Here's the deal, I swapped a JDM D15B into my DX. I have a socketed P28 ECU, and 5 aftermarket chips, plus one custom made for the D15b's off the D15B's basemap.

The problem is, I had one tank that i actually got 17 mpg, thats really really low, also other evidence of a rich A/F mixture is i am getting carbon deposits on my rear bumper. Mind you ive had this exhaust for about 3-4 yrs, with no carbon deposits on the stock D15b7 engine. Also with any of the 6 chips, i'm idleing at about 900 rpms, with the stock setup it will idle at about 500-600 rpms..

Any thoughts on what could be causing the bad A/F mixture?

 

try adjusting your throttle... it might be to closed for all the chips you have set in the computer