Vibration in drive 900 rpm

dla442

Original poster
Member
Mar 31, 2012
249
grand rapids, mi
When the trailblazer is in drive , foot on brake and you throttle to 900 RPM there is a very harsh vibration noise coming from somewhere in the center of the vehicle up front. Could this be motor mounts or a transmission mount? I had just replaced the whole front end on the vehicle today. I don't suspect the control arms or tie rods. It does not do it in reverse with the foot on the brake. When at a stop sign and you hit the accelerator that vibration is present just for a brief moment as it passes that 900 RPM.
 

Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
Usually a bad motor mount vibrates at idle in drive. It might be possible. Maybe try stopped with the foot on the brake and bring it up to that RPM. Maybe try to watch the engine while doing this. Could be something in the exhaust hitting something.
 

mrrsm

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Supporting Donor
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Oct 22, 2015
7,639
Tampa Bay Area
A Failed Transmission Mount can also present with similar symptoms. Not sure if you can just "Jack Up" the 4L60E with its Cross-Member Design to see if the Rubber portion of the Mount has separated from the Metal Portion. Others may have to chime in if they agree that looking here is worth this effort.
 

dla442

Original poster
Member
Mar 31, 2012
249
grand rapids, mi
Usually a bad motor mount vibrates at idle in drive. It might be possible. Maybe try stopped with the foot on the brake and bring it up to that RPM. Maybe try to watch the engine while doing this. Could be something in the exhaust hitting something.
Tks. With foot on brake in drive 900 rpm is the loudest. 1k rpm goes away. Ill peek at mounts
 

dla442

Original poster
Member
Mar 31, 2012
249
grand rapids, mi
A Failed Transmission Mount can also present with similar symptoms. Not sure if you can just "Jack Up" the 4L60E with its Cross-Member Design to see if the Rubber portion of the Mount has separated from the Metal Portion. Others may have to chime in if they agree that looking here is worth this effort.
I'll take a peek at it today thank you
 
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dla442

Original poster
Member
Mar 31, 2012
249
grand rapids, mi
hard to tell if mount is failing. when car warms up and driven for a bit the noise is gone. sure makes me think the mount warms up and expands a bit????
 

gmcman

Member
Dec 12, 2011
4,656
A few thoughts:

When cold, remove tension from the serpentine belt and see if the fan is loose/wobbly.

Trans fluid level good? Likely not the issue.

Harmonic balancer looks good when rotating, any noticeable wobble?
 

dla442

Original poster
Member
Mar 31, 2012
249
grand rapids, mi
Replaced driver side mount after an extensive investigation...lol. it looked ok but slightly cracked. Replaced the mount and significantly quieter and engine doesnt move so much when put into drive. I think ill do the passenger side next just to be clear. The vibration is gone now though. Diagnosis, bad driver side mount.
 

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