Insulating the Engine Cover on a Hummer H1 Diesel
Updated April 12, 2012
I had an opportunity to take the engine cover off my 1998 TD wagon to install the rear head cooling kit . While running the engine with the cover off to test the system for leaks it became VERY apparent the most of the noise coming from the truck is through the engine hole in the firewall. I had a bunch of Dynamat that a friend had given me a couple of years ago so I decided to experiment. I removed the doghouse and engine cover and went to work. Below shows exactly what I did. While I don't have a spl meter (sound pressure level) I firmly believe that the truck is much quieter.
Dynamat makes some products that I look like they will do a better job of silencing the diesel noise. In particular they make a product that has lead sheet in it called extreme liner.
I cleaned the engine cover very well with degreaser and then wiped the aluminum face with solvent. I drilled out all the rivets that held the large fender washers in place. I sprayed the surface with some adhesive just to make sure that the dynamat would stick. The Dynamat was cut and formed into the contours.
The job took about 2 boxes of 18" x 32" mat.
I kept away from the flat areas of the cover because I wanted to make sure that the push and turn fasteners would work. If the area around the fastener is too thick It would cause problems getting the engine cover back on. I re-drilled the holes through the Dynamat and replaced the washers using 10-24 screws with nyloc nuts.
The next step was to install the mat around the cowling in the hole in the firewall. I cleaned all the dirt and grease from the surfaces and applied the mat.
This shows the top surface on the left side of the truck over the fuel pump. Notice the fender washers and screws. I didn't want the mat to fall off once it got hot. Carefully drill pilot holes for the screws in the aluminum panel that lines the cowling. There maybe wires running up there that you don't want to damage. 
This shows the right side (passenger) of the engine opening. I took the mat and ran it behind the existing foil/ fiberglass insulation. There were already 2 big washers holding this to the body. I removed them and reinstalled them on top of the new insulation.
