Replacing the UPPER motor mount in an LS4 with a polyurethane upper motor mount

Special Thanks!

Special Thanks to 06montess for his help with this project, both in the actual car work and the reviewing and editing of content. Thanks, man!

Disclaimer

Just so you know, I'm not responsible for any inaccurate information, bad advice, or stupid puns which may or may not be present in this document. I'm not responsible for anything that may happen to your car as a result of doing anything, especially anything which may be described here. I'm responsible for mowing my lawn and that's about it.

Summary

General Motors designed the upper motor mount on a GM LS4 engine to have a good amount of play in it; the laudable goal of this was to reduce the Noise / Vibration / Harshness (NVH) associated with driving these W-body cars (2006+ Chevrolet Impala SS, Chevrolet Monte Carlo SS, Pontiac Grand Prix GXP, Buick Lacrosse Super). They wanted to give the passengers a nice, smooth ride without feeling every gear shift, hearing (and feeling) the vibration of the motor resonating throughout the chassis, etc.. This is accomplished by designing motor mounts that absorb some of the NVH through the flexibility of the motor mounts; that is, the rubber motor mounts are soft enough to absorb the shocks without transmitting those vibrations through the chassis.

So, when you stomp on the gas, when the transmission shifts, etc.. that is, whenever the engine moves around a little, some of the energy from that movement is absorbed by the rubber motor mounts. The flexibility of the motor mount also means that the engine is allowed to shift around a little during transitions between states (again, acceleration, shifting gears, etc..). This can be hard on certain parts of the car, including the exhaust, not to mention the fact that you are absorbing energy produced by the engine with the rubber motor mounts. As a performance enthusiast, I want as much of that energy delivered to the wheels as I can get.

Polyurethane motor mounts are substantially harder and less flexible than the OEM rubber motor mounts. As a result, they transmit more vibrations through the chassis of the car. They still have some flexibility, but they're substantially less flexible than the original rubber motor mounts. Also, because they're made of polyurethane, they won't decompose as rubber will.

So, I've replaced my OEM rubber motor mounts with new polyurethane motor mounts. You can find poly motor mounts for sale on:

They're generally sold for 2005 and older Impalas (with the 3.8L engine); however, those cars have two upper motor mounts while the 2006+ W-bodies only have one upper motor mount. So, if you purchase a package, you'll generally get two of which you can transform the unused one into a paper-weight, pencil-holder, a self-defense weapon.. or, you can give / sell it to a buddy. I recommend selling on to a friend with a similar car although the idea of bouncing one off of the forehead of the next guy who argues with me that my 2006 Impala SS is a rear-wheel drive car is quite amusing to me.

This project took about 30 minutes to complete with a basic set of tools and a LOT of WD-40.

What you need

You'll need several tools:

  • slotted / flat-head screwdriver
  • vice
  • 2x 15mm wrench
  • 1x 13mm wrench

You can get by with open-ended or close-ended wrenches or go with sockets and a ratchet in which case you'll probably want to use a deep-socket 15mm.

You will need a lot of WD-40 and it's partner in crime, paper towels. You will be making a mess.

Instructions

Remove the bolts securing the dogbone to the block and the radiator bracket

I started by using my 15mm wrenches to remove the bolt closest to the engine although you could just as well start with the bolt closest to the radiator. The bolts were stuck a little, so I had to hose them down with WD-40 and let that penetrate for a few minutes to loosen up a little.

First bolt to remove

Remove the remaining bolt securing the dogbone to the engine block

Then, remove the other bolt. Again, you may wish to use some WD-40 to loosen seized bolts.

Second bolt to remove

Remove the dogbone

Here is the situation with the dogbone removed.

Dogbone removed

Remove radiator bracket

Now that the dogbone is pulled out, unbolt the bracket that attached the dogbone to the radiator. This bracket contains the rubber motor mount that we'll be replacing. The two larger bolts are 15mm and the smaller, middle bolt is 13mm.

Unbolted radiator bracket
Side view of original motor mount in bracket

Remove the rubber motor mount

This is the fun part. You have two options on how to proceed from here:

  • use screwdrivers to pry the rubber motor mount out without destroying it
  • use a saw to cut the rubber motor mount, destroying it in the process

If you cut the rubber motor mount, obviously, you'll be destroying it. Generally, I like destroying things, but I wanted to be able to back out if I had to, so I opted to remove the rubber motor mount without destroying it.

I put my bracket into my vice, hosed the entire thing down in WD-40, and used the screwdrivers to pry it out. This is surprisingly difficult to do and it's messy. It wasn't -hard- but it was more difficult that it initially appeared. Using a hammer didn't work well for me, but I've heard that some have had luck with that approach. I managed to wedge one screwdriver in between the bracket and the rubber, pry it a little, wedge the other screwdriver in and pry it a little further and just keep working it until you get a corner out. Once you've got a corner out, it's all over but the crying.

Original rubber motor mount has been removed
The two mounts next to each-other

Insert the new polyurethane motor mount

This part took longer than I thought as well and it turns out that the two aforementioned manufacturers have slightly different pieces. The 3800performance piece comes with two endcaps while the intense-racing one is a single, solid piece. I purchased that intense-racing piece, so I had the additional work of cramming the motor mount in. Others may have a slightly different experience if they go with the 3800performance piece.

With the intense-racing mount, the only thing holding the motor mount in is friction and some shaping to match the bracket; since the forces subjected to the motor mount are parallel to the bracket (as opposed to perpendicular--that is, pushing / pulling on the sides of the motor mount), they need to have a good, tight fit.

With the 3800performance mount, the endcaps prevent the mount from sliding out, so their mount is a little easier to slide in.

Anyways, I worked in reverse of the rubber motor mount extraction--I got one corner in, then used screwdrivers to encourage the polyurethane motor mount edges to slide inside of the bracket. I then reoriented the bracket with respect to the vice so that the clamping pressure of the vice pushed the poly motor mount into the bracket. With enough force and lubrication, you'll be able to make it fit.

The motor mounts, again

Reinstall bracket and dogbone

Finally, do the opposite of what you did to remove the bracket and the dogbone. I found that reattaching the bracket and the dogbone to the bracket were both very easy. I had to put some pressure on the engine block in order to get the bolt attaching the dogbone to the block to slide in. Make sure you only push on the block, not on anything that's not the block. Don't push on anything made of plastic or any wires or the throttle body or anything else like that. It turns out that the engine doesn't run very well when there's air leaking through a crack or somesuch.

Make sure you put an appropriate amount of force into tightening the bolts--you don't want the engine wiggling loose after a few heat cycles coupled with crazy vibration. Make sure you do so safely though--you don't want to slip and chip a tooth or strip your threads.

The poly mount has been installed

Go gentle on it until you're sure everything is seated properly and securely. You'll feel your shifts more and there's less of a lag when you change engine state (accelerate, for example). I've heard some refer to this as being "more responsive," but I personally tend to reserve that term to the responsiveness of the engine to commands rather than the car's responsiveness to what the powertrain components are doing.

I hope this helps! Please direct comments to Wes -at- LS4Store -dot- com.

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