You want to add fueling only when needed. And thats only at open loop, otherwise you mess up the fuel trims. You add fuel above 7PSI to get an AFR 12 and you'll be flying!
For fueling I suggest this:
First clamp the MAF signal at 4.6V
then add fuel with this
https://splitsec.com/product/aic1-g2...or-controller/
using something like this:
http://www.ebay.com/itm/Weld-on-Addi...-/261843239446