check engine light

my light came on after a couple days of driving since my new jdm b18c swap ... it would come on with the old motor that i threw a rod in , but when acura looked at it they got an signal saying emissions leak ( prolly due to the gas cap they said ) .... i took this new motor back to the shop and it was the same thing .... the shop said since i have the same ecu it is prolly getting the old signal again .... also they said it could be due to a vaccum line ?? i just dont want to throw another rod so i need any advice you can give me ... what do yalll think ??

you have a differnt motor but same ecu? why not get a new ecu with your new motor?