last week I swapped to an 04+ 2 bar map on a L26 top swapped L67 engine. when I did the swap my fuel gauge showed empty with the low fuel light on, I got a code for P0463, and I got a check engine light with code P1635, which is for 5 volts reference for the tps, map, maf and fuel gauge. so this week, to verify the map was causing the issue, I put the L67 map sensor back in, and the codes went away and the fuel gauge started working. I don't understand why a map sensor would cause that issue.