2 issues with 98 jgc

[partsmanctc]

hope someone can help
when ever i put gas in the engine light comes on and the jeep has no power,i can barely make it up a hill.
gas cap has been replaced twice,1 oem and 1 aftermarket,i don't have access to a code reader.
once i can get the engine light out{disconnecting the battery}it's seems like i can smoke the 4 tires,there's that much power.this light will come on ONLY when i puts gas in.
the second issue is i'm installing a trans. cooler and i don't know which line is the return line from the rad, is it the upper one with fitting or lower without the fitting.any suggestions on both issues.
thanks in advance