A couple of weeks ago I installed the aam spacer, definitely noticed the power increase but during daily driving I have noticed that my gas mileage has decreased significantly, this is without getting on the throttle (non spirited driving).
Anybody ever experience this with the plenum spacers?

 

Reset your ECU

 

Is this procedure necessary after a mod install or will the ecu reset itself after drivetime??

 

I disconnected the battery, then after installing the spacer I turned on the ignition and held the brake pedal down for 30 seconds.
Then reconnected the battery. That should be sufficient for clearing the ECU.
It should 'learn' fairly quickly. I have not noticed any change in mpg except when I like to put my foot in it.. which is more now!
Make sure you don't disconnect the throttle body connection or move the plate. If you do, you need to run the procedures for calibrating those. Should be in your instructions.