Drivetrain Vibration

[Joe K. 99 Zeee!!]

I've been trying to track this down. Maybe some of you can see if I'm on the right track.

I had a new LS7 clutch/pressure plate and new slave cylinder installed in November. I've been running a poly trans mount for about a year now. Where I drive I usually don't get up to 65mph, but over the holidays I noticed a significant amount of vibration. I did some testing and it looks mostly speed dependent. It maxes out at about 67mph in 6th gear at around 1,500 RPM.

I took the car out on the highway today to try to eliminate some possibilities. I accelerated up to 85mph in 5th and 6th. I checked 1st through 5th through 1,500 RPM at different speeds. The vibration seems has a moderate frequency...I'm guessing at least 25 Hz.

I had my tires balanced last week. That didn't help, so I went back to have the shop double check them. The vibration remains. I've had the poly mount on a while, so I'm used to what that feels like. This vibration is similar, but seems more pronounced.

So, here's what I'm thinking. It's not the clutch/pressure plate as it occurs only at a particular speed and not rpm. Maybe the shop had to disconnect the driveshaft to get at the slave cylinder and didn't install it in the same position as it was removed. Or, maybe the pinion angle is off.

What do y'all think?

[Joe 1320]

So, here's what I'm thinking. It's not the clutch/pressure plate as it occurs only at a particular speed and not rpm. Maybe the shop had to disconnect the driveshaft to get at the slave cylinder and didn't install it in the same position as it was removed. Or, maybe the pinion angle is off.

What do y'all think?

Could be either. I read somewhere that the poly trans mount is just a touch taller than stock, there may be something to that. Since they also disconnect the driveshaft to do the clutch, there may be an issue with that as well. Just for curiosity, is there any difference with a full tank of gas vs. near empty? If there is, I would suspect pinion angle.

[Joe K. 99 Zeee!!]

The transmission mount is from Energy Suspension. They're supposed to be a little less harsh than Prothane. Also, I didn't use the preload plate and ground down the two bumps on the top so it was closer to the height of the stock mount. It never gave significant vibration till I got up to around 75mph.

I hadn't thought about checking a full tank vs. empty. I'll give that a shot. I usually run at at least 1/2 tank.

Thanks Joe!

['STANGETR]

Just my 2 cents....I never had any vibration on the interstate until I had my new tranny put in with the tougher polyurethane mount. I could be cruisin' down the interstate at 70-75mph, and if I talked (or sang to a song!!) it would sound like I'm stuttering That would be my guess for the vibration.

[97Bandit]

Harmonic balancer?

I dont know if this is ur problem, but on my lt1 (i have a 97 Trans am with a 6 speed and) i had a wierd shake in my shifter handle and it would vibrate when driveing at higher speeds -really noticable at over 100 when backin out of it- and all rpms when floored first i thought it was a miss of some sorta. I too wondered if it was my poly mount. well i was stumped and pissed so one day in shop class i was just replacing my very worn out belt and i notice a sound that was similar to a powersteering pump struggling or out of fluid after the new belt was on...but the powersteering worked fine. I crawled back under it and check things over. I pulled on my balancer and the dang thing would move cause the rubber in the middle of the ring was mostly gone. With the belt off the outside would almost slide off where the bolt up was only there... it was bad... but i have 135,000 miles and it was original so yeah.. anyway just sharing some info for weird problems and now my poly mount and everything works fine its as smooth as a cadillac, lol