaced at the last rebuild 6 years ago. I do have energy suspension motor mount inserts on my shelf in the garage that I'm going to swap in just as soon as time allows.
 
Like I said, remove belts and check. Now that we know for sure it's engine RPM, time to check out flexplate, converter problems OR LOOSE TRANSMISSION
 
Looked at motor mounts. Looks like right side is shot; noticed a small bolt wedged in under the mount itself to hold the motor up somewhat as it should. the insert is shot. I have never noticed this before and when the motor was dropped in upon last rebuild (46,000 mile ago), someone must of mickey moused that in. Im sure this put extra abuse on the left mount and its probably shot now as well. Luckily, I have new motor mount inserts. Could be the source of the vibration; we shall see.
 
Had a similar moderate vibration on a /6 rebuild recently; vibration was the same running or sitting still..... replaced the pressure plate and clutch disc and it's smooth now. So if the MM's are not it, then look at the trans/TC area.
 
With this vibration (light to moderate) most notable at 2700-3100 rpm, does any one see an issue driving it for a few days to work? Hasn't changed in a 1000 miles and my other two cars are down at the moment suddenly and loaned my daily out to someone with a more major Running issue (I know great timing....). ATF looks clean and unburnt and oil pressure is great. If it's something with the flexplate or converter, is there any risk of a dangerous explosion driving on the street if something comes apart (when I think explosion I mean like a 727 that let's go at the dragstrip)? I'll try to check the flexplate bolts first tomorrow before driving. Won't have time to check more components until Sunday.
 
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